Furnace sizing and Goodman GMH95 questions

[Homeowner314]

I'm in the process of buying a foreclosed home with a missing furnace and have an affordable quote for a Goodman GMH950703BX, but I have a few questions. Since it's February in Michigan, I'll need to be have the contractor do the furnace install as soon as I get possession. I do not want to pay for central air.

House is 30x40 ranch, built around 1980, with triple pane vinyl windows, which I assume are nitrogen filled. Insulation is unknown, doors insulated steel. There's a double pane sliding door, but it opens into an unheated 3 season room, so I assume it's no worse than a triple pane slider. Home has no discernible drafts on a windy day. The windows especially seem to be very high quality. It's in lower Michigan, 7000 degree days, can expect five to fifteen days a winter when temps drop below zero at night. -20 might happen every ten years or so.

In sizing, he used the 50 BTU / square foot rule and rounded up to 70k. I'll have to use propane, so it looks like actual output is 59k. The next size smaller is GMH950453BX, which puts out 39k on LP.

I've spent some time with a few online heat load calculators and with conservative assumptions like R10 walls and R20 ceiling and one air change per hour, I'm getting around 35-40k, worst number I can recall is 42k BTU at 92 degree temperature differential. I ignored the attached garage and the three season room, which together cover about 30% of the outside wall area, so I really think the heat loss should be less than I calculated.

Here's my main question:
Is there anything wrong with asking for the 45k furnace, if it might run 80-100% of the time on a really cold night? The vast majority of the time from October-May I would expect it to be running on the low stage. Seems like using a 70k would defeat the purpose of having a two stage burner.

With LP around $3 a gallon, I'm thinking that if a 45k furnace that burns 1/2 gallon an hour can't handle the load, I need to vigorously seal and insulate, not step up to the 70k furnace.

My next question, although the GMH95 is a two stage furnace, it appears you use it with a single stage thermostat.
And you can set it to run single stage, low stage for five minutes followed by high stage, or auto, which can vary from 1-12 minutes on low stage as needed? Does the furnace itself decide how long to run on low stage when you set it for "auto", and is this going to be the best setting to use?

Finally, is GoodCare extended warranty worth buying? None of the brochures on Goodman's web site describe it in any detail.

Edited to add: If I ask for a 20x25x4" media box on the air inlet and use MERV 12 filters to help with allergies, would this harm system performance?

[JonesHVAC-R]

I'm in the process of buying a foreclosed home with a missing furnace and have an affordable quote for a Goodman GMH950703BX, but I have a few questions. Since it's February in Michigan, I'll need to be have the contractor do the furnace install as soon as I get possession. I do not want to pay for central air.

House is 30x40 ranch, built around 1980, with triple pane vinyl windows, which I assume are nitrogen filled. Insulation is unknown, doors insulated steel. There's a double pane sliding door, but it opens into an unheated 3 season room, so I assume it's no worse than a triple pane slider. Home has no discernible drafts on a windy day. The windows especially seem to be very high quality. It's in lower Michigan, 7000 degree days, can expect five to fifteen days a winter when temps drop below zero at night. -20 might happen every ten years or so.

In sizing, he used the 50 BTU / square foot rule and rounded up to 70k. I'll have to use propane, so it looks like actual output is 59k. The next size smaller is GMH950453BX, which puts out 39k on LP.

GMH950703BX input rating is actually 69,000BTU and output is 66,400BTU per manufacturer spec. To maintain you would be using about 60% of the 66,400BTU. Propane burns @ 93,000BTU/1gal.

I've spent some time with a few online heat load calculators and with conservative assumptions like R10 walls and R20 ceiling and one air change per hour, I'm getting around 35-40k, worst number I can recall is 42k BTU at 92 degree temperature differential. I ignored the attached garage and the three season room, which together cover about 30% of the outside wall area, so I really think the heat loss should be less than I calculated.

Based on your geographical location and a house built in 1980 I calculate approx. 46,800BTU. Now, you also state that you have triple pane sliders. I say your right in stating that they are filled with nitrogen as every window manufacturer I know that offers the triple pane they fill them with nitrogen. Based on that I would be willing to bet that they didn't go cheap with insulation but for conversation sake we will say R-13 as that is the standard for a 2"x4" wall. We cannot ignore the two attached unconditioned spaces as there will be a loss there.

Here's my main question:
Is there anything wrong with asking for the 45k furnace, if it might run 80-100% of the time on a really cold night? The vast majority of the time from October-May I would expect it to be running on the low stage. Seems like using a 70k would defeat the purpose of having a two stage burner.


Nothing wrong with asking, although remember you are hiring a professional in your geographical area. Personally I would go with his recommendation if it were my home.

With LP around $3 a gallon, I'm thinking that if a 45k furnace that burns 1/2 gallon an hour can't handle the load, I need to vigorously seal and insulate, not step up to the 70k furnace.

My next question, although the GMH95 is a two stage furnace, it appears you use it with a single stage thermostat.
And you can set it to run single stage, low stage for five minutes followed by high stage, or auto, which can vary from 1-12 minutes on low stage as needed? Does the furnace itself decide how long to run on low stage when you set it for "auto", and is this going to be the best setting to use?

You can use single or two stage stats. This will be set up by the contractor. It should be setup to offer you the full benefit of the furnace you are installing.

Although others believe differently, I feel your better off with a 2-stage. The "controlled" studies by engineers show that aside from the comfort you truly have an energy savings. Some techs believe that a single stage is better because your using full resources instead of 60% to maintain temp. Personally and in my professional opinion I agree with the "controled" studies.:.02:

Finally, is GoodCare extended warranty worth buying? None of the brochures on Goodman's web site describe it in any detail.

This will extend your parts/labor to 10years and pays for all the labor, unless there is a call that you just needed to replace the batteries in stat, to include the call charge as well as the parts. However you need to keep up on your yearly PM's.

Edited to add: If I ask for a 20x25x4" media box on the air inlet and use MERV 12 filters to help with allergies, would this harm system performance?

I say no.

:.02:

[beenthere]

Better off with the GMVC. It can use a thermostat to control staging, and provide you with the comfort you will be paying for.

Remember, your house is not a controlled lab/study. When set up with a combustion analyzer, its easy to tell that first stage burns less efficient then second stage.

[Homeowner314]

GMH950703BX input rating is actually 69,000BTU and output is 66,400BTU per manufacturer spec.

From the spec sheet I downloaded, it's 66,400 on NG and 58,995 on LP. If you're saying play it safe and go with the 70k, the lower output on LP would sway the decision towards the bigger unit.



This will extend your parts/labor to 10years and pays for all the labor, unless there is a call that you just needed to replace the batteries in stat, to include the call charge as well as the parts. However you need to keep up on your yearly PM's.

My yearly maintenance would be out of pocket, not covered by GoodCare, right?



Based on your geographical location and a house built in 1980 I calculate approx. 46,800BTU. Now, you also state that you have triple pane sliders. I say your right in stating that they are filled with nitrogen as every window manufacturer I know that offers the triple pane they fill them with nitrogen. Based on that I would be willing to bet that they didn't go cheap with insulation but for conversation sake we will say R-13 as that is the standard for a 2"x4" wall. We cannot ignore the two attached unconditioned spaces as there will be a loss there.

A couple of points of confusion.
1. I've got double hung triple pane throughout, except for a 6x7 double pane patio door which opens into unconditioned enclosed space. Windows appear to be clear/uncoated and I'm 99% sure there's no argon/krypton in them.

I guessed that I could throw the 42 square feet of patio door in with the triple pane windows at R-2 because it had enclosed space on the other side. Maybe I should separate it from the other window area and call it R-1.5?

2. When I said I ignored the unconditioned spaces, I meant that I treated the walls with unconditioned space on the other side as if they had the great outdoors on the other side of them.

3. I forgot to mention that I really didn't know what to do with 1200 sq feet of subfloor and carpet over a basement, so I could be quite a few BTU off in my calculations. Your 46k might very well be more accurate than my 38k. I really should break into the attic and at least figure out the correct R-value up there before he orders the unit.


Beenthere, you think it's worth an extra $ for a basic two stage thermostat and the GMV furnace? I should mention that this is a $25k house that's taking up most all the money I've got, I'm not keeping it for more than 2-3 years, and when I'm done rehabbing it I'll probably get no more than 70k for it. I can't pay $ for the proper Goodman communicating thermostat, I paid less than that for the computer I'm typing this message on.

[beenthere]

Prices for equipment service or installs isn't permitted.

Yes, its worth it.

[Homeowner314]

Yes, its worth it.

From another thread, I see that the GMH is 'the only 2 stage that can't use a 2 stage thermostat' and 'as the house heats up, the 2nd stage kicks in on time delay and blasts you out'.

So, there's a reason why the GMH comes factory set for single stage operation?

And I should either size it small and run it as a single stage or get a full fledged 2 stage with variable blower, and the latter is a much better way to go?

It's looking like the GMH70 would not be as comfortable as a GMH46 or a GMVC70.

The cheapskate in me is still thinking of going with the GMH46 and planning on improving the house if I need more heat. But since it'll be late February or early March before the system is running, I won't know until next winter if it works.

BTW, I just found some place on the net that gave +1°F as the heating design temp for Lansing, Michigan. Is it really that high?

[beenthere]

Don't know what your design temp is. Could be that high.

Get the smallest GMVC, and let the thermostat control staging. make improvements to your home as time and money allow.

[Homeowner314]

Get the smallest GMVC, and let the thermostat control staging. make improvements to your home as time and money allow.


Sounds like a plan, as long as I don't need ComfortNet. Those t-stats would be nice at 1/2 or 1/3 the price I was quoted.

Do you have a specific suggestion for a 2-stage t-stat, or just buy whatever looks good and has the features I like?

BTW, I visited the house today. Still don't know what's in the walls and attic, but I found out the large center part of the living room is only double pane, and all the windows are 1994 manufacture.

I previously assumed that the rest of the house was equal to the windows. Now that I know somebody replaced all the windows in a 15-20 year old house, I'm thinking the previous owner was having trouble with heating costs.

The vinyl siding might be the same vintage as the windows, so maybe they would have upgraded the insulation while the siding was off. If not, I suppose I could take off the siding and add an inch of foam and a tyvek wrap.

[Homeowner314]

Now I'm wondering about my ducts.
I'm assuming the rectangular trunk lines are adequate to supply the small round branches. The trunks look huge compared to the branches.

I have the following branches, all 13-15' long, straight runs with a round to rectangular 90° elbow connecting to the floor grids.

2 x 5" diameter
4 x 6" diameter
1 x 7" diameter

If I assume 75cfm per 5", 100cfm per 6", and 150cfm per 7", that's 800cfm, right? Are my assumptions too conservative?

The smallest GMH and GMVC furnaces come with 1200cfm blowers.

The trunk ducts have rectangular grids in them for basement heating, but I was thinking of blocking those off and not heating the basement.

Are my ducts adequate, or do I need to replace the entire system? Baseboard electric and 200A service is beginning to look better all the time. I can install that myself.

[beenthere]

You might get that much air from them.

Electric baseboard while cheaper to install. Would cost a lot more to heat the house with.

TH5220D or TH6220D if you want programmable.

[Homeowner314]

Thank you for all your help.

When you say 'might move that much', are you talking about my 800 cfm guess or Goodman's 1200 cfm blower rating?

Electric used to be much more than LP, but 100% electric at 0.12/kWh is very comparable to 95% efficient LP at $3/gallon, and last I knew it was about $3.20 per gallon. OTOH, it might just be that my propane price isn't regulated, so it goes up weekly, and it takes a year or more to raise my electric rate. Next year's electric might be much worse than this year's propane.

[beenthere]

Is that 12 cents with all additional fees included?

[Homeowner314]

Is that 12 cents with all additional fees included?

All the per kWh fees add up to about 12 cents on winter rates. Summer rates are 12 cents for the first 600 kWh and about double for anything over 600. I didn't count the fixed monthly costs, because those are the same unless I go completely off grid.

Then there's taxes on top of the entire bill. But there's 4% sales tax on the LP that isn't included in the price either.

My LP is $2.399+tax on a forward contract. A friend paid $2.699 for his contract with another supplier. Last fill I got, they told me it would have been 3.199 if I hadn't prepaid last summer. Last fill my friend got, they 'forgot' he was on a contract and charged him 3.399. This year was the first time my contract was over $2, and I fear for next year's contract.

I can knock .20 off the propane price if I buy my own tank, but even a used tank costs more than putting 200A electric to the house.

If I were keeping the house long term, I'd put in baseboard for now and a heat pump later. I just don't think I could add the value of a heat pump install to the price of this place when I sell it. I bet I can't get the value of the variable speed blower back either, seems like an existing furnace is valued by its efficiency rating and its age. Maybe they'll look at the 46k furnace and think 'hey, this house doesn't need much heat', but probably they'll just want to see a year's worth of propane bills. Which reminds me, maybe I should call the propane suppliers and find out who delivered to my house and how much they delivered.

[BaldLoonie]

I wouldn't touch a GMH. If you want to go Goodman, either go the "real" 2 stage with a 2 stage stat or the communicating controller or go elsewhere.

Your quote sounds like maybe something I wrote. The advantage of 2 stage is long run cycles on gentle low. You cannot have that with a GMH or any timer controlled 2 stage. In the car, do you turn your heat hotter when the car is warmed up? NO, you turn it down. Think of that.

[JonesHVAC-R]

Thank you for all your help.

When you say 'might move that much', are you talking about my 800 cfm guess or Goodman's 1200 cfm blower rating?

Electric used to be much more than LP, but 100% electric at 0.12/kWh is very comparable to 95% efficient LP at $3/gallon, and last I knew it was about $3.20 per gallon. OTOH, it might just be that my propane price isn't regulated, so it goes up weekly, and it takes a year or more to raise my electric rate. Next year's electric might be much worse than this year's propane.

How big are the trunk lines (rectangular duct)? How big is the plenum (stack coming off of furnace)? How big is the return trunk and drop?

[Homeowner314]

Your quote sounds like maybe something I wrote.

I think it was you. I think I got it from searching for the word 'sizing'. So, is the GMH an OK single stage, but you'd definitely leave it on the factory default single stage setting?

Maybe I should just pay the extra for the GMVC, I'm still getting a great deal.
At least BT aimed me at a 2 stage programmable thermostat that doesn't cost more than a 1 stage and a case of Pepsi. I'd gladly pay low three digits for the communicating version, but not mid three digits.

[Homeowner314]

How big are the trunk lines (rectangular duct)? How big is the plenum (stack coming off of furnace)? How big is the return trunk and drop?

The unit is nearly perfectly centered in the house.
The plenum is 15x20 at the unit, going into a huge box that merges into a trunk going to each end of the house. I'm simply guessing here, but wouldn't be surprised if the two trunks were 8x20 or 8x24, or maybe 10 high. I'm taking a stepladder to check out the ceiling insulation Monday, I'll measure the trunks then. Normally I'm in the attic before bidding, but this time the PO glued the access port shut with Great Stuff.

The return grids are on each side of the central hallway, right above the unit. After they merge into one duct going to the unit, it's 23 inches wide, and I think eight or ten inches thick. Should I get a quote for setting up return trunks and branches so the returns function better with the doors closed? This might be getting expensive.

[JonesHVAC-R]

The unit is nearly perfectly centered in the house.
The plenum is 15x20 at the unit, going into a huge box that merges into a trunk going to each end of the house. I'm simply guessing here, but wouldn't be surprised if the two trunks were 8x20 or 8x24, or maybe 10 high. I'm taking a stepladder to check out the ceiling insulation Monday, I'll measure the trunks then. Normally I'm in the attic before bidding, but this time the PO glued the access port shut with Great Stuff.

The return grids are on each side of the central hallway, right above the unit. After they merge into one duct going to the unit, it's 23 inches wide, and I think eight or ten inches thick. Should I get a quote for setting up return trunks and branches so the returns function better with the doors closed? This might be getting expensive.

Sounds like the return and drop and trunk are sized adequately... I would add a return to each bedroom though. You'll get better air flow in doing this. Let us know about the supply trunk and we can get you some CFM ratings.

Edit: 8x20 would be 750CFM and 8x24 would be 930CFM. 10x20 would be 1060CFM and 10x24 is 1320CFM

[Homeowner314]

Sounds like the return and drop and trunk are sized adequately... I would add a return to each bedroom though. You'll get better air flow in doing this. Let us know about the supply trunk and we can get you some CFM ratings.

Edit: 8x20 would be 750CFM and 8x24 would be 930CFM. 10x20 would be 1060CFM and 10x24 is 1320CFM

So 8x20 x 2 would be more than enough on the trunks for a modern 46k unit, right?

Am I in the ballpark when I say 75cfm for a 5", 100 for a 6", and 150 for a 7" round? There isn't much info on the web for rigid round pipe.

On the returns, the back side of the walls the returns are on would be one of the bedrooms and the bath, so no new ductwork to do returns there. No, scratch that, the vanity is in the way of putting a bathroom return in. Good thing the bathroom door is only closed when somebody's in there. The other two bedrooms could each be reached with about 12' of branch off the return plenum.

[superfittertech]

So 8x20 x 2 would be more than enough on the trunks for a modern 46k unit, right?

Am I in the ballpark when I say 75cfm for a 5", 100 for a 6", and 150 for a 7" round? There isn't much info on the web for rigid round pipe.

On the returns, the back side of the walls the returns are on would be one of the bedrooms and the bath, so no new ductwork to do returns there. No, scratch that, the vanity is in the way of putting a bathroom return in. Good thing the bathroom door is only closed when somebody's in there. The other two bedrooms could each be reached with about 12' of branch off the return plenum.

you wouldn't want to put a return in the bathroom anyhow. That's what the exhaust fan is for when the door is closed. When no one is in there keeping the door open is fine.

[Homeowner314]

you wouldn't want to put a return in the bathroom anyhow. That's what the exhaust fan is for when the door is closed. When no one is in there keeping the door open is fine.

That's another thing for me to do in the spring. Code called for a window or a vent when this place was built. It has a combined laundry/bath with a small window in the laundry room end. I've got to put a ceiling vent in, because I don't think turning the dryer on is the best way to vent a bathroom. :whistle:

[JonesHVAC-R]

So 8x20 x 2 would be more than enough on the trunks for a modern 46k unit, right?

Am I in the ballpark when I say 75cfm for a 5", 100 for a 6", and 150 for a 7" round? There isn't much info on the web for rigid round pipe.

On the returns, the back side of the walls the returns are on would be one of the bedrooms and the bath, so no new ductwork to do returns there. No, scratch that, the vanity is in the way of putting a bathroom return in. Good thing the bathroom door is only closed when somebody's in there. The other two bedrooms could each be reached with about 12' of branch off the return plenum.

8x20 x2... are you saying there is 2 joints of duct?

5" = 50cfm, 6" = 85cfm, and 7" = 125cfm

No return for the bathroom. You can use panning instead of duct... use the joist space for return. You will need to open the tops of the return truck duct where you branch off though. Also, don't forget to cap the otherside of the trunk.

[Homeowner314]

8x20 x2... are you saying there is 2 joints of duct?

Plenum center of house, one trunk goes east, one trunk goes west. I like that part of the layout.



5" = 50cfm, 6" = 85cfm, and 7" = 125cfm


I've got 650cfm total on my branches, plus whatever goes through the basement grids in the trunks?
My floor grids are pretty slim too. I guess I need to add them to my list of things to measure.

Glad I didn't go with the guy who wanted to put a 70k on my existing ducts, everything he said seemed to be rule of thumb instead of specific to my house.

Installer I'm using says if I can get a Goodman for a lot less than he pays for a Gibson, I can plan it out and he'll hook it up.
Maybe I need to have him use his ductilator.

All of a sudden it looks like I need to start over from the beginning, with a room by room load calc and size my branch lines and floor openings to that.

If I've got a 1200CFM blower and 39k BTU/hr of heat to work with, do I just use 10CFM of pipe for every 325 BTU/hr of heat load? How many CFM for 14' of 8" round (not flexible)?

[JonesHVAC-R]

Plenum center of house, one trunk goes east, one trunk goes west. I like that part of the layout.



I've got 650cfm total on my branches, plus whatever goes through the basement grids in the trunks?
My floor grids are pretty slim too. I guess I need to add them to my list of things to measure. 650cfm is probably not enough...

Glad I didn't go with the guy who wanted to put a 70k on my existing ducts, everything he said seemed to be rule of thumb instead of specific to my house.

Installer I'm using says if I can get a Goodman for a lot less than he pays for a Gibson, I can plan it out and he'll hook it up.
Maybe I need to have him use his ductilator.

All of a sudden it looks like I need to start over from the beginning, with a room by room load calc and size my branch lines and floor openings to that. Good idea, lol

If I've got a 1200CFM blower and 39k BTU/hr of heat to work with, do I just use 10CFM of pipe for every 325 BTU/hr of heat load? How many CFM for 14' of 8" round (not flexible)?8" = 180cfm

Just remember you will never get a perfect duct design... cooling requires higher cfm than heating. That being said, a good rule of thumb is to make sure your return duct is sized according to your maximum cfm design output on the furnace. The supply I always down size about 4" in width for static pressure reasons. Your duct sizing will be based partly on your heat loss/gain calc. Say your maximum cfm out put is 900cfm for the heat gain/loss... I would use a supply trunk line sized at 8x20. Now 8x20 is rated for 750cfm but I want a little bit of pressure built up. Now, lets say you have a room that requires two supplies and the room requires a total of 170 cfm. I would use a 6" round duct per supply which is 85cfm/supply giving you a total cfm rating of 170cfm. Are you staying with me?

[Homeowner314]

Now 8x20 is rated for 750cfm but I want a little bit of pressure built up. Now, lets say you have a room that requires two supplies and the room requires a total of 170 cfm. I would use a 6" round duct per supply which is 85cfm/supply giving you a total cfm rating of 170cfm. Are you staying with me?

So the supply trunk you like a little tight, and you like good sized branches and returns? I suspect it won't be hard to open the holes in the trunk a little and put in bigger branches, and maybe bigger floor outlets. House would look better with new hardware on the floors too.

[beenthere]

Unless your supply trunk line is long. No need to have a static pressure build up in the supply. It can cause noise issues. Best to have a static as close to if not under .5" ESP.

Do you have central A/C.

[JonesHVAC-R]

So the supply trunk you like a little tight, and you like good sized branches and returns? I suspect it won't be hard to open the holes in the trunk a little and put in bigger branches, and maybe bigger floor outlets. House would look better with new hardware on the floors too.

The branches (take-offs) should be sized according to CFM needed. The floor outlets also sized according to CFM. All take-offs should have dampers. Your house as you stated is 30x40 so as far as beenthere's comment goes I would ask how far do your supply trunk lines extend? In most cases they go almost end to end and that is what I am assuming being your furnace is directly center of your home as you already stated. Remember to base everything off the calc.'s though.

[Homeowner314]

Do you have central A/C.

No, but I'm wondering about putting in a heat pump later.
Wouldn't see much use as A/C, but if I could size it to supply my heat down to 20-30° it could save some propane. Amana/Goodman claim 24k BTU and COP of 3 at 25° for their 16 SEER 2 stage 3 ton, but it wouldn't be cheap (by my standards).

I saw some used self contained 2 ton GSHPs for about the cost of a GMVC95045 and got excited for a while. After adding up the costs of digging a pond and upgrading to 200A service, I think I can suppress the urge to buy two of them and depend on 8 year old HPs of unknown history.


Your house as you stated is 30x40 so as far as beenthere's comment goes I would ask how far do your supply trunk lines extend? In most cases they go almost end to end and that is what I am assuming being your furnace is directly center of your home as you already stated.

Yep, trunks stop about 5' from the ends of the house IIRC. So the trunks and branches are all about 15' or a little less.

[beenthere]

The indoor coil will add restriction to the supply. Your supply trunks are short enough you don't need to worry about regaining static.

[JonesHVAC-R]

Yep, trunks stop about 5' from the ends of the house IIRC. So the trunks and branches are all about 15' or a little less.

15' each way? Which would mean your spanning 30', right?

[Homeowner314]

15' each way? Which would mean your spanning 30', right?

Yes.

[JonesHVAC-R]

That's what I thought. I would personally install as I suggested.

[Homeowner314]

Got it, thanks.

[beenthere]

15' each way? Which would mean your spanning 30', right?

The trunk line isn't additive when the furnace is in the middle of the line.

[JonesHVAC-R]

The trunk line isn't additive when the furnace is in the middle of the line.

I do not agree and a simple test with a velocitor can prove that. Your comment suggests that you will get the same amount of air flow/static pressure regardless of your lengths of trunk if the furnace is in the middle. If this were the case no contractor would have the need to down size trunk line when the furnace is placed in the center... However, I can agree that when placed in the center your loss is less than if placed to one side.

[beenthere]

I do not agree and a simple test with a velocitor can prove that. Your comment suggests that you will get the same amount of air flow/static pressure regardless of your lengths of trunk if the furnace is in the middle. If this were the case no contractor would have the need to down size trunk line when the furnace is placed in the center... However, I can agree that when placed in the center your loss is less than if placed to one side.

If the trunk line doesn't exceed 24 foot. There is no real reason to use a reducing trunk. Most common reason given is, "thats the way we have always done it"

Static pressure is not what moves the air.

[JonesHVAC-R]

If the trunk line doesn't exceed 24 foot. There is no real reason to use a reducing trunk. Most common reason given is, "thats the way we have always done it"<--Does that comment make your customers feel all warm and cuddly inside, lol? I'm not trying to be smart, it was just funny as it has been used countless times on homeowners. In the countless designs I and my father have done both residentially and commercially there has almost always been a reduction in trunk size, the exception would be like a cottage. We are very careful of how we size as we guarantee our Whole Home Comfort estimates/work.

Static pressure is not what moves the air.I wasn't suggesting that. I suppose I could have typed it differently for you.

Not the first time we have disagreed, and I'm sure it won't be the last... lol.:cheers:

[beenthere]

Thats just the excuse many people give for doing something a certain way.

So you admit there is no reason to attempt to keep a high static pressure.

[Homeowner314]

OK, did some checking.

The furnace is about 5' off center, so the supply trunks are 10' and 20', not 15' each. And they're only 8x10. Good enough for a variable speed 1200cfm, or will it ramp up and overwork itself and / or make wind noise?

The bigger ducts I remembered seeing were the return trunks, 10x14 and very short.

[Homeowner314]

I posted duct trunk sizes this morning, it's the last post on page 3.

30x40 single story, 8' side walls, basement constructed of 8x8x16" concrete block, with the top two rows of blocks exposed to air, ten rows buried. Basement is not living space, furnace and water heater are down there. Isolated from the living space only by subfloor and thinly padded carpet, with vinyl flooring on about 1/5 of it.

Out of 140 linear feet of outside walls, 22' are sheltered by the garage and 20' sheltered by the enclosed porch, 98' are exposed. I'm assuming the sides are insulated with 3 5/8" of loose fill cellulose, sheathed with cellotex and 1/8" foam under the vinyl siding. I'll have to pull some siding and check the wall fill, if it has settled I will be pumping more in to get rid of the voids.

I'd assign R10 to the exposed walls, but what number do I use for the sheltered walls on my load calculations?


Ceiling:
Probably had 6" of cellulose upstairs, I'd call it 5" now. No big deal, 30 more bags would bring it up from R19 to R44, assuming the drywall can take 1.5lb / sq ft.

Windows: I've got 101 sq ft (rough opening) of 1994 vintage triple pane vinyl windows, a 54 square foot window that's about 60% fixed double pane and 40% sliding triple pane, and a 42 sq ft aluminum frame sliding door with an enclosed porch on the other side.

Doors: There are three 3' insulated steeel doors, one has a storm door over it, the other two are sheltered by the enclosed porch and the garage.

[beenthere]

Check to see what insulation you have in those walls. Probably the same as the others.

[Homeowner314]

Does 'probably the same as the others' mean I should treat the wall exposed to the garage the same as a wall exposed to the outdoors when calculating heat loss?

If the ceiling is blown cellulose, what are the chances the walls are too, is it a near certainty?

If the walls are cellulose, they are or they will soon be packed full. I'll take off some soffits and drill the cellotex to find out if there are any voids at the top.

I think my only important unknown variable now is infiltration. Maybe a blower door service will have a spring special when the heating season ends. It looks like this house needs nothing badly enough (except for a functioning furnace) that I can't do without it for March/April/May. And I've quite firmly decided that if the house can't run on a 46k that it needs to be fixed until it can.

BTW, if I cheap out and get the GMH against all advice, is there any difference other than staging between a TH6110D and a TH6220D?

[beenthere]

Yep, treat it the same as any other exposed wall.

Nope, just staging.

[JonesHVAC-R]

Thats just the excuse many people give for doing something a certain way.Ahhh... I got ya.

So you admit there is no reason to attempt to keep a high static pressure.No, I don't. Taking velocity, cfm, etc... I still stand behind my recommendation.

Length of duct absolutly plays a factor.

[JonesHVAC-R]

OK, did some checking.

The furnace is about 5' off center, so the supply trunks are 10' and 20', not 15' each. And they're only 8x10. Good enough for a variable speed 1200cfm, or will it ramp up and overwork itself and / or make wind noise?YIKES! 8x10??? That will allow approx. 310cfm and YES will definately create a wind noise along with whistling. These absolutely need upsized.

The bigger ducts I remembered seeing were the return trunks, 10x14 and very short. This allows approximately 670cfm, they need upsized as well.

You have some issues there that need addressed

[JonesHVAC-R]

Does 'probably the same as the others' mean I should treat the wall exposed to the garage the same as a wall exposed to the outdoors when calculating heat loss?

If the ceiling is blown cellulose, what are the chances the walls are too, is it a near certainty?

If the walls are cellulose, they are or they will soon be packed full. I'll take off some soffits and drill the cellotex to find out if there are any voids at the top.

I think my only important unknown variable now is infiltration. Maybe a blower door service will have a spring special when the heating season ends. It looks like this house needs nothing badly enough (except for a functioning furnace) that I can't do without it for March/April/May. And I've quite firmly decided that if the house can't run on a 46k that it needs to be fixed until it can.

BTW, if I cheap out and get the GMH against all advice, is there any difference other than staging between a TH6110D and a TH6220D?

TH6110D is 1 heat/1 cool andTH6220D is 2 heat/2 cool. Personally I would use a different model as those two are Honeywells lower end digital...

[beenthere]

Length of duct absolutly plays a factor.

Since we're talking about the OP's duct system. Its short enough that it doesn't need to reduce. Which is what I posted earlier.

[Homeowner314]

TH6110D is 1 heat/1 cool andTH6220D is 2 heat/2 cool. Personally I would use a different model as those two are Honeywells lower end digital...

OK, I'd love to see a good debate about t-stats. Looks like the only meaningful adjustment on the Pro 6000 series is that I can tell it to use 3 cycles or 5 cycles per hour.

What T-stat do you think I should buy and what would it do for me?

Should I get the White Rodgers that looks like the Comfortnet, should I kiss my budget goodbye and get the Comfortnet?

BTW, I talked to my installer yesterday, he's afraid of my undersized ducts too and wants to see the system and map out a duct plan ASAP.
He also told me I should buy a 80%, because I'd never get the money back for a 95% when I sell the place. I don't think he realizes how cheap I can source a 95% Goodman. If propane stays at $3, I can get the difference between 80% and 95% back in one winter. And I think I can get it back on resale if I sell the house this summer.

[beshvac]

. I don't think he realizes how cheap I can source a 95% Goodman. If propane stays at $3, I can get the difference between 80% and 95% back in one winter. And I think I can get it back on resale if I sell the house this summer.


You getting the equipment thru your installer or are you gonna buy online? I guess it wouldn't matter to you if you are going to sell the house and the new owners will be stuck with no equipment warranty.

[Homeowner314]

You getting the equipment thru your installer or are you gonna buy online? I guess it wouldn't matter to you if you are going to sell the house and the new owners will be stuck with no equipment warranty.

Right now, the installer is somebody who does not sell Goodman. (and his budget brand is Gibson, which costs him more than I'd pay for Goodman)

Amazing coincidence. I was looking up Goodman's warranty this morning and found that wonderful clause. I started another thread on the warranty (http://hvac-talk.com/vbb/showthread.php?p=12384811#post12384811) at exactly the same time you pointed it out to me. But thanks for the heads-up, I would have been extremely unhappy to read the warranty papers upon receipt of the unit if you hadn't pointed that out and if I hadn't downloaded the warranty.

Anyway, does buying Goodcare from either the installer or the seller fix the problem?

[beshvac]

No

I used to work in a city that had 2 goodman distributors. It was a nightmare because you had to figure out which distributor provided the product because they would only warranty what was purchased from them. So warranty claims would get kicked back by them if the unit was purchased from the "other" guy.
:gah:

[Homeowner314]

Thanks, beshvac.

If I buy from the local Goodman shop, instead of buying it online and using the (non-Goodman) installer I already know and trust, and pay the extra $$$ for Goodcare, will that be transferable to the buyer when I sell the house in a year or two?

If not, I suppose there's always a claim for 'warranty of merchantability and fitness for purpose' against the internet seller and/or Goodman for the short time I have the house. Especially if I buy it from a vendor who loudly proclaims the ten year/lifetime factory warranty in his internet advertising.

[beshvac]

I don't know....I dropped goodman after poor local support for a warranty issue years ago. Which is the reason why you let the pro hvac company use the the equipment they like-because there are issues behind the scenes like warranty support, factory support, parts availability, etc.

[JonesHVAC-R]

No

I used to work in a city that had 2 goodman distributors. It was a nightmare because you had to figure out which distributor provided the product because they would only warranty what was purchased from them. So warranty claims would get kicked back by them if the unit was purchased from the "other" guy.
:gah:

I would have been a bit pissy on that one, the Manufacture is who reimburses the warranty and being that the build date codes are in the serial of the furnace you should have pressed that one. I think the only issue I have had is when the warranty was close to build date and I was trying my hardest to find out when purchased as then the warranty started from that day. Now if the homeowner had there invoice with install date then that changes everything and the warranty starts from day of install. I think you ran into a supplier who didn't want the hassle.


Thanks, beshvac.

If I buy from the local Goodman shop, instead of buying it online and using the (non-Goodman) installer I already know and trust, and pay the extra $$$ for Goodcare, will that be transferable to the buyer when I sell the house in a year or two?

If not, I suppose there's always a claim for 'warranty of merchantability and fitness for purpose' against the internet seller and/or Goodman for the short time I have the house. Especially if I buy it from a vendor who loudly proclaims the ten year/lifetime factory warranty in his internet advertising.

I am an Amana dealer which is Goodman's top end furnace. I am meeting with my rep today and will double check the Goodcare policy... I know I have asked this before and for some reason it is sticking in my head that it is transferable one time... I'll let you know later tonight

[JonesHVAC-R]

OK, I'd love to see a good debate about t-stats. Looks like the only meaningful adjustment on the Pro 6000 series is that I can tell it to use 3 cycles or 5 cycles per hour.

What T-stat do you think I should buy and what would it do for me?

Should I get the White Rodgers that looks like the Comfortnet, should I kiss my budget goodbye and get the Comfortnet?


Honeywell actually designs the stats for Goodman/Amana for their whole home comfort systems. They design them to communicate better with the equipment. This being said, I always you the branded model that is similar to the Prestige 2.0. It is a great system and it has the capability to be monitored while your away from home with the RedLink system. You can adjust system temps and turn systems on and off from your Android or Apple based devices.

BTW, I talked to my installer yesterday, he's afraid of my undersized ducts too and wants to see the system and map out a duct plan ASAP.
He also told me I should buy a 80%, because I'd never get the money back for a 95% when I sell the place. I don't think he realizes how cheap I can source a 95% Goodman. If propane stays at $3, I can get the difference between 80% and 95% back in one winter. And I think I can get it back on resale if I sell the house this summer.

I will post back a bit later...

[beshvac]

I would have been a bit pissy on that one, the Manufacture is who reimburses the warranty and being that the build date codes are in the serial of the furnace you should have pressed that one. I think the only issue I have had is when the warranty was close to build date and I was trying my hardest to find out when purchased as then the warranty started from that day. Now if the homeowner had there invoice with install date then that changes everything and the warranty starts from day of install. I think you ran into a supplier who didn't want the hassle.



I am an Amana dealer which is Goodman's top end furnace. I am meeting with my rep today and will double check the Goodcare policy... I know I have asked this before and for some reason it is sticking in my head that it is transferable one time... I'll let you know later tonight

Oh...I did get "pissy" but it did not matter. Now they call me and wonder why I am not buying equipment from them and I have to remind them...."you remember when I purchased a DOA unit from you and you told me "that's what the first year warranty was for?" and "your guy that verifies the warranty has to come out and look at the unit....and he can be out there 1 week from today?" Sure my customer is going to wait a week so some warranty bubba who is going to come out and verify the NEW AC they paid for has to be FIXED???:gah:

[Homeowner314]

JonesHVAC mentioned in another thread that I found reduced BTU numbers for LP conversions on a factory site.

The link for that is here at Goodman. (http://www.goodmanmfg.com/DesktopModules/GoodmanProducts%5CProductFiles%5C31SS-GMH95.pdf) (page 3 of the .pdf)

They give LP output numbers more than 10% lower across the whole GMH95 model line. I assume it just means that they burn 10% less BTU of fuel on propane. Anybody with an * can post the link in this thread (http://hvac-talk.com/vbb/showpost.php?p=12384161&postcount=17) if they want.

[JonesHVAC-R]

Ok, here's the low down, lol. The Goodcare warranty can be transfered one time as I had thought.

[Homeowner314]

Thermostat question:
Is the 6220 dual fuel capable?
How about the 8320?

[beenthere]

No the 6220 is not a dual fuel thermostat. The 8320 is.

[Homeowner314]

I wish to thank everyone who generously donated their time to helping me.

There was a weekend sale, so I devoted my Sunday afternoon to deciding what I'm buying. My best estimate of design heat load is 39k, using simplified online instructions for Manuals J and D. I can knock 6k off that figure by ripping off the siding, installing Typar wrap, and sealing leaks wherever I can find them. (I get 12k BTU/hr for infiltration at one exchange per hour, and should be able to modernize it to 0.5 per hour)

Manual D tells me that I can just barely move enough air through my current ducts to heat the house, assuming outlet temps 30° above interior temp. If my temp drop in the ducts is low enough to give me warmer outlet temps, so much the better. It turns out my 8x10 trunks are more of a limiting factor than my branches, and I don't want to pay for new trunk lines if I don't have to.

I will need to have return ducts put in the bedrooms. We're still arguing over whether to put returns in the kitchen/dining/living rooms, I think the central hallway return just off the dining and living rooms should take care of that, installer wants returns in every room but the bathroom/laundry.

I'm getting the GMH950453BX, a 20x25x5 media box, and a Honeywell 8110 thermostat, mainly because the 6xxx series lacks a 'circ' setting and I want to move air through the filter periodically. The GMH can be set for medium-low on the fan and it won't try to force too much air through the ducts. I was worried a GMVC would kick up the fan too hard.

A week from now, I should have the furnace sitting on a concrete platform in the basement waiting for my installer to hook it up. It'll be interesting to see how he handles the media box, but he said I could buy the 20x25 instead of the 16x25. (furnace has a 16x25 knockout on the side for return air)

[JonesHVAC-R]

Glad we could be of help to you!:grin2:

[Homeowner314]

tedkidd should read this. My tiny furnace putting out 39,800 btu/hr on propane is too big for my 1200 sq ft, 1970's ranch house in Michigan.

I'm living in the house, getting 30% run times (10 3/4 minutes out of 35) on low stage in darkness with a moderate breeze with thermostat set to 24° above ambient. That implies I could get about 75-80° above ambient from low stage, if it were possible to run this furnace for more than 12 minutes on low stage. Only other heat sources during the measurements were 350lb of humans and an energy star refrigerator. It looks like my actual load at 0° outside and 70° inside is closer to 28,000 btu/hr than my calculated 39,000 btu, and I should be able to drop that to around 20k with a few $hundred of insulation.

The only improvement I've made to the house to date is to seal the ductwork. That dropped my basement temps a degree or two, and added maybe two or three degrees to my theoretical ability to heat the house. It's still 64° in the basement with the house at 70°, and I've got three drafty single pane steel frame windows down there, with 140' x 2' of 8" cement block basement wall exposed to outside air. I'm surprised at how close to conditioned temps the basement is holding, just from the water heater, furnace, ductwork, and floor above radiating heat down there.

Last winter my DIY manual J calcs gave me 4400btu/hr for the entire roof (1200 sq ft, 70° design temp, R19) and 8400 btu/hr for the exposed part of the basement walls (280 sq ft, 60° design temp, R2).

I plan to increase the ceiling insulation to R38 and add R6 of foam sheet to the top 2' of the foundation, as well as insulate the joist pockets above the block wall. If my calculations above are correct, I'll save more from $100 worth of foam and glue than I could by taking the ceiling to R-infinity. Still, $250 worth of attic cellulose to drop my design load by 2200 btu/hr should be well worth doing when I'm paying $2.20 a gallon for propane.

A friend who's a block mason strongly recommends insulating the outside of a basement rather than the inside, says that by insulating the inside you can drop the wall temp so that the earth up against the wall can freeze and push the wall around.