Furnace sizing and Goodman GMH95 questions
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?
Last edited by Homeowner314; 02-04-2012 at 10:35 AM.
Originally Posted by Homeowner314
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.
Originally Posted by JonesHVAC-R
My yearly maintenance would be out of pocket, not covered by GoodCare, right?
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.
A couple of points of confusion.
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.
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.
Last edited by beenthere; 02-04-2012 at 12:58 PM.
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.
Prices for equipment service or installs isn't permitted.
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'.
Originally Posted by beenthere
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?
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.
Originally Posted by beenthere
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.
Now I have a duct question.
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.
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.
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?
Originally Posted by Homeowner314
The unit is nearly perfectly centered in the house.
Originally Posted by JonesHVAC-R
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.