Furnace sizing and Goodman GMH95 questions

[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.

[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.

[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.