Furnace sizing and Goodman GMH95 questions

[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.