Furnace sizing and Goodman GMH95 questions

[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.

[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.