I'm building 50x18 (4000 sq ft) shop with 18ft high ceilings. The walls and roof are sprayed with 1.5 inches of closed cell spray foam insulation. I have three insulated garage doors (2) 12x12, (1) 12x14. We are installing a HVAC system with gas heat and we are trying to determine how many tons needed to heat and cool the space. One contractor recommends (two) 5 ton units and another recommends a 5 ton unit and a 3 ton unit. Anyone have any experience with hvac systems with high celings?
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50x80 shop HVAC sizing help
- Thread starter matt74
- Start date
theoldwizard1
Well-known member
Location ?
If you have not poured the floor yet you may really want to think about in floor heating. Heat rises so with your tall ceilings the heat source starts out at the lowest point the hottest and will go up to the ceiling from there. With a 18' ceiling that would be an excellent way to go.
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Location would help a lot.
I come up with 120,000 BTU furnace for heat, so if you are going with 80% you will need two units.
If it were me, I would put in one 120,000 BTU unit and plan for the possibility of a second smaller unit if you need it, you might not. Slap 5 ton of AC on it and see what happens. If you need more add a another smaller unit. I would think a 45,000 BTU with 2 tons of cooling.
With that ceiling height, you will want a couple good ceiling fans. Not some Walmart junk.
I come up with 120,000 BTU furnace for heat, so if you are going with 80% you will need two units.
If it were me, I would put in one 120,000 BTU unit and plan for the possibility of a second smaller unit if you need it, you might not. Slap 5 ton of AC on it and see what happens. If you need more add a another smaller unit. I would think a 45,000 BTU with 2 tons of cooling.
With that ceiling height, you will want a couple good ceiling fans. Not some Walmart junk.
Sorry, I should have stated I’m in central TN, (Zone 2 or 3). The concrete is already poured and I didn’t think about radiant heat in the floor until it was too late. I’ll probably add a wood stove to supplement the heat when it’s real cold but I definitely don’t want to rely on it. I will just use the wood heat when I’m down there for long periods of time in the winter. Here in TN we have to worry about cooling as much if not more than heating.
I also built a room inside the shop 16x31 (496sqft) with 9ft ceiling. So we will zone one of the units for that room and once the room it’s up to temp it will switch and dump everything into conditioning the rest of the shop. The top of the room is all open with 8ft clearance to the ceiling and will be conditioned with the rest of the shop. It’s still just a total of 4000 sq ft space being conditioned.
Everything is residential and we are installing all of the ductwork in the trusses before the metal goes on the ceiling. So ideally we would get it right on the front end and not have to go back and add more later after the ceiling goes up.
There is also the element of closed cell insulation not breathing so it doesn’t need to be oversized where the units aren’t allowed to run enough to expel the moisture from the air.
I also built a room inside the shop 16x31 (496sqft) with 9ft ceiling. So we will zone one of the units for that room and once the room it’s up to temp it will switch and dump everything into conditioning the rest of the shop. The top of the room is all open with 8ft clearance to the ceiling and will be conditioned with the rest of the shop. It’s still just a total of 4000 sq ft space being conditioned.
Everything is residential and we are installing all of the ductwork in the trusses before the metal goes on the ceiling. So ideally we would get it right on the front end and not have to go back and add more later after the ceiling goes up.
There is also the element of closed cell insulation not breathing so it doesn’t need to be oversized where the units aren’t allowed to run enough to expel the moisture from the air.
You're warmer down there than here in Iowa but according to the design temperature, only a few degrees. With a well insulated building I'm still thinking 5 ton would be enough. What kind of indoor temperature are you expecting?
I'm not sure how well it's going to work trying to zone a big system to heat or cool only a small room.
I'm not sure how well it's going to work trying to zone a big system to heat or cool only a small room.
I agree. A 5 ton will be close to 2,000 CFM for a room that needs 400 CFM. You can zone it, but you will need a large bypass damper, and probably still dump air into the shop, but partially close that damper. That is a lot of air to choke off.I'm not sure how well it's going to work trying to zone a big system to heat or cool only a small room.
danski0224
Well-known member
Load calculation?
Then you will know the answer.
Then you will know the answer.
1 1/2" of insulation at R6 is only an R9.
That is grossly insufficient. I would triple that on the walls and quadruple that in the ceiling. Then put in a 3 ton heat pump mini-split.
My garage is half the size of yours and only 12' ceiling height but I can heat it with a 750w small heater. It is has R28 in the walls and R60 in the ceiling with an insulated sill.
That is grossly insufficient. I would triple that on the walls and quadruple that in the ceiling. Then put in a 3 ton heat pump mini-split.
My garage is half the size of yours and only 12' ceiling height but I can heat it with a 750w small heater. It is has R28 in the walls and R60 in the ceiling with an insulated sill.
1 1/2" of insulation at R6 is only an R9.
That is grossly insufficient. I would triple that on the walls and quadruple that in the ceiling. Then put in a 3 ton heat pump mini-split.d
spray foam salesmen tend to pretend they're selling magic, and ignore the actual performance of their stuff, because it's so expensive to do proper depth spray foam, and doing a hybrid installation removes a bunch of the perceived advantages of foam.
dcg9381
Well-known member
We need to know:One contractor recommends (two) 5 ton units and another recommends a 5 ton unit and a 3 ton unit. Anyone have any experience with hvac systems with high celings?
1) You location
2) You use case (heating, cooling, full time, or part time)
I don't know manual-J math that works with 1.5 inches of foam and 36K unless it's in California.
TN, there's no way that will heat on 36k.
Make the HVAC guys show you the math (manual-J). If you have big uninsulated doors, that's another consideration.
danski0224
Well-known member
Plenty of them are math adverse.Make the HVAC guys show you the math (manual-J).
As are many of the people that pop in here, looking for free design work.
dcg9381
Well-known member
As someone who has gotten a lot of great electrical advice here, I resemble that remark.As are many of the people that pop in here, looking for free design work.
I am even more convinced that a 120,000 BTU 80% will heat it, I wouldn't be surprised if 5 ton will cool it too.Sorry, I should have stated I’m in central TN,
If 5 ton doesn't keep up for the few days of extreme heat, would it be an issue? So you have a week or two where the AC runs, but the indoor temp never gets to set point, who cares? I bet it will work fine 340 days out of the year and the days it might not keep up, it might 75-80 degrees, but no humidity.
There is no way you need 10 ton of cooling! And I will bet you don't need over 5 ton of cooling either.
?????spray foam salesmen tend to pretend they're selling magic, and ignore the actual performance of their stuff, because it's so expensive to do proper depth spray foam, and doing a hybrid installation removes a bunch of the perceived advantages of foam.
I have used spray foam on many multi-plex's and my own home. It is magic and it works. Good insulation is worth the money for performance, comfort and cost savings.