HVAC Recommendation

Greatwhitewing · May 15, 2026

36x22 two story garage style shop. Zone 1 is 716 SF slab first floor PLUS 470 SF upstairs with air exchange via 42" stairwell need to keep above freezing and cool max upper 80's. Second floor is mostly wood and tool storage and perhaps the finishing room which will have a suitable exhaust fan.
Zone 2 is 420 SF in separate second floor as a finished rec room with say 50 F min and max 80 F. Finished means drywall, trim, floating floor, ceiling fans etc.
All 2x6 walls with fiberglass batt insulation. New construction, insulated 8' garage door and a few windows on second floor.
Located in south central MA with 0 F as a peak low temp and 100 F peak max. Typically 20-85 F.
My WAG is 2 ton heat pump with electric backup heat. Current plan is two zone with air handlers just inside the wall the heat pump will be located with very short line sets.
Keeping the first floor air handler as far away as possible from dust with plans for robust dust collector.

Questions
1. Is that the right sized HP?
2. Can/should I add a second air handler for zone 1 on the second floor with a long line set run around 30-40'?
3. Any reasonable options for additional heat source using scrap wood/pellets/used motor oil or is this more trouble and risk than it saves.

PoorUB · May 15, 2026

I question the space sizes? 36x22 is 792 sqft, and you second floor rooms add up to 890 sqft

Keep in mind most mini splits recommend a minimum line set length, something like 15 feet.

I would be tempted to stick a single 12,000 BTU in the ground level, and a 18000 BTU two head unit in the two second floor spaces as they would heat and cool similarly.

You need to consider heating capacity and lowest temp it will heat, more than the cooling capacity. Al manufacturers are a bit different. Even though you say it only gets to around zero in the winter, your minisplits need to heat much lower than that or you will not have much capacity at zero. Get units that advertise heating down to around -20F. You might need to go with an 18,000 on the ground floor and a 24,000 on the second floor, again, it depends on the units. If you size them correctly there will be no need for backup electric heat, unless you just want to be safe in case the minisplit fails.

Greatwhitewing · May 15, 2026

PoorUB said:
I question the space sizes? 36x22 is 792 sqft, and you second floor rooms add up to 890 sqft

Keep in mind most mini splits recommend a minimum line set length, something like 15 feet.

I would be tempted to stick a single 12,000 BTU in the ground level, and a 18000 BTU two head unit in the two second floor spaces as they would heat and cool similarly.

You need to consider heating capacity and lowest temp it will heat, more than the cooling capacity. Al manufacturers are a bit different. Even though you say it only gets to around zero in the winter, your minisplits need to heat much lower than that or you will not have much capacity at zero. Get units that advertise heating down to around -20F. You might need to go with an 18,000 on the ground floor and a 24,000 on the second floor, again, it depends on the units. If you size them correctly there will be no need for backup electric heat, unless you just want to be safe in case the minisplit fails.

The SF are rounded numbers with a second floor overhang on three sides with over 100 additional SF

I posted what I thought the outside extremes and normal ranges and min heating and cooling capacities which are not even close to usual habited spaces. 0-100 for extremes and more normal range of 20-85 outside

A second unit is option I would like to avoid if possible

PoorUB · May 15, 2026

Greatwhitewing said:
The SF are rounded numbers with a second floor overhang on three sides with over 100 additional SF

I posted what I thought the outside extremes and normal ranges and min heating and cooling capacities which are not even close to usual habited spaces. 0-100 for extremes and more normal range of 20-85 outside

A second unit is option I would like to avoid if possible

Well, heating capacity numbers are reasonable, but a minisplit often doesn't heat full capacity in cold weather. You might need a larger unit to get the heating capacity you want. You need to pick a unit and check the specifications for it. Some will barely heat to 20F, let alone zero. I am not going to do your shopping for you!

You could go with a three head unit, or if you are unconcerned of the storage area and letting the temperature swing, a two head would work too. But keep on mind heat rises, so any heat produced in the winter will roll right up the stairs and the cold will fall down the stairs and create drafts. I would condition the space if it is open the the garage.

HVAC Recommendation

Greatwhitewing

Well-known member

PoorUB

Well-known member

Greatwhitewing

Well-known member

PoorUB

Well-known member