45K or 75K

[HDFlattrack]

I have a 3 stall garage in southeastern Minnesota so it goes without saying that it can get very cold and stay there for weeks at a time. I have a trusted professional who will be installing the gas line due to come in the next week. Throughout our conversation on which size heater to buy in his opinion 45k BTU would be enough but would cycle to the point that it may be hard to keep up at times. The local Menards had a 75K BTU unit that was recommended for a 3 stall of our size and that is what I purchased due to the fact that 45 & 75 were the only choices. Did I make the correct choice or is too much not a good idea in this case? I plan on setting it to maintain 45-50 degrees when unoccupied and to knock off the cold when Im in there a few times a week at 55-60. Any constructive input is appreciated.

 

[CNGsaves]

Sounds fine for your environment in MN to have 75K Btu hanging NG heater.

Didn't mention what all insulation you have and heat losses??

 

[HDFlattrack]

Sounds fine for your environment in MN to have 75K Btu hanging NG heater.

Didn't mention what all insulation you have and heat losses??

Thanks for the reply. The walls and celiing are insulated to code...what exactly that is Im not sure but I will find out. The double door and single stall door I have insulated personally with foil backed styrofoam which has helped to the tune of maintaining 35-40 degrees even when the temps dip below zero. There is some heat loss, which will be rectified shortly, at the weather seal around the doors. I have some work to do to make the space "air tight" this summer which Im looking forward to.

 

[dave67fd]

Depending on what your 3 stall is in actual sq/ft, I would guess your best off with the 75.

 

[HDFlattrack]

Depending on what your 3 stall is in actual sq/ft, I would guess your best off with the 75.

I apologize for not having exact numbers but we are just short of 900 sq ft.

 

[Jackfre]

A heat loss generated on the Manual J platform is based upon data from the Federal Binn numbers. BN's are a 25 yr compilation of weather statistics by hour by area. I used Minneapolis weather data. According to that data the weather in Minneapolis has an average of 61 hrs/yr below -10, 240 hrs from -10-5f, 177-5-10f, 219 10-15f, 288 15-20f, 357 20-25f, 526 25-30f, 526 25-30f, 598 30-35f, 666 35-40f & about 2200 hrs above 40.

IF you run a heat loss on your building you use as a base number the "design temperature". In Minneapolis it is probably -10f or maybe -15. Once your heat loss calculation is done it is essentially saying that if you select your heating plant to match its "design temp calculation" then when the temperature is -15f the unit will run constantly but will maintain 70f in the space. You are in the very low temp ranges usually around 10% of the heating season. Therefore, your heating plant is typically only grossly oversized the rest of the heating season. Then there is generally the oversizing by selection that happens because we do not want to be undersized. What an oversized heating plant gives you, depending upon your distribution system, is rapid temp rise. The down side is that the to big unit is short cycling like crazy the majority of the heating season.

I'd run the gas line for future expansion and probably go with the "trusted professionals" recommendation.

It is freakin' cold there!

 

[ChargedUp!]

I just went. Through this with my build. I'm actually in Plymouth, MN and built 1000 sq ft attached and insulated. I was ready to buy the bigger unit because of what I guessed was the right answer. I ended up trusting the heating guy and went with the 45k sized unit. He said I will have no problem heating the garage to 70 if I want. I said OK since he was the pro and knew more than I. He did state several of the reasons above as well so I'm confident in going with his recommendation.
Good luck.

 

[Shop Specialties]

I would say 45k would be adequate.

 

[pop pop]

Too large will cause condensation and corrosion in the heat exchanger on short cycles. Go with the knowledge, not the guess.

 

[RKA]

I'd go with a 45. You're saying you're only trying to maintain it 10 degrees higher than it stays right now with no heat. That's nothing. And the days you're out there, you'll bump it 20-25 degrees. I'd prefer the longer cycle times on the smaller unit for the reasons mentioned.

Be careful about trying to keep that space too tight. If you're pulling cars in with snow that's a LOT of moisture to trap in that heated space. That moisture will freeze on windows and cause corrosion issues on tools and such. Based on the garage temps you're describing, it's probably pretty well sealed already (or heat from the house is bleeding into the garage).

 

[Steevo]

I used a 75k for my 960 sq ft 24x40 with 12' ceiling.
I have R21 walls and R60 ceiling insulation.
I keep it set at 50 degrees 24/7, and the heater only cycles a few times a day if the door is kept shut.

 

[bd8134]

I have a 45k propane for ~1,400 sqft with mostly 12' ceiling, 10' for a 3rd. R21 walls, R30 ceiling, set also 50 degrees 24/7, runs 68 or so when working in there.
When less than say 20 degrees outside it takes ~90 minutes to get to temp but it can certainly get it hotter if necessary. My heater stays on longer and will cycle less than a larger unit, but that is what I had, I will go bigger if this one is no longer serviceable.
The other advantage of a smaller unit is during the fall or spring. It might be necessary to only raise the temp 5 or so degrees. With a larger unit it might keep cycling possibly create condensation in the tubes.
The smaller unit should be fine for you. Might depend though on how cold you will let your garage get if you are not in there and what you want to get your temp to.

 

[skyking]

I have a 150,000 btu in my 50x40x14. Insulated .

 

[Chetter]

I run a 45000 btu unit in my 768 sq ft garage and it is fine. I have 10ft ceilings and I insulated the ceiling, walls, doors and around doors. Heat it 24/7 at 50* when I'm not in there and when I do go out and turn it up to say 56-70* takes about 20 minutes to bring up to temp.

 

[cowboyjosh]

In my 1228 SQ FT 4 car garage; I have a 60k Reznor. The HVAC guy did the heat loss calc, all my walls and ceilings are insulated and my CHI garage doors are R-17. Too many people think the biggest furnace is the best, the best furnace is the proper sized furnace.

 

[dave*99]

45K should be fine based on your usage. I have 40K in a similar sized garage in NJ. I only turn it on when I am going to work out there. It brings the temperature up 1 degree per minute from a 40 degree start point. Although it rarely gets that cold due to the insulation in the walls and ceiling. You said you would run the heat full time at a lower temp and bump it up when you are there. This means you don't need the rapid temperature rise the 75K would offer.

 

[KPSquared]

You are not going to regret the 75k. I have a 75k in my 30x36 and it works great. Once it's heated up, it barely has to kick on to bring the place back up to temp.

When I did my heater research, the 75k "rated" for 900 - 1400 sq ft or something like that.


When it gets to -40 and I have to open the doors, having the big beast fire up is great. . .