We are building a new construction house with a 27’x42’ 4 car attached garage. Being in Minnesota it will have a heater. But what size is recommended? I will be building some kit airplanes in there which are multi year projects. So comfort is important. It will probably be a mix of leaving the heater off and having the temp set at 50 degrees during the winter; with just turning it up to 70 while working out there. I’m currently leaning towards a 200K btu 80% garage heater. For a quick blast up to temp. Any thoughts?
Heater size?
- Thread starter N969DP
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You are at least 3 times too large!
I have a newer 24x38 and heat it fine with an 80% 45,000 BTU furnace.
The largest I would put in is 60,000 BTU, and I believe a 45,000 BTU heater would be plenty large.
For what it is worth, I live in North Dakota, I also have worked in HVAC for 20 years.
I have a newer 24x38 and heat it fine with an 80% 45,000 BTU furnace.
The largest I would put in is 60,000 BTU, and I believe a 45,000 BTU heater would be plenty large.
For what it is worth, I live in North Dakota, I also have worked in HVAC for 20 years.
I heat my 420 sqftx10' shop with a 27k just fine. I as well think you're wayyyy oversized/.
I maintain 40° when not in there & crack it up when I'm working to 65° ish .
I maintain 40° when not in there & crack it up when I'm working to 65° ish .
toyotadriver
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I heat my shop 30x40x10 well insulated in a warmer climate than yours with a 80k Mr Heater Big Maxx. I don't heat it all the time so it cools down to as low as upper 40s without heat. It takes a lot of BTUs to raise the temp from upper 40s to mid 60s when it's cold outside and all the tools and equipment is at that same temp. A 60k would do it but Mr. Heater didn't make a 60k. I initially installed a 45k and it would heat it but it would also take a long time to get up to temp. I'm very happy with my 80k.
200k will DEFINITELY be grossly oversized.
200k will DEFINITELY be grossly oversized.
mike93lx
ALLIANCE MEMBER
How did you come up with 200k btu?
ripperd
Well-known member
I have that size attached garage in MN. I have a 45k btu unit and it works great. Just insulate well and there is no reason to go much larger.
Stuart in MN
Well-known member
This is the correct answer. If your supplier won't do a heat loss calculation, you can find a number of online calculators - run the numbers through several of them to get a consensus.
I am thinking a 200K btu so I can blast it up to temp in a few minutes when I want to work out there. And just leave it off most of the time. The occasional time I want to keep the garage at a consistent temp it will most likely be only set at 50 degrees. I understand wanting a small heater to run nonstop on the coldest day of the year just to keep up. But since I don’t plan to keep the garage harm all winter long I figured 200k would warm it up within 5 mins for the days I want to work on the plane even if I only have 30mins to work on it. As for insulation walls will have R19 or R21 (not decided yet) and eventually the ceiling will probably have R50 blown in like the rest of the house. I’m not planning any ceiling fans and with 11’ ceilings I don’t want to wait long to have it heated up.
I wouldn't go bigger than 75-80,000. 200,000 will warm up the air in a couple minutes but everything in the garage will still be cold.
A heater blowing warm air is comfortable. Even if the temperature is less than normal. If the heat shuts off but all the objects are still cold then it feels cold, even though the air is warm.
A heater blowing warm air is comfortable. Even if the temperature is less than normal. If the heat shuts off but all the objects are still cold then it feels cold, even though the air is warm.
ripperd
Well-known member
For some reason I misread what your plan was. I thought you were keeping at 50 and bumping up as needed.
But the other poster is right. 200k is not going to be comfortable either, because its going to feel like a blast furnace while it is on (and probably be loud).
I actually don't know my garage usage since I put it in within a month of us moving in. But based on the size of the house and the total gas bill I bet it only adds $15-40 keeping it at 50 all the time.
When its about 25F out I think my 45k heater can get it from 50 to 65 in about 15-20 minutes. When its -25F i would double that time basically.
Thanks ripperd. After doing more research into full time heating that just sounds like a lot better all around without really costing to much more. -25 outside going from 50-70 in an hour don’t sound to bad either. Do you have a standard 80% ceiling hung model? If so that is sounding like the better way to go. Guess I didn’t do as much heater research as I should have before asking the initial question.
Buy a smaller heater. Much smaller. Get up in the am and go out and fire thing off and go in and have breakfast and coffee. Dress in layers when out there and shed the layers as it warms. You have gotten good advise so far on sizing. You can have heat or you can have comfortable heat. You want the comfort.
toyotadriver
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200k will fail fairly quickly due to never getting to normal temp for long. You'll get condensation in the heat exchanger.
Generally, for most people it's better to slightly oversize a heater for a garage vs what you would put in a house. The key word is SLIGHTLY. 200k for you is GROSSLY oversized.
I believe for you, a 45-50k will work fine but take awhile to bring it up to temp. I'd personally go 60-80k.
Generally, for most people it's better to slightly oversize a heater for a garage vs what you would put in a house. The key word is SLIGHTLY. 200k for you is GROSSLY oversized.
I believe for you, a 45-50k will work fine but take awhile to bring it up to temp. I'd personally go 60-80k.
ripperd
Well-known member
No problem! Yeah mine is a typical 80% ceiling mounted unit heater. Beacon Morris BRT45. They used to sell it at Menards, but it looks like they don't sell that line anymore.
mike93lx
ALLIANCE MEMBER
Why do people choose 80%? To not deal with condensate? Cost? Something else?
Just curious.
Just curious.
Bigshow0003
Active member
I'm in northern WI near the MI UP. I have a 40x30x10 and use a 75K Modine Hot Dawg ceiling mounted. (A local contractor recommended I use a 60K, no more than a 75K). I kept my shop at 50 degrees all winter and bumped it up to 60-65 on occasion for projects. It didn't take long for the temp to reach that desired level. Probably less than 30 minutes which seems on par with another post above.
We are on propane here; and this winter was more mild than other winters. I have a dedicated 330 gal tank for the shop. Started the season full, filled once mid winter and I'm at about 50% now with heating season nearly done. (But again, a bit milder winter).
My slab is insulated; R19 batts in the walls; 3" of closed cell spray foam on the room deck. OSB walls and ribbed dura panel steel ceiling.
We are on propane here; and this winter was more mild than other winters. I have a dedicated 330 gal tank for the shop. Started the season full, filled once mid winter and I'm at about 50% now with heating season nearly done. (But again, a bit milder winter).
My slab is insulated; R19 batts in the walls; 3" of closed cell spray foam on the room deck. OSB walls and ribbed dura panel steel ceiling.
There isn't very many high efficiency unit heaters. The ones that do exist are expense, 2-3 times the price of an 80%. The other choice if a high efficiency furnace, again cost.
Also you need to deal with condensate.
I have a 1952 poorly insulated 925 sqft rambler, and a newer detached garage about the same size, North Dakota. A 95% furnace in the house, a 80% in the garage. My worst utility bill, electricity and gas last winter was $229, although right now natural gas is cheap. I heat the garage to 55F. My electric bill runs about $100 or a bit more, so less than $125 for gas for both buildings, plus a gas dryer in the house. If NG doubles in price, $250 for both.
Thanks all, I’m waiting to hear back quotes for some 45K and a 60K units. Since I have been convinced to just keep the garage at a consistent 50 degrees all winter long. I’d guess the 45k will be quieter? Any reason I should still consider a 60K model? Maybe since the door is 20x9 which would let a lot of heat out when open, or since the 45k unit’s throw listed isnt the full length of the garage?
finn
Well-known member
I am using a 75k in a 32x75’ portion of my shop as a booster to supplement the radiant heat and another in my 32x54’ garage.
Something on the order of 45k would be adequate for the op.
High efficiency condensing heaters aren’t generally a good fit for outbuildings unless there is a good way to handle the condensate. Can’t just run it out the wall, as the line will freeze. Similarly, the building has to be heated to a safe level all winter or the condensate handling system may be damaged.
Something on the order of 45k would be adequate for the op.
High efficiency condensing heaters aren’t generally a good fit for outbuildings unless there is a good way to handle the condensate. Can’t just run it out the wall, as the line will freeze. Similarly, the building has to be heated to a safe level all winter or the condensate handling system may be damaged.
toyotadriver
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I'd go with the 60k personally. A 20x9 door is huge and even well insulated garage doors **** compared to the insulation level of a wall. The 45k will heat it successfully but will take awhile to bring up to temp when you start working out there. The 60k will make you much happier in my opinion.
Amnashaikh
Member
I think 75k to 80k would be just fine. No idea how you came up with 200k though.
Damiean
Member
I know this is an old thread, but thanks for the great information. I do relate to what the OP stated... I did a quick search for a calculator to determine how large of a heater I needed using this site: https://www.newair.com/blogs/learn/how-many-btus-does-it-take-to-heat-a-garage
It recommends 225,000 btu for a 30x40x12 shop, warm-humid, normal insulation.
But it sounds like instead, 60k-75k is the group consensus.
Edit for more info: I'm outside Atlanta... we only have 'cold' weather from Dec-April... goal is to keep it at 40-50 when not in use, crank it up to 70 if I'm spending time working on anything.
It recommends 225,000 btu for a 30x40x12 shop, warm-humid, normal insulation.
But it sounds like instead, 60k-75k is the group consensus.
Edit for more info: I'm outside Atlanta... we only have 'cold' weather from Dec-April... goal is to keep it at 40-50 when not in use, crank it up to 70 if I'm spending time working on anything.
mike93lx
ALLIANCE MEMBER
12' ceilings gives you 50% more volume than most houses, plus all the heat ends up high.
Fans to push hot air down would help a lot.
225k btu feels way overkill for Atlanta. I heat my entire 3500 sq ft house with less and I don't have any kind of special insultation or air sealing.
Costs me around $40-50 a month in winter to keep my garage at 50*. Usually kick it up to 60-65* if I'm in there for a while.
Hard to say exact cost since it's an attached garage and the house uses gas as well. Mostly heat the house on wood though.
About 5500 cubic feet
Since this post got brought back up. I have since decided to go with a 60K 80% house furnace with 10’ish of ductwork mounted up at the ceiling for hopefully a quieter forced air option than a unit heater. The house is still under construction and when it’s done and with some time in there I can post if it was the right choice.
Currently a zenith kit plane. The next one will either be a bearhawk or a Vans RV15
I wouldn't trust that calculator. I've ran across that one before and there has to be something wrong with how its coded. Every other calculation I run from other sources say I need somewhere in the neighborhood of 20-30k BTU for my garage. That calculator says I need 72k if I'm in a 'mixed humid' climate, or 86k BTU if I'm in a 'cold' climate. An 80k BTU heater in my 24x24 would be ridiculous.
That calculator is comical. I can heat my 28x40x9 garage that has walls and a ceiling but NO INSULATION with 75,000 btu. I live in Iowa and it gets cold, even when it's below zero outside I can heat it up to the mid 50's and work comfortably.
kelpaso1
MEMBER EMERITUS
I agree that calculator is way out to lunch. Says I need just shy of 80K BTU in my fully insulated 22x24x8 garage 
What is cold in Atlanta? 30F?
Assuming reasonable insulation, 1200 sqft, outdoor temp never gets below 20F, 30,000 BTU will do fine. If you want to go crazy, 45,000 BTU at the most! Seriously, here in North Dakota we would install a 45,000 BTU unit heater and it will do fine at -30F. My neighbor heats 900 sqft garage with 30,000 BTU.
60,000-75,000 BTU is insane in your part of the country
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Like everyone else I think 200k BTU is overkill. If you want to keep it at a lower temp but still warm the relatively large space quickly, maybe look at two stage unit heater. Sterling offers two stage valves in their heaters from 60k-120k BTU. I don't know the BTU of the first stage, but maybe a 120k BTU Sterling with two stage valve would work for you, more heat to get up to temp quick and lower to maintain. Keep in mind oversizing the heater too much will result in rapid temp swings (not comfortable) and shorter heat exchanger life. A two stage unit can mitigate this.
I have a Sterling 45k but in retrospect I would have probably been happier with a 75k and 2 stage valve but the 45k is adequate for my use in a 24x30x10. When I got the place it had a 115k BTU furnace in the garage and it was nice to go from freezing to 70s super fast, but it cycles frequently and was less comfortable at a given thermostat setting than the 45k BTU heater.
I have a Sterling 45k but in retrospect I would have probably been happier with a 75k and 2 stage valve but the 45k is adequate for my use in a 24x30x10. When I got the place it had a 115k BTU furnace in the garage and it was nice to go from freezing to 70s super fast, but it cycles frequently and was less comfortable at a given thermostat setting than the 45k BTU heater.
Vintage Car Guy
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I realize this is an older post, but appreciated the insights and hoped for advice on how to size a natural gas heater for a new construction garage at 30 x 40 x 13 attached to the house in northern Kentucky. It does get down to 20 - 30 degrees F at times through the winter. Well insulated - garage attic R-30, garage exterior walls R-13. I would like the ability to get it up to 60 - 65 but can be patient with temp rise time.
Sounds like most recommendations above were 45k - 60k BTU. Also wondered if people had more success in a corner facing out diagonally or in the center near the ceiling?
thanks in advance
Sounds like most recommendations above were 45k - 60k BTU. Also wondered if people had more success in a corner facing out diagonally or in the center near the ceiling?
thanks in advance
T VETTE
Well-known member
I would like to know how his heater choice worked out after all the debate and the change of btu choice. Vintage guy, I think the corner facing out works best so as to not short one side or the other too much. Getting ready to install a Maxx 50 shortly here. But I am only in zone 9
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