Heater Size Advice

[xrogers]

Hi all,

I've been searching the history and find similar questions, but every situation is a little bit different...

I'm in St Paul, MN, and am getting ready to install heat in my garage. It is 24x26x9, 2x4 outside walls, fiberglass insulated, with two single-car insulated doors and one small window in an insulated man door. The back 26 foot wall is shared with the house, as is about 14 feet of one 24 foot wall. There's also a heated room over the ceiling.

When it was zero F outside a couple weeks ago, my garage was 37 F. Tonight it is 30 degrees F outside, and my garage is about 50 degrees F (this is with no added heat at all). I figure the the most I need to raise the temperature in the garage is 35 degrees F.

I want to be able to do winter woodworking in the garage, perhaps 2 nights a week (total 10 hours). Otherwise, I plan on leaving the heat off. If it's too cold to work outside a week or two a year, that's not a big deal.

I have an estimate from an HVAC company who has all this information and recommends a 75000 BTU natural gas Hot Dawg.

I was also considering doing electric heat. I can put in two 30A 240 circuits, and connect one to a 5000 watt Fahrenheat ceiling heater. I can add another 5000 watt heater if after using it the one heater seems insufficient (or use two to get the garage warmed up initially, and then run one). Purchase and install (even of two heaters) would be much cheaper (by about $1500---I'm more comfortable with electric DIY than gas). Gas is certainly cheaper to run than electric, but the amount of time I'm running the heat just doesn't amount to that much, so the price difference would take a long time to recover (nearly 10 years).

Is 75000 BTU overkill? Is a 5000 watt electric nutty small for this? How about 2 of them? Or do I need to get comfortable with DIY gas? Or should I shut up and pay the $2200 estimated install price?

Thanks!

 

[Will S.]

I think 75,000 btu is too much for that shop, based on what you presented.

 

[Steevo]

I think 75k BTU is overkill for your situation. I have 24x40x12 heated with a 75k, and it is more than enough. That is twice the cubic feet of yours. I am sure I could have gone 45k and been fine.

 

[Highpsi]

That 75k BTU is definitely overkill IMHO. I heat a 34x26x10 garage just fine here in frigid central Wisconsin with a 45k natural gas Beacon Morris heater. My walls are 2x6 insulated and also insulated ceilings.

Keep in mind that having the heater run too infrequently is not a good idea either. The heat exchanger needs a chance to get nice and hot and burn off the condensates inside or your exchanger will experience premature wear.

For the little that you want to run that heater, you might consider those electric heaters. I had a single 24k 240v electric heater in my last two car insulated garage and it heated it up in dead of winter very nicely.

The price difference of a 45k vs. 75k is very minimal so I was tempted to get the bigger one since "bigger is better" right? In this case, not true... I'm very satisfied with how warm my garage stays and the frequency of the heater running to maintain 55 deg. in the dead of winter. (I can adjust it up to 65 or 70 and it will heat it up to that temp. in less than 10 minutes) Once that concrete slab stays that temp, you have a MUCH better comfort level in the garage. (Especially those of us guys rolling around on a creeper under the car etc.)

My friend heats his insulated garage of similar size with a 24k electric heater, but doesn't run it always. I was amazed the other day when I went to hang out with him doing some woodworking and my feet nearly froze and he was running the heater full blast. Makes you realize how nice it is once that slab is warm.

I did the entire install of mine except running the gas line from the connection in the basement. Feel free to message me if you have any questions.

 

[Kevin54]

I have Hamilton hanging heaters, and for a 2-2 1/2 car the recommended size was 45,000 and for a 3-4 car and up they recommended a 75,000

Here is a calculator I found for garage heaters to determine the size http://www.ultimategarageheater.com/propane-garage-heater/search.php

 

[volaredon]

huh I have a 100K BTU Modine (not Hot Dawg) in a 30x36x10 with 6/12 pitch roof and some days it is barely enough.

 

[xrogers]

Thanks for the advice so far. I just couldn't see how my garage needed that much heat, and appreciate all your information, thoughts and opinions.

When I run the online heat calculators, the most plausible result I got was about 24000 BTU, which matches well with what you experienced folks are saying.

After these responses, I'm leaning strongly to two electric heaters (and maybe wearing lined boots for the cold slab). It seems like it would be enough, and having two separate sources could be handy.

Thanks again, all, very helpful.

 

[manwithtools]

Similar sized and insulated shop attached to my house. I've got a 6000 watt electric heater and it does just fine. I've used it down to 7 degrees and it keeps it at 70. Electricity here is cheap and it does not add any moisture to the room; metalworking shop so that's important to me.

I've got a 30 x 40 x 10 outbuilding that is not insulated nearly as well and I only chose a 60k BTU unit heater for that. I've not tested it yet so can t say for sure how it will work. I'd say your HVAC contractor needs to go back to to school. A unit heater need to run long enough to help evaporate the condensation in the flue. Bigger is not always better

 

[zmaxmotorsports]

I think 75,000 btu is too much for that shop, based on what you presented.

 

[lakeroadster]

I used two of the 5000 W 240V Fahrenheat ceiling mounted heaters in my previous shop (7500 cu ft 2x6 construction fully insulated). The electric heat worked great and having two heaters was nice to direct the heat to specific working areas.

I only heated the shop when I was planning to do work. If I was working on a vehicle it took about 3 hours for the shop to get "comfortable". Without a vehicle it took about 2 hours.

The vehicle ('65 C10) is a large mass to warm up.

Comfortable to me is wearing cold weather socks and boots and a light jacket. My experience is the slab takes about all day to warm up.... which is ok, as long as you dress appropriately.

Since you're doing woodworking, a heat source that doesn't have a flame in the work area would be a good choice, IMO.

 

[xrogers]

Thanks for the additional replies. I appreciate you all taking time to read this and respond. I hadn't considered the possibility of condensation from too short of cycling, or the possible advantages of electric for dusty work.

It seems pretty clear that the speced heater is too big, and that the electric should be fine. I'm going to go the electric route, and I'll let you all know how it works out.

Thanks again!

 

[Alan Grudzinski]

I have a 20x20x9 garage fully insulated with one shared wall to the house.
Insulated garage door also.

I just recently installed a 30K BTU Beacon Morris Natural Gas Heater. It is plenty for the space. Seeing how little it runs now, if available, I could have installed a smaller one.
Granted it has not been that cold this winter here in SE Michigan but it handles the space with out issue. Even if I have the garage door open for some period of time, it recovers quickly.

Based on your space I would think a 30K would do it...45K no problem....75K (as others have said) is overkill.

As has been mentioned you don't want to oversize causing the heater to short cycle and have condensation issues.
Don't know if you checked this site out but it can help give you a baseline to start at as far as BTU size goes.
http://www.ultimategarageheater.com/natural-gas-garage-heater/search.php

Here is a link to my install
http://www.garagejournal.com/forum/showthread.php?t=316473

Alan

 

[xrogers]

I installed a single 5000 watt Fahrenheat electric heater, and it works great.

I've been out in the garage when the outside temp was 0 degrees F, and been perfectly comfortable (mid sixties, at least). At low temps it quickly takes the edge off the cold (I can work just fine in the garage after 15 minutes or so), and takes an hour plus of continuous running to really heat up the space. After that, it starts cycling on/off, but clearly is running a lot.

On more typical 20-30 degree days, it starts cycling much sooner, and seems to run less than half the time. I haven't actually measured any of this precisely, so YMMV.

This is everything I wanted, at a fraction of the contractor cost.

Thanks for the advice, everyone!

 

[bzinsky]

I think 75k would be too much if it was a continuously heated space, but when you have a space that your primary goal is use it only when needed, and to turn an unheated space to a heated one, I think more btu's the better.

I have an 80k btu gas heater in my detached 2 car garage. I would not like it any smaller since it would take longer to warm up.

As for the short cycling, even if it was a continuously heated space, he could always set a large temperature swing for heat call.

I personally don't use a thermostat in my garage, just a turn dial 1 hour light timer. 20-30 minutes before I'm ready to work I just turn it to 60 minutes. After a couple hours I'll turn it on again. Not for everyone, but would certainly take care of any short cycling.

With on-demand garage heating, I think one of the most important things is airflow. Even though it takes a short time to heat the room, everything in the garage takes a lot longer to warm up. Air circulation would help with that.

 

[Jackfre]

Given the common wall with the house and the temp characteristics you are seeing of 37* indoor when 0* outside you should not be over 40kbtu for your garage. You are getting the garage tempered by the house and anything over the 40 is going to be drastic overkill. Essentially if you are going to be in the garage in the morning turn it on when you get up, go have breakfast and it will be warm when you get out there. If you want the mass of the tools/equipment warm too, then turn it up the night before. A 75 will drive you crazy in that size space and the fan will just blow up dust. I'm like a broken record on this , but the best you can do for your space is the Rinnai EX38C. Not cheap, but nothing better. I'm doing 960 sq ft with the EX22C very nicely...but I'm in the foothills of the Sierra. Moderate temps! I was in Minneapolis and Wausau two weeks ago and 2* with that cutting breeze. Yikes!