Help with sizing heater

[cdsvt]

Hi everyone. I was wondering if I could get some advice on sizing a garage heater.

I would like to do a Big Maxx suspended heater, as I had one in my last garage and it worked great for my purposes. And they are pretty reasonably priced at Northern Tool.

I've never heated a garage all the time, and don't suspect I will with this one either. I mostly want to be able to heat it up to tolerable conditions so I can fix and tinker on cars in the winter.

We live in Vermont with a northern climate with reasonably cold winters. I plan to use LP because that is what I have available on site.

The garage is attached to the house - 28x28 with ~10' ceilings, the walls are 2x6 construction with fiberglass insulation and 5/8 x-rock on all the walls. It has two 10x8 insulated garage doors, a single 3' wide insulated man door leading to the outside, and two 3'x4' windows (2 yr old Integrity, same as the house). It shares a common wall with the house 12' wide, full height - that wall is made from SIPs with 5.5" EPS foam insulation core having an OSB face to the garage (no studs) - covered with 5/8" x-rock.

The ceiling is not insulated yet, but will be shortly, once I wire it I can get to the insulation. It is 14" deep or so - truss joists. I plan to use fiberglass batts and 1/2 sheetrock over it.

What I am looking for help with is heater unit sizing. In this case I think I have two choices - 50K and 80K BTU.

I want to buy the one that is closest to being "right sized" - if 50K BTU is sufficient, I don't want to spend extra on the 80K BTU unit. But if 50K is undersized, I don't want to have to spend twice.

Access to LP and power is a non-issue. I have easy access to extend my LP lines and I have 100 amp service to the garage (as of this weekend, thanks to advice from GJ on another thread in the electrical section).

Would anyone here be willing to provide some guidance?

Thanks in advance.

Charlie

 

[regguy1]

The 50K will be just right. I have a 45K natural gas in my 26' x 30' and it heats it just fine.
8' 10" ceiling / R13 in walls and ceiling.

See it here: http://www.garagejournal.com/forum/showthread.php?t=140674&highlight=26'+30'+Garage+photos+videos

 

[blueplate]

Hi everyone. I was wondering if I could get some advice on sizing a garage heater.
Charlie

User 'rshadd' had some good things to say in another post:

A simple formula that can be used to determine heater BTU requirements:

Cu. Ft. of Area X .133 X Desired Temp. Rise °F = BTU Size Needed.

Example:
Area: 50' X 25' x 10' = 12,500 Cu. Ft.
Desired Temp Rise: 30°

12,500 (Cu. Ft.)
X .133 (Factor)
1662.5
X 30 (Temp Rise)
49,875 (Proper Heater Size)

Answer: A 50,000 BTU heater should be selected for this application.

Or plug your numbers into this BTU calculator:
http://www.calculator.net/btu-calculator.html

 

[samert111]

The 50K will be just right.

Yep, what regguy said.

My Pole Barn is 28x48x15 with R19 in the walls and R30 in the ceiling, 9x7 & 12x14 insulated overhead doors a man door and 4 ft Lexan clear insulated panels running the full length both side for lots of natural light and I have a 75K BTU Beacon Morris and it's just right.

 

[cdsvt]

Thanks all. I am not confident in making a call here, since it is awfully close to size of the smaller one and I don't understand how heater efficiency plays into this.

I'm not sure about the temp rise part - it can get down into the negative degrees in the winter, but if I were to say 10 degrees is the lowest for arguments sake, and 40 degrees is the rise (so I could get up to 50 degrees F on fairly cold days), then I get 41,7088 using that formula.

That's a little bigger than 40K, so by itself that suggests to me that I can get away with a 45 or 50K unit.

However, isn't there something about efficiency in there? Like the 50K unit actually puts out 80% of that listed number? If so a 50K would only put out 40K (not enough) and a 45K would only put out 36K (not nearly enough)?

And also, doesn't the R-value of the walls and ceiling and the number and size of openings matter? Or would that play into the "factor" - if so, is the .133 value correct for the kind of insulation I have? Or does it help me get away with a smaller heater since I have R19 walls and (if I recall correctly) the formula came from a posting where the original poster had uninsulated block walls? Does my insulation give a different factor?

Sorry of these are stupid questions or I am over analyzing it. This is a pretty big purchase for me, so I want to do it right.

 

[jvitez]

You're not over analyzing it at all, you're being careful!

How do plan to use the garage? Do you want to keep it heated all winter, then maybe bump up the temp when you want to work (play....) in there? Or do you plan on keeping the heat off, and only turn it on when you want to work in there?

This is important because if you plan on the latter you'll need more heat input to get a rapid heat rise. If you can plan your work then you'll need less heat input, ie "I'll be working there tomorrow morning so I'll turn on the heat before I go to bed," vs spur of the moment work.

If you plan on keeping the garage heated to 50 F all winter and if you're OK with working at 50 F, a 45K or 50K btu LP heater will do you fine. You actually want the heater to be running more than less, as it takes some time for fuel burning heaters to reach steady state efficiency. Think of wildly oversized heater: the thermostat calls for heat, the heater turns on for 2 minutes and blasts enough heat to satisfy the thermostat, then it won't turn on for another hour. Hot air rises, your floor is colder, so all the hot air is at your ceiling and you're cold. It's called heat stratification.

Now imagine a heater that's "just right." In a perfect world your fuel burning heater would be running 100% of the time on the coldest day of the year. That would give the best efficiency as the heater is at steady state efficiency for most of the time, plus the fan is always on which circulates the air to keep more of the hot air down where you want it.

 

[cdsvt]

Sorry for the delayed reply, I was out of town away from computers.

My thinking is changing as I read up on all the posts on this site. I think I will heat it to a low temp all the time and then bump it up when I want to work/play in the garage - probably about 1 or 2 nights and a weekend day per week average.

Thanks for the advice.

 

[cdsvt]

I opted for a 50,000 BTU Mr Heater Big Maxx (propane version) from Northern Tool.

I got an email that it was on sale for $399, plus I had a $20 off coupon that offset half of the shipping cost, so I figured I am not going to see a better deal on this thing.

I grabbed the Mr. Heater thermostat too, while I was there. I didn't get the vent kit, since last time I put one in a garage I bought theirs and it had a bunch of pieces I didn't even need, and it felt overpriced for what they provide. I thought I'd try to piece it together myself this time.

Thanks for the help!