BTU requirements?

[StaggeringGoat]

I have a shop condo slightly bigger than 40x60 (2500 sq ft) and although it was 90 degrees today, I will need a heater before long. There is currently a Hot Dawg gas heater but that is being removed. (long story)

I think I'd rather go with electric than gas. I have calculated it to actually be cheaper in the "off peak" hours when electricity prices drop. Then again, during the "peak" hours it is more expensive. I also feel like an electric heater is safer in a shop space with possible flammable fumes and dust, am I wrong?

Question is how many BTU do I need? The shop is pretty well insulated and it doesn't get THAT cold here, (40 is pretty low) I also don't like to keep the shop all that warm when I'm working, maybe 60 degrees or so. Would be nice if I had enough power to crank it up to 70 inside when it's snowing outside though.

My little office and bathroom have their own 1000w wall heaters but I have a feeling they will just barely keep those rooms warm...

 

[Tom McDermott]

There's too many variables to answer. Need to know the types and size of walls, windows, doors, floor, insulation, temperature differential, etc.

Here's an on-line calculator that will let you input the parameters and it will estimate the BTU's required:

http://www.dimplex.com/customer_support/heat_loss_calculator

-- Tom

 

[BadgerBoilerMN]

Off-peak is rarely a bargain and "estimate" is right. Think about having an experience professional perform a proper and accurate heat load and then think: gas-fired direct vent infrared. It is not sealed combustion, but close enough in non-explosive environments.

 

[Charles (in GA)]

Fairly large building, but how high is the ceiling? or is it a steel building without a ceiling? This makes a huge difference as you are not only concerned with square footage, but with cubic footage.

My steel building is 60x60, thus 3600 sq/ft, but it is 66,600 cu/ft due to the 16ft eave and 21 ft roof (2/12).

I know I'm beginning to sound like a broken record to some, but the use of ceiling fans for garage and shops, especially since many are ten foot and over, is an effective way of preventing temperature stratification, and getting that expensive hot air back down near the floor where it will do you some good.

Charles

 

[StaggeringGoat]

Off-peak is rarely a bargain

Well, 1 therm of natural gas cost about $1.20. Gas heaters are about 80% efficient. Add in the extra gas charges and it comes close to $1.50/100,000 BTU.

1 "therm" of electricity (100,000BTU) is about 29 kilowatts or about $1.03. It is 100% efficient.

That said, that's only off peak hours, but that's when I normally work. On peak times electricity is over $3/therm. I'm considering some sort of hydronic heat, thus I could heat up the water at off peak times and use it during on peak. Too bad I can't put PEX in my slab....

The advantage with this is that I wouldn't have to have natural gas service to the shop at all, and pay the extra meter fee, but, I would have to run a big *** cord to an electric heater....

Fairly large building, but how high is the
ceiling? or is it a steel building without a ceiling?

It slopes from about 12' on one end to 18' on the other end. I definately think a ceiling fan would be a worth while install on the higher side. The ceiling is just the steel roof, with a thin insulation blanket underneath. Couldn't be more than R6....

 

[Charles (in GA)]

It slopes from about 12' on one end to 18' on the other end. I definately think a ceiling fan would be a worth while install on the higher side. The ceiling is just the steel roof, with a thin insulation blanket underneath. Couldn't be more than R6....

Flat roof with a slope? no peak? somewhat unusual on a building that large, but certainly not out of the ordinary.

Charles

 

[StaggeringGoat]

Yeah, it does have a bit of an odd roof. No peak, it just slopes for 60 feet. It's part of a larger shop condo complex so the entire building is probably about 15,000 sq ft.

 

[StaggeringGoat]

Although the roof insulation is pretty poor, I believe the walls are pretty good and the place is very tight. During these last hot days of summer it can be 95 outside and about 80 inside if I don't open the doors.

If I can maintain that temperature differential in the winter I'd be pretty happy, it stays 40-50 here most of the winter.

 

[stingry]

Off-peak is rarely a bargain and "estimate" is right. Think about having an experience professional perform a proper and accurate heat load and then think: gas-fired direct vent infrared. It is not sealed combustion, but close enough in non-explosive environments.

I'm with him on this one. Infared tube heaters are the best possible type of heat for your building. To try to heat that with electric is not practical at all in my opinion. The infared will heat the floor and other inanimate objects and keep the heat down where you are working. any type of forced air will send the heat to the ceiling and out thru the poorly insulated roof.


Cheers
Steve

 

[MoonRise]

Punching in your given dimensions and 'guessing' at a few other parameters (uninsulated concrete slab-on-grade, two insulated metal doors of 25 ft2 each, R6 walls and R0 ceiling, all walls 'exposed' to the outside, poor draft sealing) to fill in the heat-load calculator at the Dimplex website and it estimates 68kW (68243 watts) to heat to a 72F design temp diffference ( 0F exterior).

Change the DTD to 40F (30F outside and 70F inside) and the heat estimate is then 37913 watts. With 240V service, that comes to a wopping 158.33 amps to run the heater(s).

That will spin your electric meter!

Rarely is electric heat ever cost-efficient (subsidized hydro-electric or TVA-type rates maybe).

Only need 'spot' heat so that you feel warm? Go for the IR heaters.

Need to have the entire space warm for the machines/materials/processes as well? Painting, adhesives, whatever? Then you need to heat the space as well.

And R6 ceiling insulation is pretty much R-zero. Any heat you pump in will rapidly just go up-n-out through that roof. And if the ceiling is that poorly insulated, I can't imagine that the walls are any better either.

You mentioned that sometimes when the air temp is 95F outside that it can still be 80F inside. I think that is due a lot to the huge 'cool' slab of concrete on the ground.

And we don't know where in the world you are, because you don't have any location info in your user information and you haven't mentined it in your posts here. So all our 'help' is mostly just (educated) guesses.

Ceiling fans or other air movers will certainly help in moving warm/cool air around, so are almost never a bad idea.

About the only way I can guess that electric-powered heat would be somewhat cost-effective (barring the aformentioned very low rate subsidies) if your climate really is that mild in the winter (40-50F, you mentioned?) might be some heat pump system. Which also gets you cooling as well.

Get a local HVAC estimate for heating/cooling loads and you will know much better what you actually need and what some local choices would be.

 

[Charles (in GA)]

Yeah, it does have a bit of an odd roof. No peak, it just slopes for 60 feet. It's part of a larger shop condo complex so the entire building is probably about 15,000 sq ft.

So not all of your side walls are exposed to the outside? This will make a difference in calculations. Need to get a professional at this point to at least do the calculations.

Charles

 

[StaggeringGoat]

Yes, only 3 of the 4 walls are exposed to the outside. One 60' wall joins another heated unit. I *think* the walls are steel 2x4s filled with insulation and then drywall.

Right now there's a Modine hot dawg explosion proof gas heater installed, but it's being removed because I don't need an explosion proof heater and it was installed in the wrong unit my accident. I looked all over and inside the damn thing for a BTU rating, but couldn't find anything, not even a model number.

Do you guys think it's reasonable, if I can keep a 15 degree temperature differential in the summer with no A/C, should I be able to stay 10-15 degrees warmer than outside during the winter? Like I said it's pretty mild here, (oregon) I only need about 15-20 degrees of heat.