Suggestions on furnace

[airwalk]

So I've decided to go with electric heat in my new garage currently under construction, but now I have no idea what size to buy or where.

My garage is 32x40x10 with 3 car bays. It will be insulated (R20 in walls, R40-50 in ceiling). I want to keep it above freezing all winter, and turn it up to a comfortable temp likely just on the weekends. I'm thinking of installing 2 forced air units at either end on the short walls for two reasons, one is to heat up faster, and two is so I can just heat one end depending on what I'm doing to save a little energy.

So does anyone have any suggestions as to how many BTU's/Watts the furnaces should be, and where to buy them?

I tried out a heat loss calculator and got almost 10000 Watts based on a design temperature difference of 90F, although I have no idea what to do with this number.

Thanks for any help,
Ben

 

[Sokoloff]

If the proper design rise is really 90*F, which equates by the heatloss calculator to 10KW, then you need a 10KW heater (which is a LOT!! of juice).

I'd suggest that you think about what your actual design requirements are. When heating a house, you need to handle the design temp for sure. When heating a shop, if your area's design temp is -25*F, you might decide to only design for a 55*F rise, meaning you'd just about be able to keep the place from freezing, and on a 10* day, you could heat the shop to 65*F, but on a -20* day, you could only reach 35*F, so you might want to skip working in the garage that day. For a house, that's unacceptable. For a garage, it's probably OK. That will just about cut your heat load (and furnace sizing) in half.

You'll find that 10KW or 34K BTU/hr is probably not realistic to attain from electric heat alone. That's almost 100A of 110VAC.

If I were you, and was stuck on "it's got to be electric", I'd consider electric radiant panels mounted overhead. They won't do as much to protect the shop from freezing, but even 40* air feels OK if it's still (doors closed) and you have a radiant heater "shining" on you (feels like the sun on a winter day). If you need to keep the shop from freezing, you could run the radiant panels all the time, but they're better at heating "things" than air.

 

[airwalk]

Now I understand the calculator's numbers, and you are right about the design requirements, I guess I'm not living in there! A 55 to 60F rise is probably more realistic (I could probably say my design temp is -30F or more as I get direct wind all the time), and I very likely will skip working in the garage on days like that!

My reasoning for the electric decision is I will have to wait until next year for gas installation, and that is if it is even economically possible. This way I can have heat as soon as the insulation is installed.

I understand the radiant heaters, and they would be good when I'm actually in there, but I think I would prefer some sort of forced air system instead. I would like to keep it warmish (above freezing) in there partly for my dog, and partly to speed up how long it takes to warm up to a working temperature. I actually just assumed there would be an electric equivalent to a direct-vent forced air NG furnace, such as a Reznor.

So with this kind of design rise in mind, I could probably be comfortable with a couple of 3-4 kW heaters? I would go with 240v so this will cut my amperage in half.

 

[IDASHO]

Where the hell did the 90 degrees come in?

Even here in northern Idaho, with my insulated 24x30 it takes VERY little to keep it above freezing.

Even in the dead of winter during a heavy windy storm I can manage to keep it 40+ degrees with nothing more than a single 6kw electric heater.

I can see 40-50 degree differences if I keep the heater on high for a couple of hours.


Just make certain your shop is insulated and air tight as possible. You will be surprised how little heat it takes to keep it from freezing.

 

[airwalk]

The 90 deg is calculated as the difference of the coldest outside temperature to the whatever you want the inside temperature to be. Here in my neck of the woods (Canadian prairies) it can get down to -30 C (-22 F) for extended periods, so at a room temperature of 72 F that is a 90 deg difference. I'm not saying I want my garage to be room temperature all the time!

 

[pseudorealityx]

Using electric heat is a terrible idea, energy use wise. Even if it costs a bunch to bring in gas, you'll likely save that in just a few winters.

 

[APEowner]

I'd be inclined to wait till I could get the gas in there and use a portable propane heater for now. If you're really going to be working in there you'll want it comfortable and if you're going to be knocking the heat way back during the week then bumping it up on the weekends you'll want extra capacity so you it doesn't take half the day to get the temperature up. Two other things to consider it recovery time after the door is opened and the length of time it takes to heat a vehicle that's been outside.

I don't know the numbers but my father in law sized the heater in his shop based on the part time use low temperature parameters you're talking about and it works OK if you bump the heat up the night before you want to use it and you don't open the overhead door. However, if you run a vehicle in from outside on a Saturday morning in the 30s and the building hasn't been pre-warmed it'll be 2:00PM before the temperature gets up to 50ish and the car and tools will still be pretty darn cold to the touch.

 

[airwalk]

Using electric heat is a terrible idea, energy use wise. Even if it costs a bunch to bring in gas, you'll likely save that in just a few winters.

This was my argument originally, but then my wife and I got to thinking about how much I will actually use the garage and it will likely be the odd Saturday/Sunday, not too heavily, so we thought the energy savings would be fairly minimal.

-->APEowner, I am only making assumptions there are stand-alone electric forced air units that would work the same as a gas forced air unit. I know there are house furnaces that work the same. Wouldn't an electric unit that puts out say 15000 BTU's heat up an area, and recover that area, just as fast as a NG unit putting out 15000 BTU's?

My other problem is where to find units like this, there must be such a thing.

 

[APEowner]

Wouldn't an electric unit that puts out say 15000 BTU's heat up an area, and recover that area, just as fast as a NG unit putting out 15000 BTU's?

Yes, 15,000 BTU/hour is going to heat the same regardless of the source. I was suggesting that you might find that your original 10Kw/34k BTU/hour calculation might not be that far off. If that's the case then you may find that electric is just not practical. It's really a question of how warm you want it to be how often you're going to open the doors and how quickly you want it to recover.

Will the shop have it's own electrical service? If your running something like a 50 Amp sub panel for the shop you could end up using good chunk of that just for heat if you go electric.

There are electric forced air furnaces. I put a unit made by Williams in an office area in the middle of a factory where we couldn't easily get fuel to it. I don't remember the exact specs but it was something close to 10Kw. I think someone on this board put a similar unit in their shop but I have no idea where that thread is.

 

[airwalk]

You are right, there are still things to think about, such as open doors and recovery, and 10kW could prove to be a little too costly, but I need to figure out how gas compares to this to really prove that (or not).

The garage does have it's own 100A electrical service, so as far as available space I don't think I'll have a problem there.

I'll have to hunt for that thread, I wouldn't mind at least finding out some prices and some experience with a heater like this.

 

[airwalk]

By the way, thanks for the responses so far everyone.

 

[jvitez]

Try these:

http://www.ouellet.com/commercial-heating-specs.aspx?i=17

or

http://www.ouellet.com/fd.aspx?h=/documents/products/catalog_10_en.pdf

Ouellet heaters are sold in several places around here, they're made in Quebec, and the second link's heater goes to 25 KW on 240V!

You need to compare electric and NG rates to get an operating vs capital cost sense. The first link's economy heater runs around $275 here, so if 10W is OK, you'd need two, with two 40 amp branch circuits. Your 100 amp service gives 24 KW, so you'd still have lots left over.

Also remember that electric heat is considered 100% efficient at the heater, but make sure you look at the output ratings of NG heaters, most of which are around 80% efficient.

 

[IDASHO]

Using electric heat is a terrible idea, energy use wise. Even if it costs a bunch to bring in gas, you'll likely save that in just a few winters.

Not always.

Around here is is much cheaper to heat with electric than gas.

Simple cost per therm.

 

[nate379]

I know for sure I would not set it up that the heater is too small to keep the building comfortable like a few people are suggesting.

I want to work in my garage comfortably whether it is 50* or -40* outside... and truthfully, it would get used more when it's cold as I usually work on stuff in the driveway when it's nice out.

How about use the smallish electric heat to keep the temps above freezing (I'm guess you have water plumbed into the building?) and use a wood stove when you are working in there??

My Dad heat's his shop with a wood stove and it will cook you out the place if you don't watch out and throw a couple too many logs in.

 

[APEowner]

Just as a point of reference; my shop is 24' x 40' with 8' ceilings and I'm heating it with a 57k BTU/hour oil fired forced hot air furnace. The programmable thermostat knocks it back to 55F at night and it comes back up to 65F in about an hour even on the coldest of days. I get nice recovery after the doors are open as well. I experimented with knocking the heat back further but the tools took along time to warm up and I hate working with gloves on. I ran a duct down the center of the shop and I've got a pair of ceiling fans that keep the air moving around.

I'm sure I could get away with a smaller unit but I heated the shop so that I could work in there and I hate being cold and working in gloves and a coat.

 

[pseudorealityx]

Not always.

Around here is is much cheaper to heat with electric than gas.

Simple cost per therm.

Given, I've yet to do any work in Idaho, but that amazes me, especially considering the amount of heating you guys need.

 

[pseudorealityx]

FWIW, a 10KW electric strip heater is going to pull ~50 amps at 220V.

 

[compman25]

Given, I've yet to do any work in Idaho, but that amazes me, especially considering the amount of heating you guys need.

It's the same here in Spokane WA, cheaper to use electric.

 

[pseudorealityx]

What's the price per therm out there?

 

[airwalk]

You need to compare electric and NG rates to get an operating vs capital cost sense. The first link's economy heater runs around $275 here, so if 10W is OK, you'd need two, with two 40 amp branch circuits. Your 100 amp service gives 24 KW, so you'd still have lots left over.

Thanks for the links, that is exactly the type of heater I was thinking of! I'm thinking 2 5kW units, although I would think more along the lines of a 30A branch each as they would only draw about 20A, or am I missing something.

Also remember that electric heat is considered 100% efficient at the heater, but make sure you look at the output ratings of NG heaters, most of which are around 80% efficient.

Yeah, this makes it tricky to determine a cost comparison, not that that wasn't tricky enough. I really should determine the cost of running gas to the garage, just for comparison's sake.

FWIW, a 10KW electric strip heater is going to pull ~50 amps at 220V.

Shouldn't this be more like 42A?

What's the price per therm out there?

In Canada gas is sold in gigajoules, which is at $5.21/GJ right now, this works out to approximately 55 cents/therm. Electricity is currently 10.49 cents/kwh.

It is cheaper to use gas here from what I understand, but the cost savings don't seem to be substantial, unless I'm underestimating how long a heater will run.

I don't have water plumbed into the garage, keeping it above freezing is maybe better put keeping it "roughly" above freezing. I have a small diesel tractor I use for pushing snow and it starts SO much easier when it's warm, as well as much easier on it.

 

[pseudorealityx]

Shouldn't this be more like 42A?

I looked up a Carrier accessory heater for a fan coil unit. It was rated a little over 10 KW IIRC, so maybe that's the difference. Either way, that's the ballpack you're looking at.

In Canada gas is sold in gigajoules, which is at $5.21/GJ right now, this works out to approximately 55 cents/therm. Electricity is currently 10.49 cents/kwh.

Yeah, no idea WHY the HVAC industry is stuck with english units here in the States.

 

[jvitez]

My bad: 5000W/240V=20.833 amps x1.25 continuous service rating=26.04 amps, so you're right, 2x30 amp circuits would be correct.

When you look at NG or propane heaters, they all have input and output ratings. Look at the output rating when doing your calculations. The input ratings are important when calculating size and length of NG pipe runs.

Here in Manitoba, our POCO has a current price fuel cost comparison:

http://www.hydro.mb.ca/your_home/home_heating_comparisons.pdf

At today's prices, the actual fuel cost to heat their "average" home by mid efficient NG is 67% that of pure electric. Most NG heaters I've seen are 80% efficient. So it's back to capital cost vs operating cost comparisons.

 

[pseudorealityx]

There are plenty of 90% and above efficient residential furnaces these days.

 

[jvitez]

There are plenty of 90% and above efficient residential furnaces these days.

Absolutely correct. It's actually hard to find mid efficiency residential furnaces anymore, really as it should be since the condensing technology has been around for quite a while now.

I meant most NG unit heaters are mid efficiency, since that's what the OP is considering.