Garage Heating Dilemma

[Daner]

Hey Guys,
I’ve been working on heating my garage for some time and I cannot decide on the best way forward. Let me know if you can help. Here is the situation:
The garage; 22x24 double attached. Spray foam insulated walls and roof (~R20). I am located in Manitoba (design temp of -10°F). Temp is held around 50°F. Used as shop and to warm cars. Currently using 4800W construction heater with success. Have used only radiant convection oil heater in shoulder seasons. I turn up the heat to work once per week and the big overhead door is opened a couple times a day minimum. Ice/snow melt from the vehicles pools in two low spots in front of the doors.Also to note, the cost of natural gas is about half of electricity per unit of heat output.
The problem; the construction heater should only be used temporarily so I’m looking for options as to my permanent solution. What makes this situation unique is that I have already ran tubing for radiant heat…..but due to a disagreement with the contractor I wasn’t able to get in to insulate under the slab or at the slab edge. I realize now that this was a major oversight, but I need to move forward and work with what I have. I’ve done the heat loss calcs and I estimate a little over 30% more cost versus forced air options to heat the space.
Here are the solutions that I’ve come to (feel free to add any others):
1. Forced Air Natural Gas (ie. Scrap the in floor tubing) – unit heater or even IR
2. Radiant with Electric Boiler
3. Radiant with Gas Boiler
4. Combo Electric Radiant/Forced Air NG
*Tankless and standard HWT options for radiant have come on and off the table. One major drawback to the standard tank is that garage space is at a premium, not to mention the equipment isn’t exactly designed for this application….but costs are good, though.
Option 1 is the least expensive for unit, install and operation. Also, recovery is pretty good. The drawbacks are noise and comfort. Ceiling mount won’t help much with pooling water.
Option 2 is a little higher cost for components and install than a unit heater. Comfort is high, will help with pooling water, no noise. Recovery/Ramp will likely be an issue especially with insulation losses. Also, this is by far the most expensive to operate.
Option 3 is about half of the operational costs of option 2, but the wall mounted boiler is around 3 times the cost. Comfort is high, will help with pooling water, no noise. Recovery/Ramp will likely be an issue especially with insulation losses. (I realize that a standard floor mounted boiler is about half the cost of a wall mounted version, but floor space is precious. It is still worth considering though.)
Option 4; Until recently this was seen as a ridiculous option, but when presented with the costs of the wall mounted gas boiler it became more viable. Some electric costs are mitigated. Recovery/Ramp is improved. Less noise than unit heater only. Comfort of radiant heat. Some assistance with water pooling.
Hopefully I’ve presented my situation. I’m sorry for the novel, I just wanted to make sure all the details were out there.
Thanks in advance for your time and thoughts.

 

[scoob8000]

I'd probably got with forced air natural gas. Mostly for price and simplicity.

I would also think it would warm the garage more quickly on-demand.

 

[StupidSheet]

I have a buddy that has a 30x30 that doesn't have insulation underneath or on the side and is currently running infloor. He has been for about 6 years. I asked him what a cost was per month and all he could come up with was it was about the same price it used to cost him to run a hot tub outside all winter. He took out the hot tub and did the infloor in his shop the same year so didn't notice a difference in his electric bill. I think he said around $100 a month was what the hot tub was costing him.

On another note I just picked up a 5kw Dimplex heater which is DESIGNED to run constantly. This is the 4kw https://midwest.westburne.ca/heater...4031-ccx-work-shop-heater/product/CCXDGWH4031 and the one I picked up has adjustables louvers and is 5kw. https://midwest.westburne.ca/heater...mpact-unit-heater-almond/product/CCXCUH05B31T

I picked up the 5kw to help for the recovery time of the infloor when doors get opened and such. I may pick up one more depending on how it keeps up but my shop is also a bit bigger than yours.
Hope this gives you some options as I am also in Manitoba.

 

[p_mori7]

If the tubing is in place to have a heated floor, use it ! Once it's warm, the thermal mass of the slab is the best way to maintain constant 50*. I would use a NG boiler.

Use the construction heater to kick up the temp on the weekends.

 

[jmlcolorado]

I have a very similar sized garage, with less than adequate insulation. Walls are 2x4 insulated, arric is just bunches of scrap r-19 previous owner tossed in.... Kinda. So really, nothing in the attic.
I have a 100k btu updraft house furnace in my garage and it's epic!
I keep it al 40* at all times! except when I'm working out there. I can crank it up to 70 in about 6 minutes .
I wish I knew the cost to run it as I out it in the day we moved into the house.
IR would take much longer to heat the garage, but it would hold heat much better when doors open/close.

 

[James-W]

Having a heated floor would be really nice to have, but if it is going to cost you 30 percent more than with forced air, then I would hesitate about doing it. I guess it would depend on just how much that 30 percent equates to in dollars. In any case, if the cost to heat the concrete is excessive, then I would go with the natural gas forced air heater.

 

[jdepiero]

I just finished my 30 x 60 x 12 ft ceiling detached garage and installed a 90% efficiency horizontal NG furnace in the attic. I spread out 10 ducts in the ceiling to minimize cold spots.
I have it at 50 degrees. I will have to see after this winter how the NG bill averages.

 

[JCByrd24]

If it were me I'd be keeping that space a little cooler, I'd do everything I could do to air seal the overhead doors (double seals etc), and I'd start digging in the spring to install foam to cover any exposed concrete and go down at least 18" below grade (your contractor did do you a huge disservice if you were planning to heat this 24/7 in Canada). With no under slab insulation I wouldn't use the radiant for constant heating, so my vote would be NG unit or radiant tube heater. I'd look hard at the Modine Effinity units, 93% efficient vs 80% of all others.

 

[burhead]

I wouldn't consider anything but radiant heat if you work out there at all. My dad has a slab on grade 30 x 30 that he only heats when we are working in it. He heats it with a 10k electric heater. You can heat it to 80 degrees but when the floor is still 40 degrees, you still feel cold. I just completed my radiant heat and am getting to use it for the first time. I had the temp set at 60 and it was too warm. You could feel the heat coming up your body. I now have it set at 57 and it is great. If you open the door to move something out side, the garage temp recovers in minutes. I have been monitoring mine with a data logger and I estimate that it is going to add about 2 dollars per day to my electric bill. If you have gas available, I know there have been some that have used a on demand water heater with great success.

 

[LsFarm]

As long as you didn't build on top of a swamp or running underground spring, your heat loss into the ground will be minimal once you get the slab up to temp. But you will be loosing heat around the perimeter. For this winter, can you lay foam board on the ground around the outside of the walls, and tarp over them, then weigh them down with straw bales or similar? Just to reduce the heat loss to the outside ?

The big difference with an insulated slab is rate of temperature change in the slab. As long as the sand/gravel under the slab is DRY, it won't conduct heat away, it will become part of the heat sink and heat storage.

As mentioned in an above post, a heated slab is MUCH more comfortable to work on, and you will never be as comfortable with any form of forced air heat. Radiant floor heat is the way to go, and your perimeter insulation can be installed when weather permits.

I'd hang a HWT heater on the wall, just make a very sturdy stand to put it on, and put a tool box, or cabinet under it, so the legs of the stand don't interfere with the floor space. Or, choke down the bill for a wall mount gas fired on demand water heater. Since you will be recirculating your water, not adding fresh water and it's minerals, the on demand heater should last a very long time. The depositing of minerals in the tubing is what kills the on demand setups.

Hope this helps.
LsFarm

 

[ksj9393]

Daner - You really have 2 issues to contend with - the short-circuited, uninsulated slab, with a well insulated garage above it.

Have you done a Manual J heat loss calc? Given your volume, and delta T of 60F, you may find that you only need 20K to 25K BTU/h to heat it, if doors well sealed. (Try this crude calculator for fun: http://www.calculator.net/btu-calculator.html) Hard to find direct-vent, HE natural gas usits that small should you choose the corner mounted, forced air option.

As for in-floor radiant, I agree with others... you can't beat the overall comfort of a warm slab to lay on! Also far more efficient to heat slab than air if overhead door are frequently cycled. But there is no doubt that your perimeter heat loss will be significant and I wouldn't even consider the option unless you put rigid foam around the perimeter. Tell us, do you have a frost wall set on footings? Or simply slab on grade with edge thickened slab? If frost wall, then insulate down to footings with R10 XPS and go for the in-floor radiant. Heat the water with propane. Maybe a tankless on-demand unit.