Calculating BTU for Heating a Garage from 40 to 60°F in 4 Hours

[lurch85]

As to the wattage, heat only gets to the garage whatever can be transferred through a one-inch text line at 22gpm

 

[lurch85]

By the way, the wood box is 32 inches x 32 inches x 36 inches. You can get as big of a piece of wood in there as anybody could ever lift

 

[gjonesy]

Question for lurch85, just to get a better picture, is there a radiator/heat exchanger in the garage? Or are you totally relying on heat transfer from the pipes running under the floor? 165000 btu input would be more then adequate to heat that space if the source was direct (ie above the floor). Please paint a detailed picture of what you are working with.

 

[lurch85]

The north 3rd of the concrete has half inch X running under the floor at what I think is 9 inches apart. I also have a forced air round radiator with a blower in the in the center. I have not been able to find out the heat output. I am probably going to take another heat exchanger/ plenum and put a blower behind it and make my own. What I'm trying to figure out, is if I should have one big one, or two mediums, or some sort of an idea on how many BTUs I need before I go buying anything. A heat exchanger that should be able to put off 200000 BTU and put a blower behind it, but if I would need more than that, I may be better off putting into smaller ones into 2 corners.

 

[gjonesy]

The short answer is 200000 btu should be plenty. 60x40=2400x55= (132000 btu/h output) 132000 / 0.8= 165000 btu/h input.

 

[lurch85]

How does that figure sort of aspect for time? Given being as decently sealed building??

 

[gjonesy]

How does that figure sort of aspect for time? Given being as decently sealed building??

Good question, that I don't have an answer for. The information I found doesn't deal with the time it takes to warm up. Just gives the btus needed to heat up a space by sqrft.

 

[OmCheeto]

R 7-10 is probably more likely... 1 1/2 inch of low grade foam

I come up with an aggregate R-value of 7.8, based on the energy stored in the heated portion of the concrete floor.

Assuming an 8" slab thickness:
(8" slab) * (the heated area of 800 ft^2) = 533 ft^3 of concrete.
Changing the temperature of that much concrete by 20°F required 322,000 BTU's

24 hr * 5200 ft^2 * 20°F  
------------------------- = 7.75
      322,000 BTU

 

[gjonesy]

I'd tie into the boiler with the 200000 btu heat exchanger, I calculated 165000 btu/h input for an above the floor heat source at a btu factor of (55) per square foot. that's actually over kill, cause you already have heat coming from the floor which keeps your garage at 40 degrees constantly. You could probably go lower, (say 30 btu/h per sqrft) keep in mind that if this system also heats your home it will have a subtractive effect on the rest of the system. So even when the blower system isn't (on) as long as water is going through the exchanger it will give off some of its heat. Once its installed you may find that you hardly need to turn it on to keep that space warm.

 

[russ_watters]

How does that figure sort of aspect for time? Given being as decently sealed building??

Time is generally not considered in this, since people rarely cycle the heat like this. But it shouldn't make much of a difference: If you've properly sized your heater, with the appropriate safety factor, the time to heat the room will be fairly short. No more than an hour and probably much less.

However, a quick calculation of the time to heat just the air (at 1.08 BTU/F/cu ft) tells me you'd need 12,000 BTU to heat it in an hour.

Let's also assume you have infiltration in the amount of a quarter of an air change per hour. That would be 140 cubic feet per minute, or another 6,000 BTU/hr.

Previously, we calculated the heat loss through the walls at 9,000 BTU. I'd size your heater for (6,000+9,000)*1.5=22,500 BTU (6.6 kW).

So with a heater that size, you'd heat the air in the room up to your desired temperature in about 35 minutes, then over the next hour or two the objects in the room would heat up to that temperature.

 

[lurch85]

Thanks guys for all of your help in this... I'll keep you posted..
I don't think I'm going to have much time in the next 2 weeks between fixing my bulldozer/skidder and cutting my firewood for the rest of the winter (i leant it out, had to fix it, and got behind and will be at work almost steady)
But I will keep you posted, hopefully have some real world numbers to give you guys in a while

Btw, If i give the boiler a steady stream of wood, I'm pretty confident it could handle both the garage and house... (even if not, I have a kicker propane boiler for the house it all)

 

[lurch85]

for those interested, its a Taylor Woodstove, http://www.taylorstove.net/taylorhomestoves.htm ...T 750 H