Can't Get Enough
Join Date: Oct 1999
Location: SW Montana, USA
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Re: Heating a Garage
The most important thing to remember when heating anything, is delta T. That is: Temperature inside - Temperature outside. The higher you keep the temperature inside, the bigger your heating bills will be. If the garage is well insulated, (especially the slab, since you have a radiant slab,) keep it at 55-60 degrees.
On your comment, once the slab is warm, it will still take heat input to keep it warm. Consider your slab as just a big low temperature radiator. it is designed to give up heat to the heated space. As fast as the heated space gives up heat, the slab has to provide it. Because there is so much mass there, it does take some time for any change in thermostat set point to affect the room comfort. On that same thought, if you have the temp. set point at 50 degrees, and you go in and turn the temp up, the heat required from the boiler will be way in excess of the heat required from a radiator or fan coil unit. But, after the heat is in the slab, if you turn down th t-stat, the room will stay warm for a long period of time while the slab cools down. That is the down side to radiant slab heat. Not real good for varying indoor temps in the short term. Also, if you have your heat turned down, come in and crank up the t-stat, because the slab takes so much heat, and responds so slowly, the slab will tend to overheat, and shoot past the set point on the t-stat, further adding to your heating costs. This can sometimes be mitigated with a "smart" t-stat that learns the heat loss characteristics, and compares inside and outside temps to prevent overshooting set points. In reality, Slab heat should be used in a steady state mode, with very little temp. variance.
Just a suggestion on your construciton. If, you don't have the slab insulated very well, I would suggest doing it pronto. If not, your efficient heat will be used to thaw the ground around you more than heat your building. As a minimum, I would suggest an R-10 foam board extending vertically from the slab to 2 feet below the footing or 4 feet deep, whichever is more. Excavate along side your building 4+ feet and install this tight against your foundation. With slab heat with out insulation under the slab, you effectively are heating the entire block of earth under the slab. The perimeter of the slab loses the majority of the heat. Your goal with this insulation is to stop this loss.