I recently fired up a radiant system in Helena, Montana in December. The 2000 square foot 4" slab had 1500 feet of 1/2" pex and R-4 beneath the slab with an R-8 perimeter. The system is heated with an electric boiler and the house was unoccupied (vacation home). The thermostats (2 zones) were set to 68 degrees and the system used 1025 Kw to bring the 'house' to a stable 68. This assumes [(Dec.07@1823 Kw)-(Dec.06@810 Kw)]=Energy required to bring system to a steady state. There were 14 days of operation, out of a 30 day billing cycle. The weather was consistient with the previous year, with low temps of -15F and hight temps of 34F. The building has a design heat loss of 23475btu/h @ -16F, so it is well insulated.
Answer to the question: How much energy did it take to bring this 2000 sq.ft. building to room temp and hold it for 2 weeks= 1013 Kw or 3.456 mil.btu
Assuming that the electric boiler is 100% efficient it was a good measure of heat loss for the building. If I had been heating with propane and using a boiler with an efficiency of 82%, I would have used an additional 52 gallons of propaneto bring this to temperature and hold it for 14 days.
You have many factors to consider. The beginning temperature. This was at 40 F air temp, as there was an electric forced air backup to keep this building from freezing up in the winter. The soil conditions under the slab. Well drained and dry will conduct less heat away from the slab than a wet clay type soil. The wind velocity or air movement over the slab will increase its' heat loss. Whether the slab was fully cured can also make a difference to some degree. Heat Transfer is predictable, if you have all of the pieces to the equation.
It is rare to have such a controlled environment, as we did, for the building in Helena. We used 300 gallons of propane to start up and heat a 4200 ft home (2800 sq ft of slab) from Thanksgiving to Christmas Day (you remember when you run out on Dec 25th). We were still working on the house and it was -30F for most of this time.
I hope that this is of some help. If any one else has an easy answer, I am all ears. |