I was looking for an alternative to electric back up, the garn has ports for 9 elements about 27 to 36 kw.I don,t need the back up to keep the house a normal temp , just keep it from freezing if I got away for a week or 2 in the winter. I have propane , would a propane instant hot water heater run through wtw exchanger work or would this exceed it's duty cycle?