I live in central California in a location that can dip below 32F on occasion. The previous homeowner downplayed this and just indicated that on the nights that are forecasted to dip below freezing, he would run the pool pump for a few hours during the coldest part of the night. Well this is easy enough to accomplish on the main pump, put the spa pump is not on a programable timer. The separate spa pump is in a closed circuit that goes through a gas heater....which is the component that I am worried about.
There is a drain valve on the heater manifold to drain the water out of the heat exchanger, but based on the layout of the pool/spa (spa spills over into the pool) the drain valve is actually lower than the water level in the spa....so the spa water drains out of the heater valve when the drain valve is opened.
There is not an easy way to disconnect the heater from the circuit and close off the pipes, just too many glued joints and splices.
Any ideas or conventional wisdom to solve the dilemma and prevent water from freezing in the heater?