have you been flowing water through the heater as part of your normal circulation or do you have a bypass on the heater?
Also, follow these guidelines when operating in cold weather - this is from the Manual
Operating this heater continuously at water temperatures below 68° F. (20° C) will cause harmful condensation and will damage
the heater and void the warranty.
• If the heater has been drained for freezing condition, do NOT turn "ON" until the system is circulating water.
• Water trapped in the heat exchanger can result in freeze damage to the exchanger or headers. Freeze damage is specifically not
covered by the warranty.
• When starting the heater for the swimming season with a water temperature below 50° F (10° C), the heater may be used to heat
the water; however, make sure that the heater operates continuously until the water temperature reaches the heater’s minimum
setting of 68° F (20° C). During cold weather, if there is no danger of freezing, operate the filter pump continuously even if the
heater is not operating. If air temperatures are expected to drop below freezing (32° F/0° C), shut down the heater and winterize
it.