If the gas heater is left outside in cold weather (below 0 degrees), there is a great risk that the pipes will freeze and the pump and the heat exchanger are most vulnerable.
A cracked pipe (copper) in the heat exchanger can be repaired by soldering:
Or the entire heat exchanger can be replaced. The article code for it is 330HGHE. If the heat exchanger is replaced, the pictures show how the one to be replaced is removed and the new one mounted in the opposite order.