Pool and Spa
Questions about Pool Repair? Ask an Expert Online.
The sounds you heard was the water in the heater heat exchanger boiling from lack of flow. It is a violent banging and shuddering. In most cases when this is happening the pool pump is not flowing water but it sounds like yours is, so next we go to the INTERNAL bypass valve located in the inlet/outlet header. All heaters are designed with this bypass valve as a safety if too much water is flowing into the heater this bypass valve open a little to allow some water to bypass the heater. Now if the bypass valve is broker or stuck it will bypass too much water and the water already in the exchanger starts to boil. The sounds you heard is most likely pieces of this bypass breaking loose and flowing through the pipes. You need to replace this bypass.
This video will give you an idea where the valve is located. It all depends on your make and model heater.
Were you ever able to resolve the issue?