Why does your pool heater not run when the filter is on then when the filter is switched off the heater will warm up for a few seconds then cut off not even warming the pool?

The issue could be due to a flow switch that prevents the heater from operating if there isn't sufficient water flow, which can happen when the filter is on if there are clogs or issues in the filtration system. When the filter is switched off, the heater may briefly detect some flow but then shut down due to inadequate water pressure or temperature readings. Additionally, there could be a malfunction in the heater's thermostat or internal components that causes it to stop heating. It's advisable to check for any obstructions in the filter and ensure the heater is functioning properly.