hayward pool heater millivolt 350 type

Thermocouples put out around 25mv. Thermopiles, which is probably what you have, should be able to put out as much as 750mv. What is the problem with your heater? Does the pilot stay lit? If not, make sure the pilot is clean, not a lazy flame, little or no yellow. If it does stay lit, the problem is probably in the "thermostat" circuit. The usual culprit is the pressure switch. If the pump isn't putting out enough pressure, the heater won't fire. This is usually caused by the filter needing back washing. You can jump the pressure switch momentarily and if the heater fires, the problem is the switch. Either the filter needs backwashing or a bad switch. Check for continuity with the pump running. If the switch closes, the filter needs cleaning.