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.
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