What are the different ways to check the thermopile?
I read somewhere that its
1 open circuit
2 Close circuit with open thermostat
3 Close circuit with close thermostat
Can somebody explain this please?
You need a multimeter capable of measuring mV (millivolt).
Open Circuit test = The test is carried out by removing the thermopile leads from the TP/TH and TP terminals and then placing your meter leads on the two wire terminals of the thermopile. 325mV on an open circuit is usually okay but you may read up to 750mV.
Then reconnect the thermopile to gas valve and measure the mV on the gas valve with the thermostat/switch on and off.
The closed circuit test will tell you the voltage drop across the main operator coil. With the switch or thermostat closed, place your meter leads on the TH and TP terminals and measure your drop in mV. A SIT 820 valve will require 145 mV to pull in while the Honeywell VS8420/21 valve should have a minimum of 155 mV.
https://youtu.be/PkSgj14oqDg