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Closed Circuit Thermopile Test

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 mVDC.

A SIT 820 valve will require 145 mVDC to pull in while the Honeywell VS8420/21 valve should have a minimum of 155 rnVDC. It is also a good idea to measure the voltage drop across the control circuit to ensure that it is not excessive. This could be caused by a poor pilot flame, a worn out switch, long wire run, undersized wire or loose connections. By placing your meter leads on the TH and TP/TH terminals, you should get a small reading that, when added to the closed circuit, should be well below your open circuit reading.

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