Another quick check you can carry out is the system resistance check. This will verify whether your control circuit has too much resistance in it.

Simply disconnect the control wires from the TH and TP/TH terminals, close the switch (thermostat, remote control) and note the reading you get. It should be less than 1.7 R. If it is greater, check that all connections are tight, that the wire gauge is heavy enough for the length, and that the switch itself does not offer too much resistance.

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There are two instances where you will measure resistance when checking a valve for faulty main burner operation.

The first is measuring the main operator resistance in order to verify that there may be a problem with the gas valve. By disconnecting all wires from the TP and TH terminals, and then placing your meter leads on these terminals with your multi meter set to resistance (R) you will get a reading that should fall within the following parameters:

• SIT 820 ‐1.75 to 2.75 R
• Honeywell VS8420 ‐3.1 to 3.6 R

Before taking a resistance reading, zero your meter by touching the two leads together. If the reading is not '0', subtract this number from the number you get when measuring the Resistance.

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