SIT 820 Nova millivolt valve: main burner will not come on or will not cycle

What is going on

Pilot is fine but the burner will not light with the switch/thermostat, lights intermittently, or the flame looks wrong.

Likely causes

• Knob not on ON, thermostat not calling
• Too much resistance in the millivolt circuit (wire length, thermostat, corroded connections)
• Weak thermo-generator (thermopile)
• Failed main operator
• Wrong orifice, inlet or outlet pressure, partly blocked burner, bad regulator/diaphragm

Check in this order

1. Make sure the knob is on ON and the wall switch/thermostat is calling.
2. Millivolt circuits are very sensitive to resistance: use a thermostat rated for millivolt, keep connections clean and tight, and keep total thermostat wire length within limits (14 ga 100 ft, 16 ga 64 ft, 18 ga 40 ft, 20 ga 25 ft, 22 ga 16 ft).
3. Thermopile test only the thermopile leads on the operator, pilot lit about 3 minutes, TPTH to TP open circuit should be 500-750 mV; adjust the pilot to reach it.
4. Closed-circuit jumper TPTH to TH - reading must stay above 300 mV. Then remove the jumper and reconnect the thermostat/switch - must stay above 175 mV (below 150 mV = too much resistance in the thermostat circuit; find and fix it).
5. Operator test remove all operator wires and measure resistance TP to TH; out of spec (see SIT table) = replace the valve.
6. Burner lights but flame is poor check the regulator matches the fuel, orifice size, inlet and outlet pressure, partial burner blockage; replace the valve if the diaphragm is bad.

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