Pilot light won’t stay lit after installing a new gas valve? If it only stays lit while you’re holding the knob in, but shuts off when you release it or turn it to ON, the thermocouple safety circuit isn’t being held open. One of the most common causes is the thermocouple block being installed in the wrong port, which can interrupt the millivolt signal even when the thermocouple is still good.
Because this problem shows up right after a valve replacement, it’s easy to think the new valve is defective. In reality, a quick port check often solves it, confirm the thermocouple block is in the correct port and the probe is sitting properly in the pilot flame.
The sections below cover how the thermocouple circuit works on systems using valves like SIT 820 Nova Series, how to select the correct thermocouple port, and what to check when the pilot won’t stay lit.
How the Thermocouple Works:
The thermocouple is a safety device that detects whether the pilot flame is lit.
When heated by the pilot flame, it generates a small electrical signal (often around 20–30 millivolts) that the gas valve uses as proof of flame. On valves like the SIT 820 Series, that millivolt signal keeps the safety magnet open so the pilot can remain lit. If the pilot goes out, the thermocouple cools down, the signal drops off, and the valve closes to prevent gas from flowing into your home.
On the SIT 820 Series gas valve, the thermocouple lead routes into the thermocouple block/connection point on the valve body, where it energizes the safety solenoid (magnet) that keeps the pilot circuit activated. If the block is installed in the wrong port, not seated correctly, or the probe isn’t sitting properly in the pilot flame, the signal can be weak or intermittent. The result is a pilot that lights while you’re holding the knob in the pilot position, but won’t stay lit when you release the knob or turn the control to the ON position—even when the thermocouple itself isn’t actually bad.
Understanding Thermocouple Ports
Gas valves like the SIT 820 Nova Series typically feature two thermocouple ports: one positioned at the top of the valve and another at the bottom. This dual-port design provides flexibility for different fireplace configurations and routing paths.
In many installations, the top port provides a cleaner route that helps keep the thermocouple probe seated in the pilot flame. The bottom port is often useful when fireplace clearance is tight, the thermocouple lead length favors a different routing path, or the appliance design limits standard mounting.
Thread sizes can vary depending on the application:
- Vented systems: Commonly use 11/32" thread sizes
- Vent-free systems: Often utilize M8x1 threaded ports
Having both a top and bottom port lets you choose the connection that matches the fireplace layout and keeps the thermocouple probe consistently in the pilot flame.
Always refer to the old gas valve when determining which port will be used on the replacement.
The thermocouple block being located in position 1 or position 2 will depend on the following:
- Clearance
- Thermocouple lead length
- Specific fireplace design
Critical Installation Tips:
Thread sizes can vary depending on the application:
- Vented systems: 11/32" thread
- Vent-free systems: Often use an M8x1 threaded port
Tools Needed
- -11/32" wrench (or M8 wrench for vent-free systems)
- Fine emery cloth — removes carbon buildup and oxidation from the thermocouple probe without damaging the surface
- Clean rag — wipes away loosened debris so it doesn’t end up in the pilot assembly or gas valve port
Proper Thermocouple Block Installation
- Ensure the thermocouple block is in the correct port
- Clean the thermocouple bulb and metal plate
- Remove any corrosion or debris
- Install the thermocouple and hand-tighten
- Use a wrench to give a final 1/4 turn
Wire Placement is Crucial.
IMPORTANT: Always tuck the thermocouple wire away to prevent heat exposure and potential melting. Exposed wires can cause serious safety hazards and lead to system failure.
Troubleshooting: Pilot Light Won't Stay Lit:
If your pilot light won't stay lit after replacing the gas valve or pilot assembly, the thermocouple block position might be the culprit.
How to fix:
- Turn off the gas supply and allow components to cool
- Gently wiggle each connection point to identify any looseness
- Hand-tighten any loose fittings
- Use a wrench to give a final 1/4 turn where appropriate
- Avoid overtightening, which can damage threads or crack fittings
- Test the pilot light operation after securing all connections
Quick Diagnostic Checklist:
- The thermocouple block is in the same port as the old valve
- Thread size matches your system (11/32" for vented, M8x1 for vent-free)
- All connections are hand-tight plus 1/4 turn
- The thermocouple probe is fully engulfed by the pilot flame
Gas fireplaces and other hearth appliances should only be serviced, repaired, or installed by trained professionals
