Replace a Gas Valve (Furnace)

Shut off + lock the gas and relieve pressure, match the valve (NG vs LP, redundant/combination, inlet size), use approved gas-rated thread sealant (skip the first two threads), leak-check with bubbles or an electronic detector — never a flame — then reset/verify manifold pressure under a call and confirm ignition + a clean flame.

Approx. 90 min

Before you start

•Valve confirmed failed (no 24 V output to nothing? then it's upstream; 24 V present at the valve but no gas/erratic gas = valve) and the manifold-pressure problem traced to the valve, not the regulator/supply.
•Correct replacement valve: same fuel (NATURAL GAS vs LP/PROPANE — they are NOT interchangeable without the right valve/orifices and regulator setting), redundant/combination type, voltage (24 V), and inlet/outlet size + flow capacity.
•Gas can be shut off and locked at the appliance shutoff (and the meter/tank if needed).

Tools & materials

Manometer (inches W.C.) · Pipe wrenches, gas-rated (yellow) thread sealant or PTFE rated for gas · Electronic gas leak detector + leak-check bubble solution · Multimeter (confirm 24 V to the valve) · Combustion analyzer (verify the flame/combustion after)

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Shut off & relieve gas

Shut off and lock the gas, then relieve pressure
Close and lock the appliance gas shutoff. Kill and verify the furnace electrical power too. Relieve the trapped gas pressure in the line before opening any union/fitting.
⚠ Lock the gas off and relieve the line before opening it. No open flames, no smoking, ventilate the area.
Disconnect and remove the old valve
Photo/label the 24 V wiring and the manifold orientation. Break the union, remove the valve from the manifold and supply, and note the inlet/outlet direction (gas flow arrow).
Tools: Pipe wrenches

Install the new valve

Confirm the valve matches fuel, type and size
Verify the new valve is for the correct FUEL (NG vs LP), the correct redundant/combination design, the right inlet size, and the right regulator capability. An NG valve on LP (or no LP conversion) will overfire dangerously.
Target: Fuel (NG/LP), redundant/combo type, inlet size + capacity all match
Thread it on with approved gas-rated sealant
Use gas-rated pipe dope or gas-rated PTFE tape on the male threads only, and DO NOT apply it to the first two threads (so nothing extrudes into the valve/orifice). Make up the connections to the correct flow direction (arrow toward the burners) and don't over-torque the valve body.
Target: Gas-rated sealant on male threads, NOT the first two threads; flow arrow toward burners
⚠ Never use non-gas-rated sealant — it can fail and leak gas.

Leak-check & set pressure

Leak-check with bubbles or an electronic detector
Restore gas and check every joint you opened with bubble solution and/or an electronic gas detector. NEVER check for a gas leak with a flame. No bubbles / no detector alarm before you fire it.
Tools: Bubble solution, Electronic gas leak detector
Target: Zero leaks at all disturbed joints (bubble + electronic)
⚠ NEVER leak-check gas with a flame — use bubbles or an electronic detector only.
Set and verify manifold pressure under a call
Connect the manometer to the manifold-pressure tap, fire the furnace on high, and set/verify the manifold pressure to spec for the fuel, adjusting the valve's regulator if needed.
Tools: Manometer
Target: Manifold pressure ≈ 3.5" W.C. on NG / ≈ 10–11" W.C. on LP (verify the data plate)
Confirm ignition and a clean flame
Watch a full ignition cycle and confirm a stable, clean burner flame (no lifting/roll-out/yellow tipping). Run a combustion check to confirm proper combustion after the pressure set.
Tools: Combustion analyzer
Target: Stable clean flame; combustion within spec (no CO/incomplete-combustion flags)