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Heating · Gas ControlsFurnace Gas Valve Replacement
Furnace gas valve replacement costs $500–$1,000 installed and takes one to two hours, including setting the manifold pressure, a leak test and a combustion check. A gas valve is replaced only after it is proven bad: 24 volts reaching the valve with no gas coming through, or a valve that cannot hold steady manifold pressure. In Ontario the work is restricted to a TSSA-licensed gas technician, and gas valves are never repaired, cleaned or rebuilt in the field.
The valve is the last thing blamed and the last thing tested, because most no-ignition calls are something cheaper. The igniter, the flame sensor, the pressure switch and the control board all sit ahead of it in the sequence, and any of them will stop the furnace from lighting. The test that matters is at the valve terminals during a call for heat: if the meter shows 24 volts and the manometer on the outlet tap shows no pressure, the valve has failed closed. No voltage there means the valve is fine and the fault is upstream; why an igniter glows but the burners never light sorts those causes.
The second failure is a valve that opens but will not regulate. Manifold pressure — the gas pressure delivered to the burners — is set with a manometer to the figure on the furnace's rating plate: about 3.5 inches of water column for natural gas and about 10 for propane. A valve that drifts, sags as the burners run, or cannot be adjusted into range gives a lazy yellow flame, delayed ignition with a bang, or a furnace that overfires. We check the supply pressure coming into the valve first, since a problem at the meter, the regulator or an undersized gas line is not a valve fault.
Gas valves are replaced, never repaired, because the valve is the furnace's safety shutoff. It holds two internal valves in series and a pressure regulator, sealed at the factory, and manufacturers neither supply internal parts nor permit field repair. A valve that has been under water in a basement flood is replaced as well, even if it still works, since corrosion inside it cannot be inspected. The replacement has to match the furnace: single-stage, two-stage and modulating valves are not interchangeable, and propane furnaces in rural Flamborough, Kilbride or Caledon need the correct conversion kit.
The work falls under CSA B149.1, the gas installation code Ontario adopts, and the job is not finished when the burners light. Every joint that was disturbed is leak-tested with detection solution or an electronic detector, manifold pressure is set under fire, and a combustion analyzer in the flue confirms the burner is clean. Those three results go on your invoice. If you ever smell gas at the furnace, do not wait for an appointment — follow what to do if you smell gas.
Signs that point to the gas valve
- The igniter glows orange for several seconds, no flame appears, and the furnace tries again and locks out
- A click at the valve but no gas — or no click at all with 24 V confirmed at its terminals
- Burners light late with a bang, or the flame is lazy and yellow instead of crisp blue
- The flame pulses or the furnace overheats and trips its limit, and manifold pressure will not hold steady
- The furnace sat in flood water — the valve is replaced regardless of how it behaves
How it works
Voltage at the valve terminals, inlet pressure and manifold pressure are measured during a call for heat. If the board is not sending 24 V, or the supply pressure is low, a new valve would change nothing and we say so.
Gas and power are shut off, the old valve is removed from the manifold, and the matching valve for that furnace model is fitted with fresh sealant on the pipe threads.
Joints are leak-tested, manifold pressure is set to the rating plate with a manometer, and a combustion test confirms a clean flame. The furnace is cycled several times before the panel goes back on.
Common questions
Can a furnace gas valve be repaired or cleaned instead?
No. The valve is a sealed safety device and there are no serviceable parts inside it. Tapping a sticking valve to make it open, a trick some people try, only proves it is failing; a valve that sticks shut today can stick open tomorrow. Once testing shows it is at fault, replacement is the only repair a licensed technician will make.
Is it worth replacing the gas valve on an old furnace?
On a furnace under about 12 years old, yes. Past 15 years, weigh it: a $500–$1,000 valve on a unit that also has a tired inducer or a rusting heat exchanger is money better put toward a new furnace at $4,500–$7,500. We give both numbers in writing, and the furnace repair page explains the half-the-cost rule we use.
Can I replace the gas valve myself?
Not legally and not safely. Ontario restricts work on gas appliances to TSSA-licensed gas technicians, and the valve is the one part where a mistake means unburned gas in the house. Setting manifold pressure also requires a manometer and the rating plate figure. You can check any technician's certificate; how to verify a TSSA licence shows how.
How long will I be without heat?
The swap itself takes one to two hours once the valve is in hand. Common single-stage valves are widely stocked, while two-stage and modulating valves for specific models sometimes have to be ordered. If there is a wait in cold weather, we tell you how long and help you plan safe temporary heat — call or send a request with the furnace model number.
Why does the valve work sometimes and not others?
An intermittent valve usually has a weak solenoid coil that opens when cold and fails once it warms up, or the reverse. It is also the signature of a loose wiring connection or a control board with a cracked solder joint, which is why we test voltage at the valve during a failed attempt, not during a good one.