Gas Control Valves and Flame Failure Devices (FFD) Explained
What does a gas control valve do?
Two jobs, and it is worth separating them because they fail differently.
- It meters the gas — you turn the knob, the tap opens, gas flows to the burner. This is the part you operate.
- It carries the safety cut-off — the flame failure device, which shuts the gas off automatically if the flame goes out. This is the part that keeps the kitchen safe.
Most "my burner won't stay lit" faults are the second job failing, not the first.
What is a flame failure device (FFD)?
An FFD — sometimes written FSD, flame supervision device — is the safety interlock that stops gas flowing when there is no flame to burn it.
It works as a chain:
- The pilot flame heats the thermocouple.
- The thermocouple generates roughly 25–30 millivolts.
- That tiny current energises an electromagnet inside the valve — the magnet unit.
- The energised magnet holds the gas port open.
Break the chain anywhere and the magnet de-energises, a spring slams the port shut, and gas stops. That is exactly what should happen if a flame blows out. It is also what happens if any component in the chain fails — which is why a healthy-looking flame can still cut out.
My burner won't stay lit — is it the valve or the thermocouple?
Diagnose in this order. It saves buying the wrong part.
| Symptom | Most likely cause |
|---|---|
| Pilot dies the instant you release the knob | Thermocouple — replace that first, it is the cheapest part in the chain |
| Pilot flame is weak or yellow | Pilot burner or a blocked jet — the flame isn't heating the tip |
| New thermocouple fitted, tests at 25–30 mV, and it still cuts out | The FFD magnet unit in the valve has failed |
| Gas leaks past the tap, or the tap is stiff / seized | The valve body itself — replace the tap |
| Pilot holds fine but the main burner won't fire | Valve, or a thermopile system — see our thermocouple vs thermopile guide |
The key test: if a known-good thermocouple producing a healthy 25–30 mV still cannot hold the valve open, the fault is the magnet unit. It can no longer hold on the voltage it is being given.
Can I replace just the magnet unit?
Often, yes — and it is far cheaper than a whole valve.
We stock the PEL Gas Valve Magnet Unit FFD/FSD for 22S and 23S taps at £16.99. If your tap is otherwise sound — turns smoothly, doesn't leak, meters gas correctly — and only the safety cut-out has failed, replacing the magnet unit fixes it for a fraction of the cost of a new tap.
Replace the whole valve when the body is at fault: gas passing when the tap is off, a seized or sloppy spindle, cracked castings, or damaged threads.
Which PEL valve do I need?
We stock PEL Pintossi — the make fitted to a very wide range of commercial gas cookers, ranges and wok burners.
| Part | Use | Price |
|---|---|---|
| PEL 23S safety tap | High output — wok cookers | £86.00 |
| PEL 35S safety tap | Heavy series | £99.00 |
| PEL 36S safety tap | Heavy series, no pilot | £99.00 |
| PEL 25ST thermostatic valve | Capillary probe | £108.00 |
| PEL 24ST thermostatic valve | 100–300°C — griddles, ovens | £109.00 |
| FFD/FSD magnet unit | Safety cut-out for 22S / 23S | £16.99 |
| Gas tap control knob | Black | £4.99 |
To identify yours: the type number is stamped on the valve body. Quote it and you will get an exact match. Note the difference between a safety tap (on/off with FFD) and a thermostatic valve (modulates to hold a set temperature — used on griddles and ovens).
What is a thermostatic gas valve, and how is it different?
A plain safety tap is on or off. A thermostatic valve holds a target temperature: a capillary probe sits in the oil or against the plate, senses the temperature, and modulates the gas to hold it.
So if your griddle or oven runs hot, runs cold, or will not hold a set temperature, the fault is in the thermostatic valve or its capillary probe — not in the safety system. A kinked or broken capillary is a common failure and the valve cannot be repaired around it.
Can I fit a gas valve myself?
No. This is not the same as swapping a thermocouple. A gas valve is a pressure-carrying gas component: it must be fitted, leak-tested and the combustion checked.
In the UK, work on commercial catering gas appliances must be carried out by a Gas Safe registered engineer qualified for commercial catering. Buy the right part, have it fitted properly.
Frequently asked questions
What does FFD stand for?
Flame Failure Device. You will also see FSD (Flame Supervision Device) — the same thing. It shuts the gas off when there is no flame.
Why does my burner cut out after 30 seconds?
The classic signature of a chain that is *almost* working: the thermocouple produces some voltage but not enough to hold the magnet, or the magnet has weakened. Test the thermocouple output first — if it is healthy at 25–30 mV, the magnet unit is your culprit.
Can I bypass the FFD to keep the appliance running?
No. Never. The FFD is what stops unburnt gas filling your kitchen when a flame blows out. Defeating it is illegal, uninsurable, and how people die. If the FFD is faulty, fix it — the magnet unit is £16.99.
How fast can I get one?
PEL valves and magnet units are held in UK stock. Order before 10am for next-day delivery, free over £25. Call 073 8051 5838 with the type number stamped on your valve.
Need a valve identified?
Send a photo of the valve body with the type number visible and we will match it.
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