What a diverter valve does
A combi boiler has one burner and one heat exchanger but two jobs: heating the radiators and heating tap water. The diverter valve decides which job the hot water from the boiler goes to. With no hot tap open, it sends water round the radiators. When you open a hot tap, a flow sensor signals the board and the valve switches the boiler water through a small plate heat exchanger that warms your tap water.
The valve has two main parts: a motor or actuator head that does the switching, and the valve body with a cartridge, seals and a spindle inside. Either part can fail. On some boilers the switching is done hydraulically rather than by a motor.
System and regular boilers
System and regular boilers do not have an internal diverter valve. The same job is done by motorised valves on the pipework, usually a three-port mid-position valve on a Y-plan system or two-port zone valves on an S-plan system. They fail in similar ways and cost a similar amount to fix, and we include typical prices below.
Symptoms of a failing diverter valve
- Hot water but no heating: the valve is stuck in the hot water position, so the radiators never get hot water.
- Heating but no hot water: the valve is stuck in the heating position. This is covered in detail in our guide to no hot water when the heating works.
- Radiators warm up when you run a hot tap: even with the heating off, a sign the valve is not closing fully.
- Lukewarm hot water: some heat is leaking to the radiators, so the taps never get properly hot.
- Humming or clicking from the boiler as the motor tries to move a stuck valve.
- A drip from under the boiler when the spindle seal has worn.
Diverter valve faults rarely produce a specific fault code. Most boilers simply carry on in the wrong mode, which is why the symptoms matter more than the display.
Typical diverter valve replacement cost
| Repair | Typical cost, fitted |
|---|---|
| Combi diverter valve motor or actuator head | Typically £100 to £200 |
| Combi diverter valve cartridge or internal parts | Typically £120 to £250 |
| Full combi diverter valve replacement | Typically £150 to £350 |
| Motorised valve head (Y-plan or S-plan) | Typically £100 to £200 |
| Full motorised valve replacement (Y-plan or S-plan) | Typically £200 to £350 |
What pushes the price up or down
- Motor or valve: if only the actuator head has failed, it can often be swapped without draining the system, which keeps the job short.
- Draining down: replacing the valve body means draining the boiler or the system, refilling and bleeding it, which adds time.
- Access: on some combis the diverter valve sits deep inside, behind the pump or plate heat exchanger.
- Boiler brand: genuine parts for some brands cost noticeably more than others.
- Region: labour costs more in London and the South East.
Sludge and scale are common reasons a valve sticks. If the system water is dirty, the engineer may suggest a clean or a magnetic filter so the new valve does not stick too. Our powerflush cost guide covers the options.
Checks to do before calling an engineer
- Check the thermostat is set above room temperature and the timer or programmer is calling for heating or hot water.
- Check the heating and hot water settings on the boiler itself have not been turned down or switched off.
- On a system or regular boiler, check the programmer for both heating and hot water, as a wrong setting can look exactly like a valve fault.
- Check the pressure gauge reads 1 to 1.5 bar with the system cold, and top up if it is low using our guide to repressurising a boiler.
- Switch the boiler off and on at the fused spur, then reset once if a code is showing.
The diverter valve is inside the boiler casing and connected to the sealed system. Do not try to free or replace it yourself. Book a Gas Safe registered engineer for any work inside the boiler.
Faults that look like a diverter valve problem
An engineer should test before replacing the valve, because other parts cause similar symptoms:
- Flow sensor: if it does not detect the hot tap opening, the boiler never switches to hot water.
- Plate heat exchanger: scale inside it can make hot water weak or lukewarm in hard water areas.
- Pump: a weak or stuck pump can leave the radiators cold even though the valve is fine.
- PCB: if the board is not sending the switching signal, the valve cannot move.
- Controls and wiring: on Y-plan and S-plan systems, a faulty programmer or wiring centre can stop the valve opening.
For a wider view of part prices, see our boiler repair cost guide.
What happens during the repair
Once the engineer has confirmed the valve is at fault, the job usually goes like this. If only the actuator head has failed, it unclips or unscrews from the valve body and a new one is fitted, often without draining any water. This can take well under an hour.
If the valve body, cartridge or seals need replacing, the engineer isolates the boiler, drains it down, removes the old valve and fits the new one with fresh seals. The system is then refilled, air is bled out and the pressure is set back to around 1 to 1.5 bar. Expect this to take one to two hours on most combis, and longer where the valve is hard to reach.
Finally, the boiler should be tested in both modes. The engineer runs a hot tap to check the valve switches to hot water and the radiators stay cool, then turns the heating on to check the radiators warm up properly. If you notice the pressure dropping in the days after the repair, a little air may have worked its way out through the radiators, and a quick top-up is usually all that is needed.
Is it worth repairing?
A diverter valve is a mid-priced repair and is almost always worth fixing on a boiler under about ten years old. On an older boiler with other problems, compare the cost with a replacement using our guide to repairing or replacing a boiler. A worn valve that has been weeping for a while can also damage parts below it, so do not leave a drip unchecked. Our guide to a boiler leaking water explains what to do.