How much does it cost to replace a diverter valve?

It depends on which part has failed. A motor head swap is the short version of the job; a full valve body means draining and refilling the boiler and takes longer. The part price varies by make and model. Labour is charged by the hour, so ask any engineer which version of the job their figure assumes. Our rate is on the pricing page.

Can I keep using my boiler with a faulty diverter valve?

Usually, yes, but not comfortably. A valve stuck on the heating side leaves you with heating and little or no hot water, and one stuck part way gives you weak versions of both. It is not normally a safety issue, but the boiler works harder than it should and the fault tends to get worse, so get it looked at.

Is it the diverter valve or the flow sensor?

If the radiators warm up when you run a hot tap, the boiler has seen the demand and the valve is sending heat the wrong way. If the boiler does not fire at all when you open a tap, it has not seen the demand, which points at the flow sensor or the controls. An engineer confirms it by testing the valve while it is working.

Can I replace a diverter valve myself?

No. The valve is inside the case of a gas appliance, and work inside that case should be done by a Gas Safe registered engineer. The job also involves draining, refilling and repressurising the boiler and checking it is working safely afterwards.

Is it worth replacing a diverter valve on an old boiler?

If the boiler is otherwise reliable and the part is still made, usually yes. It is a wear part, not a sign the boiler is finished. If it is one of several recent failures, the heat exchanger is leaking or the part is discontinued, compare the repair with a replacement before you pay for it.

Diverter Valve Replacement Cost: What You Are Paying For

The cost of a diverter valve replacement comes down to which part of the valve has failed and how much of the boiler has to come apart to reach it. If only the motor head (the actuator) has failed, it often clips off the outside of the valve and the job is short. If the valve body or its internal cartridge has seized, the boiler has to be isolated, drained, opened up and refilled, and that takes a good deal longer. Parts vary a lot between makes, and an older boiler may need a part that is hard to source. The step people skip is proving it is the diverter valve at all, because a failed flow sensor, a scaled plate heat exchanger or a control board fault can look exactly the same at the tap. Our hourly rate and call-out are on the pricing page. The rest of this guide explains what sets the total, so you can read a quote properly and decide whether the repair is worth it.

What a diverter valve does, and how it fails

A combi boiler has one burner and two jobs. It heats the radiators, and it heats the mains water as it passes through on its way to a tap. The diverter valve is the three way valve inside the case that decides where the heat goes. When you open a hot tap, the boiler sees the flow and drives the valve over to the hot water side. When the tap closes, the valve swings back to the heating.

It fails in two main ways. It can stick on the heating side, so you get radiators warming up every time you run a tap and water at the tap that never gets properly hot. Or it can stick part way, so the hot water is lukewarm and the heating runs weakly at the same time. On some boilers a worn valve lets heat creep into the radiators in summer when nobody has asked for heating.

Radiators get warm when you run a hot tap, with the heating switched off
Heating works but the hot water is cold or lukewarm
Hot water starts hot then fades while the boiler keeps firing
Heating only comes on when a hot tap is running

What decides the cost of a diverter valve replacement?

Four things set the total: which component is replaced, what the part costs for your make and model, how long the boiler takes to strip and refill, and whether anything else turns up once the case is open. The labour is charged by the hour, and the rate is agreed before anyone arrives, but the number of hours depends on the boiler in front of the engineer.

Phone quotes for this job vary for that reason. One firm has priced a motor head swap and the other has priced a full valve body with a drain down. Both could be honest. Ask which one the figure assumes before you compare them.

Actuator (motor head) only: usually external to the valve, often the shortest version of the job
Valve cartridge or internals: needs the primary circuit isolated and drained
Complete valve body or hydraulic assembly: the longest version, with more connections to remake
Make and age of boiler: parts for current models are easy to get; discontinued ones may not be
Access: a boiler in a tight kitchen cupboard or a loft adds time at both ends of the job

Actuator or valve body: why it makes such a difference

On many combis the actuator is a small motor that sits on top of the valve and pushes a spindle. When the motor burns out or its wiring fails, the valve underneath is fine and the actuator is replaced on its own. There is no water to drain, and the job is mostly about proving the fault and testing the boiler afterwards.

When the valve itself is the problem, the story changes. Scale and sludge are the usual causes. Hard London water is part of it, but the bigger culprit inside the valve is the magnetite and debris carried round the heating circuit, which can stop the spindle moving or wear the seals so the valve lets by. Replacing the internals means isolating the boiler, draining it down, removing the old parts, fitting new seals, refilling, venting the air out and bringing the system back to pressure. A good engineer then runs the heating and the hot water in turn and watches the valve move.

If the valve failed because the system water is dirty, a new valve in the same water will go the same way. Check whether the quote includes a look at the system water and the filter. It is cheaper to deal with the cause now than to pay for the same repair again later.

Is it definitely the diverter valve?

Not always, and this is where a lot of money gets wasted. On a combi, no hot water with working heating has a short list of causes, and the diverter valve is only one of them. A flow sensor that does not register the tap opening, a plate heat exchanger furred with scale, or a control board that never sends the signal can all produce the same complaint. Our page on no hot water from a combi boiler goes through how to tell them apart from the outside.

A useful clue: if the radiators warm up when you run a hot tap, the boiler has seen the demand and the heat is going the wrong way, which points at the valve. If the boiler stays silent when you open the tap, it has not seen the demand at all, which points at the flow sensor or the controls. A proper boiler diagnosis tests the valve while it is working rather than guessing from the symptom, and it is the cheapest part of the whole job.

The diverter valve sits inside the sealed case of a gas appliance. Any work inside that case should be done by a Gas Safe registered engineer, and you can check how to check a plumber on the Gas Safe Register in about a minute before anyone takes the panel off.

Is it worth replacing a diverter valve?

On a boiler that is otherwise sound, yes, almost always. The diverter valve is a wear part. Replacing it is a normal repair, the same as a pump or a fan, and a boiler that is lighting cleanly, holding pressure and heating the house is worth keeping running.

The picture changes when the valve is one of several things going wrong. If the boiler has had two or three major parts in the last year or so, if the heat exchanger is leaking, or if the part for your model is no longer made, it is time to weigh the repair against boiler replacement. Age alone does not decide it. How long should a boiler last depends on the make, the servicing history and what has already failed.

If the boiler is still inside its manufacturer warranty, call the manufacturer before you call anyone else. A diverter valve failure is a common warranty repair, and a third party working inside the case can make a later claim harder.

Worth repairing: boiler otherwise reliable, parts readily available, system water in reasonable condition
Worth a second look: several recent repairs, visible corrosion or leaks inside the case
Usually time to replace: heat exchanger failing, or the part is no longer made

How long does a diverter valve replacement take?

An actuator swap on a common boiler is a short visit once the fault is confirmed. You will be without heating and hot water while the boiler is drained. A valve body or cartridge takes longer because of the drain down and refill, and on a boiler with cramped internals or seized fittings it can take most of a morning. If the part is not on the van, the first visit proves the fault and a second one fits the part.

Getting a combi hot water fault looked at

If your combi is giving you heating but no hot water, or the radiators warm up every time you run a tap, call 020 4652 5111. Tell us the make and model from the front of the boiler and what happens when you open a hot tap. We will test the diverter valve, the flow sensor and the controls in turn and tell you which one has failed before any part is ordered.

Our hourly rate and call-out are on the pricing page and agreed before we attend. The rate does not change on the day; the total depends on which part has failed and how long the boiler takes to strip and refill.

Questions we get asked

How much does it cost to replace a diverter valve?: It depends on which part has failed. A motor head swap is the short version of the job; a full valve body means draining and refilling the boiler and takes longer. The part price varies by make and model. Labour is charged by the hour, so ask any engineer which version of the job their figure assumes. Our rate is on the pricing page.
Can I keep using my boiler with a faulty diverter valve?: Usually, yes, but not comfortably. A valve stuck on the heating side leaves you with heating and little or no hot water, and one stuck part way gives you weak versions of both. It is not normally a safety issue, but the boiler works harder than it should and the fault tends to get worse, so get it looked at.
Is it the diverter valve or the flow sensor?: If the radiators warm up when you run a hot tap, the boiler has seen the demand and the valve is sending heat the wrong way. If the boiler does not fire at all when you open a tap, it has not seen the demand, which points at the flow sensor or the controls. An engineer confirms it by testing the valve while it is working.
Can I replace a diverter valve myself?: No. The valve is inside the case of a gas appliance, and work inside that case should be done by a Gas Safe registered engineer. The job also involves draining, refilling and repressurising the boiler and checking it is working safely afterwards.
Is it worth replacing a diverter valve on an old boiler?: If the boiler is otherwise reliable and the part is still made, usually yes. It is a wear part, not a sign the boiler is finished. If it is one of several recent failures, the heat exchanger is leaking or the part is discontinued, compare the repair with a replacement before you pay for it.