Faulty diverter valve: symptoms, causes and what a repair involves

What a diverter valve is, on any combi boiler

A combi makes hot water on demand and keeps none of it in a cylinder. That leaves the boiler with one supply of heat and two places to send it: out round the radiator circuit, or across the plate exchanger that feeds your taps. The diverter valve is the motorised part that picks between the two, and it picks again every time anybody in the house opens a tap. Nothing else in the boiler decides where the heat ends up, which is why one worn component can produce a fault that feels like three different problems.

Every combi has one, whatever the badge on the front. What changes from brand to brand is the hardware, not the job it is asked to do, and what every version of it shares is the wear. The valve is a moving part living permanently in hot water, so limescale, sludge and ageing seals all reach it in the end. When it stops travelling cleanly between its two positions the boiler usually keeps firing, the display usually stays blank, and you carry on paying for heat that goes the wrong way.

The symptom pattern

No manufacturer publishes a fault code for this part, so the pattern is the diagnosis. Read every sign below as an answer to one question: of the two circuits the boiler can feed, which one is actually getting the heat?

• Heat splitting between both circuits. The taps peak at pleasant and stop there however far you open them, while the radiators behave perfectly normally. The valve is not shutting the heating side off, so the tap gets only what is left over.
• Heat locked on the heating side. Showers are fine, and the radiators stay stone cold long after every tap in the house is closed and the room thermostat is still calling for heat. Controls and pumps paint the same picture, so this one gets ruled out before any casing comes off.
• Heat reaching the taps only once the radiators are already hot. Most often noticed in summer, after weeks with the heating switched off. Say that sentence exactly when you book: it narrows the job faster than any other detail you can offer.
• Heat and water both leaving the circuit. A valve can fail wet instead of stuck, in which case the pressure gauge keeps falling and nothing anywhere in the house ever looks damp.
• Not a symptom at all: radiators dropping off for a few minutes while somebody runs a bath. A combi is built to serve the taps first, so that is the valve doing its job rather than failing at it.

Why heating fine plus no hot water points at the valve

Heating fine, hot water not. That tells you the boiler is lighting and making heat perfectly well, and that the problem is where the heat ends up. If the valve will not move fully to its hot water position, most of the output carries on round the radiator circuit and your tap gets only what it picks up on the way past. Run the heating at the same time and that circuit is already hot, so warm water appears at the tap again. That is the hot-water-only-with-the-heating pattern, and it is the closest thing to a smoking gun this fault produces.

The reverse case, hot showers with stone cold radiators, means the valve is stuck the other way, and the cheap causes deserve ruling out first: a programmer set to hot water only, a thermostat turned down, or radiator valves closed since last summer. The wet failure behaves differently again. The valve body is a joint like any other inside the boiler, and it is plumbed into the pressurised side of the heating circuit, so a hardened seal bleeds pressure away with nowhere for the water to show itself. The gauge falls, you top it up, and a fortnight later you are doing it again. Our guide on <a href="/blog/boiler-losing-pressure/">a boiler that keeps losing pressure</a> works through the other causes of that.

Diverter valve, thermistor or PCB? How to tell them apart

Three faults produce overlapping symptoms, and the diverter valve gets blamed for all three. Start with the display. A temperature sensor fault, a thermistor, normally announces itself with a code of its own, and every manufacturer publishes what its own codes mean. If a code is showing, look it up in your own manufacturer's list before anybody orders a part.

A diverter valve fault usually shows no code at all, because no manufacturer publishes one: the boiler knows it made heat and cannot tell where that heat went. That does not mean a code rules the valve out. A valve stuck hard enough to restrict circulation turns up second hand, on the codes raised for poor flow and overheating. Vaillant name a stuck diverter or zone valve among the causes of <a href="/blog/vaillant-f84-fault-code/">F84</a>, and the diverter valve is one of the things an engineer checks on an <a href="/blog/vaillant-f25-fault-code/">F25</a> visit. A stuck diverter valve is a listed cause of <a href="/blog/worcester-bosch-219-fault-code/">Worcester Bosch 219</a> as well. Read it this way: a blank display plus misdirected heat points hard at the valve, and a circulation or overheat code does not point away from it.

A control board fault tends to be erratic rather than consistent. A stuck valve fails the same way every time, while failing electronics come back to life after a power cycle then stop again a week later. If your pattern is random rather than repeatable, say so when you book. Two cheaper things get mistaken for this fault too: a hot water dial knocked down by a passing elbow, and a service valve left part closed after work on the system.

Which fault is it likely to be?

How the job differs from brand to brand

The fault is the same on every combi. What changes with the badge is the hardware, the paperwork and who you ring first. Start with the hardware. A range name on its own is not enough to order against, because one range can run through more than one valve assembly over its production life, so an engineer works from the data badge instead: the model and the serial number, read off the boiler in front of them. Expect a second visit where that assembly is not stock on the van, and settle whether the second visit is charged before the first one ends.

Then the route. Manufacturers price an out-of-warranty repair in different shapes, and that changes what a diverter valve costs you rather than what it costs to fix. Ideal, for one, publishes a fixed price repair which it describes as including all required boiler parts plus labour, so a failed fan and a failed diverter valve land on the same invoice, alongside a Repair &amp; Care plan for boilers under 15 years old. Others publish a price for the part and leave the labour open, which is why two quotes for the same fault on two different boilers are rarely comparable. Our guide to <a href="/blog/ideal-boiler-out-of-warranty-repair/">an out-of-warranty Ideal repair</a> shows how one manufacturer sets that out.

Third, the guarantee. A component failing inside the casing is exactly what a manufacturer guarantee exists for, and this is still the part where owners most often discover it does not pay. The conditions decide it, and they differ from brand to brand. Vaillant, for one, makes registration, annual servicing and clean system water all conditions of its cover, and we quote its terms in full on our <a href="/manufacturers/vaillant/help/diverter-valve/">Vaillant diverter valve page</a>. Whoever made your boiler, find out where you stand before you agree to any work. Our walkthrough on <a href="/blog/how-to-check-boiler-warranty/">checking a boiler warranty</a> shows what to look for.

What fixing it actually involves

Nothing gets stripped down until the valve has been ruled in. The engineer drives the boiler through both modes and watches where the heat actually goes, because a dial somebody nudged, a pump losing its shove and a half-shut isolation valve all present identically from the kitchen doorway. Order a part before that test and you can end up paying for it twice, because a fitted part cannot go back.

A new part is not a foregone conclusion either. Scale and sludge jam a valve without ever wearing it out, and a valve in that state can come out, be cleaned and go back in, which is the cheapest ending this fault has. Some manufacturers put that in writing for their own boilers, and <a href="/manufacturers/vaillant/help/diverter-valve/">Vaillant's advice on its own diverter valves</a> is one worth reading whatever badge is on your boiler. Three findings take cleaning off the table: a valve that weeps, a motor that no longer drives it, and a valve that answers at the wrong moments. None of those is a deposit anybody can dissolve. Whether the fix is a seal kit or the complete assembly is the engineer's call once the part is out and in the light.

Either way, this is inside-the-casing work on a gas appliance, and that is not a grey area. HSE puts the law plainly: to legally undertake gas work within the scope of the Gas Safety (Installation and Use) Regulations 1998, a gas engineering business “must be on the Gas Safe Register”. Check any engineer's registration before they start, and leave the casing alone yourself.

One safety point overrides everything here. If you smell gas, stop. National Gas advises you to open doors and windows, not to smoke or light matches, not to turn electrical switches on or off, to turn the gas off at the meter control handle unless the meter is in the cellar, and to call the National Gas Emergency Service on 0800 111 999. Leave the property if the smell does not clear. If water is pooling around the boiler, turn the water supply and heating off and get an engineer out.

What it costs, and where a cover plan fits

Ask two engineers what this repair costs and the answers rarely match, because the question hides four unknowns: which valve assembly your boiler actually takes, whether the valve cleans up or has to be changed, whether a seal kit answers it or the whole assembly does, and whether the part is on the van. Only the first of those can be settled before the casing comes off. Where a manufacturer does publish a price for its own part, read it as a floor rather than a total: it buys the component and none of the time spent proving the component is at fault. Vaillant is one of the makers that publishes such a figure, and we quote it in full on our <a href="/manufacturers/vaillant/help/diverter-valve/">Vaillant diverter valve page</a>.

For the fault you have today the route is a paid <a href="/repairs/boiler-repair/">one-off call-out and diagnosis</a>: you buy the visit, an engineer attends and diagnoses the fault, and anything beyond the 30 minutes allocated to the call-out is quoted before it is done. One-off appointments run Monday to Friday, 09:00 to 17:00.

For future breakdowns, a cover plan is the other route. A failed diverter valve is a boiler and central heating repair, so it needs the <a href="/cover/boiler-central-heating/">boiler and central heating module</a> on your plan, and that module's own exclusions still decide the answer. Inside it, the plan meets the parts and the labour up to whichever limit applies to your boiler across a 12-month period: £500 while it is under 7 years old, £200 once it is past 7, and anything above that line is yours. Two limits are worth knowing before you rely on it. The terms treat failure of the heat exchanger or the PCB as beyond economic repair, which pays a contribution rather than a repair, so if the engineer traces your symptoms to the control board the plan does not put that board right. And a fault the boiler is already showing counts as pre-existing, so lukewarm taps today are a repair decision, not a cover decision.

Where the sums have stopped adding up, <a href="/blog/boiler-repair-or-replace/">repairing versus replacing a boiler</a> sets out how to judge a machine that has already cost you more than one visit.

Faulty diverter valve FAQs

Heat going to the wrong place? Talk to us.

A diverter valve sits inside the casing, so it needs a Gas Safe registered engineer, and we work with a national network of thousands of Gas Safe registered engineers. For the fault you have now, book a paid one-off call-out and diagnosis: you buy the visit, an engineer attends and diagnoses the fault, and anything beyond the 30 minutes allocated to the call-out is quoted before it is done. One-off appointments run Monday to Friday, 09:00 to 17:00. For future breakdowns, the boiler and central heating module pays parts and labour up to £500 in a 12-month period while the boiler is under 7 years old, or £200 once it is over 7. Under a cover plan we aim to get an engineer out to you within two working days, with call-outs Monday to Friday, 08:00 to 18:00. A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan does not cover, no-access visits and early annual-service requests, and any works needed in that window are quoted and paid for in advance. Cover still applies in those first 30 days. Using a module starts a 12-month agreement period. Smart Plan is a service plan, not insurance, from UK Boiler Company Ltd, trading since 2014. If you smell gas, call the National Gas Emergency Service on 0800 111 999 first.

Helpful links

• Vaillant owners: Vaillant's own diverter valve advice, guarantee conditions and part price
• No hot water: safe checks then book a repair
• Boiler losing pressure
• Repair or replace your boiler?
• Boiler & central heating cover
• Check an engineer on the Gas Safe Register
• HSE: gas work and the Gas Safe Register