A faulty diverter valve on a combi boiler usually shows up as heating but no hot water, hot water but no heating, lukewarm water at the taps, or radiators warming up whenever you run a hot tap. The valve sits inside the boiler, so beyond a few safe checks on the controls and pressure, it needs a Gas Safe registered engineer to test it and repair or replace the motor or the valve.

What a diverter valve does

A combi boiler heats your radiators and your hot water with the same burner and heat exchanger, but it cannot do both at full output at once. The diverter valve decides where the heat goes. It is a three-way valve inside the boiler with one inlet and two outlets: one to the central heating circuit, and one to a small plate heat exchanger that heats the water going to your taps and shower.

Vaillant describes it as a mechanism that opens or closes to direct hot water either to the radiators or to the taps and showers. It gives priority to hot water. If the heating is on and someone turns on the shower, the valve switches to the hot water side until the tap closes, then switches back.

Most modern boilers use a motorised valve, where a small electric motor, often called the actuator or motor head, clips onto a valve body and moves a spindle inside it. Some older boilers used a hydraulic valve moved by water pressure acting on a rubber diaphragm.

System and regular boilers that feed a hot water cylinder usually share heat between the radiators and cylinder with motorised zone valves in the pipework outside the boiler, although some system boilers have a diverter valve built in for the cylinder. This guide concentrates on combi boilers, where diverter valve faults are most common.

Symptoms of a faulty diverter valve

What you notice	What is probably happening
Radiators warm up when you run a hot tap, even with the heating off	The valve is stuck in the heating position, so the heat meant for your tap goes to the radiators
Hot water but no heating	The valve is stuck on the hot water side, or the motor cannot move it back
Heating works but no hot water, or it is only lukewarm	The valve is not moving fully to hot water, or is sitting part way
Hot water only when the heating is on	The valve is stuck part way, letting heat reach both sides
Hot water runs hot then cold	The valve is sticking or drifting, though this also has other causes
Water dripping inside or under the boiler, pressure dropping	Worn seals on the valve spindle or body are leaking
An error code about circulation or temperature	Some boilers show a fault when water is not moving where the board expects it

None of these symptoms prove it is the diverter valve. A scaled plate heat exchanger, a faulty flow sensor, a thermostat or a timer fault can all look similar, which is why the engineer tests before replacing parts. Our guide to no hot water but the heating works covers the wider picture.

Check these first

These checks are safe for a homeowner and rule out the simple causes.

1. Check the controls. Make sure the timer or programmer and the room thermostat are asking for heat. Turn the thermostat up a few degrees. A boiler that is not being asked for heating will only ever give you hot water.
2. Check the boiler pressure. With the heating cold, the gauge should read the pressure your user guide recommends, usually between 1 and 1.5 bar. If it is low, repressurise with the filling loop if your user guide allows it, and see our guide to a boiler that keeps losing pressure if it keeps dropping.
3. Open the hot tap fully. A combi only switches to hot water when it senses enough flow. A barely open tap, or a blocked tap aerator or shower head, may not trigger it.
4. Test it with the heating off. Turn the heating off at the programmer, run a hot tap for a few minutes, and feel whether the radiators start warming. If they do, the valve is almost certainly stuck.
5. Note any error code. Write down what the display shows. It helps the engineer and may point elsewhere.
6. Reset once. Use the reset button as your user guide describes. If nothing changes, leave it and book a boiler repair.

Common causes of a faulty diverter valve

Cause	What you might notice	Can you fix it yourself?	What the engineer does
Failed motor or actuator	Valve stuck in one position, no sound of it moving	No	Tests the supply from the board and replaces the motor head
Valve seized by scale or sludge	Stuck on heating or hot water, often after a quiet summer	No	Frees the valve or replaces the cartridge or body, and looks at water quality
Worn diaphragm or internal seals	Lukewarm water, heat reaching both sides at once	No	Replaces the internal parts or the valve
Leaking spindle or body seals	Drips inside the case, pressure dropping	No	Replaces the seals or valve and checks for water damage to electrics
Wiring or main board fault	Motor works when tested directly but not in normal use	No	Checks the connections and the board’s output
Something else that mimics it	Symptoms fit, but the valve moves correctly	Controls only	Tests the flow sensor, plate heat exchanger, thermostat and timer

Two things make diverter valves fail more often. The first is dirty system water: sludge and magnetite from corroding radiators collect in moving parts. The second is hard water. Across most of Hertfordshire the water is hard, and scale builds up on the hot water side of a combi, particularly the plate heat exchanger and any parts in contact with the hot water.

What an engineer will check

1. The fault history and symptoms. When the problem started, what works and what does not, and any stored error codes.
2. Controls. That the thermostat, programmer and wiring are asking the boiler for the right thing.
3. Valve movement. Whether the motor drives the valve fully from heating to hot water and back, and whether the valve body moves freely with the motor removed.
4. The electrical side. Whether the board is sending the signal to switch, and whether the motor responds.
5. The hot water path. The flow sensor, the plate heat exchanger and the hot water temperature sensor, to rule out a lookalike fault.
6. Leaks and water quality. Signs of weeping around the valve, the condition of the system water and whether a magnetic filter is fitted.
7. A full test. Heating and hot water both run and switch correctly before the engineer leaves.

Repair or replace?

A diverter valve fault on its own is almost always a repair. On many boilers the motor head can be replaced separately from the valve body, which is a quicker job. If the valve itself is seized or leaking, the valve or its internal cartridge is replaced, which means draining down part of the system.

It is worth thinking about replacement only if the boiler is old, parts for that model are becoming hard to get, or the valve is one of several faults stacking up. Our repair or replace guide explains how we weigh that up.

What it costs with Vantage

In normal working hours there is no call-out fee. Gas faults start with a fixed-price Gas Safe diagnostic visit, and if we can fix it on the same visit, part of that diagnostic charge comes off the repair. After the first hour, time is charged in half-hour steps, and parts are quoted before we fit them. Evening, weekend and overnight visits are charged at the out-of-hours rates. Every rate is published on our prices page, so you know what you will pay before we arrive.

A motor head is usually the less expensive fix, while a complete valve costs more because of the part and the extra time to drain and refill. We repair and service all the major brands, our vans are fully stocked, and our repairs carry a 12 month labour guarantee.

Preventing diverter valve problems

• Keep the system water clean. A magnetic system filter and the right level of inhibitor protect moving parts. If the system is badly sludged, a power flush may be worth it before replacing parts that will only clog again.
• Deal with hard water. A scale reducer on the boiler’s cold feed, or a water softener for the whole house, slows scale on the hot water side.
• Service the boiler every year. An engineer will spot a valve that is slow to move before it sticks.

Frequently asked questions

Can a diverter valve be unstuck without replacing it?

Sometimes. An engineer can occasionally free a sticking valve, but if it has seized once from scale or sludge it usually sticks again, so replacing the valve or cartridge is the more reliable fix. It is not a job for a homeowner.

Why do my radiators get hot when I run the hot tap?

On a combi boiler that almost always means the diverter valve is stuck in the heating position. The boiler fires because it senses the tap is open, but the heat goes to the radiators instead of the hot water.

Can I still use my boiler with a faulty diverter valve?

Usually you can use whichever side still works, as long as there is no leak and no error code. If water is dripping from the boiler or the pressure keeps falling, turn it off and book a repair.

Do system boilers have a diverter valve?

Some do, built in to share heat between the radiators and a hot water cylinder. Many system and regular boiler setups use separate motorised zone valves in the pipework instead, which can fail in similar ways.

How long does a diverter valve repair take?

A motor head swap is quick once the fault is confirmed. Replacing the valve body takes longer because the boiler has to be isolated and partly drained, then refilled, bled and tested.

When to call an engineer

If your combi is only giving you heating or only hot water, or the radiators warm up when you run a tap, the diverter valve is the first thing we test. Our Gas Safe registered engineers (registration 956889) can find the cause and fix it, often on the first visit. See our boiler repair service or call 01462 506374, answered 24 hours a day, 7 days a week.