Glossary · Boilers and boiler types
What is a diverter valve?
A diverter valve is a motorised valve inside a combi or system boiler that decides where the boiler's heat goes. It sends hot water either round the central heating circuit or to the hot water side. When you open a hot tap it switches over, which is why the radiators can pause.
Key points
- It is a motorised three-way valve. Worcester describes its valve as delivering hot water as a priority over heating.
- When a hot tap opens on a combi, the heat is diverted from the heating circuit to the hot water side until the tap closes.
- Ideal Heating says the central heating generally switches off for a moment when a tap is turned on. That is normal behaviour.
- The valve and its motor sit inside the boiler casing, so any repair is gas appliance work for a Gas Safe registered engineer.
- A boiler that heats water but not the radiators has several possible causes. The valve is one of them.
How a diverter valve works
A combi boiler has one burner and one set of primary heat to share between two jobs: warming the radiators and heating the water for your taps. The diverter valve is the part that chooses between them.
The valve has three connections. Heated water comes in on one. It leaves by one of the other two, depending on where the valve is pointing. A small motor moves it and the boiler’s control board decides when. The Worcester Greenstar 24/28/30 combi manual lists a motorised three-way diverter valve that delivers hot water as a priority, and describes an actuator driven by a stepper motor.[1]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions
In plain terms, the sequence is this. When the heating is calling for heat, the valve sends the heated water round the radiator circuit. When you open a hot tap, the boiler senses the demand and the valve switches so that the heat goes to the water for the tap instead. Glow-worm describes the same behaviour: when a hot tap is turned on, water is diverted from the primary heat exchanger into the secondary one until the tap is turned off.[2]Glow-worm, What is a combi boiler and how does one work? The heat exchanger is where the heat actually passes into the tap water.
Why the radiators pause
Because the one burner serves both jobs and hot water comes first, the radiators wait while a tap is running. Ideal Heating says that with a combi the central heating will generally switch off for a moment when you turn a tap on.[3]Ideal Heating, What is a combi boiler? This is normal. It is not a fault. The full sequence is set out in how a combi boiler works.
Which boilers have one
| Boiler | Diverter valve | Notes |
|---|---|---|
| Combi | Yes, inside the boiler | Switches between heating and hot water on demand |
| System | Varies by model | Some use a valve in the boiler, others use separate zone valves on the pipework |
| Regular | Not usually inside the boiler | Heating and cylinder circuits are normally controlled by valves in the system |
The last two rows depend on how the system was designed. Your installer or the manufacturer’s manual for your model gives the exact arrangement, so treat the table as a guide to where to look rather than a rule. The pages on system boilers and regular boilers cover the differences.
What you can and cannot do
There is nothing for a householder to adjust on a diverter valve. It sits inside the boiler casing with its motor. The same Worcester manual describes a service function that moves the valve to a centre position so that the system can be fully drained and the motor removed, which is work done with the boiler opened up.[1]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions Anyone employed to work on gas appliances in a home must be Gas Safe registered and competent in that area of gas work.[4]Health and Safety Executive, Domestic gas: frequently asked questions
If your boiler gives hot water but the radiators stay cold, the diverter valve is one of several possible causes, along with the pump, controls and others. The page on no heating but hot water works covers that symptom. An engineer needs to find which part is at fault. BASI covers this under boiler repair.
Example
Imran in Shipley runs the kitchen tap while his radiators are on and notices the radiators stop warming for the minute he is washing up. When he turns the tap off they carry on. That is the diverter valve doing its job. If the radiators were cold all day and only the hot water worked, that would be a different matter and worth a call.
What people get wrong
People often read the heating pause as a fault. On a combi it is the design: hot water takes priority over heating. A problem is when the heating does not come back after the tap closes, or when it never comes on at all.
The other common mistake is to blame the diverter valve every time the radiators are cold. It is one cause among several, and replacing a part without a proper diagnosis can cost money without fixing the fault.
The bottom line
A diverter valve is the traffic controller inside a combi or system boiler. It points the boiler’s heat at the radiators or at the hot taps and switches over in moments. A brief pause in heating when a tap runs is normal. Anything beyond that, or any work on the valve, belongs with a Gas Safe registered engineer.
Common questions
What does a diverter valve do?
It directs the boiler's heated water either to the central heating circuit or to the hot water side. A motor moves it, and the boiler's controls decide which way it points.
Why does my heating stop when I run a hot tap?
On a combi the hot tap takes priority. Ideal Heating says the central heating generally switches off for a moment when you turn a tap on, then resumes when the tap closes.
Do system boilers have a diverter valve?
Many do, though the arrangement varies by model. Some systems use separate zone valves on the pipework instead. Check your boiler's manual or ask your engineer.
Can I fix a faulty diverter valve myself?
No. The valve is inside the boiler casing and anyone employed to work on gas appliances in a home must be Gas Safe registered. Book an engineer.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Worcester, Bosch Group. Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions. Checked 11 November 2026. ↩
- Glow-worm. What is a combi boiler and how does one work?. Checked 11 November 2026. ↩
- Ideal Heating. What is a combi boiler?. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩