Diverter valve fault — no hot water (combi boiler)
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →⚠️ Do not attempt to work on internal boiler components. All gas boiler repairs must be carried out by a Gas Safe registered engineer.
A combi boiler's diverter valve is a clever little component that acts like a traffic controller for your heating system — directing hot water either to your radiators or to your taps, depending on what you need at any given moment. When it gets stuck or fails, you're left in an awkward situation: your central heating works perfectly, but your hot water comes out lukewarm or stone cold. It's a common fault that tends to catch homeowners off guard, especially during winter when you're relying on your boiler most. This guide explains what's happening inside your boiler, why it matters, and most importantly, how to get it sorted quickly and safely. If you're confident it's the diverter valve causing the problem, a Gas Safe registered engineer is your next essential call.
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Most likely cause & what to check
On a combi boiler, the diverter valve switches the heated water between the central heating circuit and the domestic hot water heat exchanger. When it fails, the symptoms are typically: no hot water despite heating working, or hot water that runs warm rather than hot.
A stuck-open diverter valve means heat goes to the radiators even when you only want hot water — and vice versa. You may notice radiators warming up when you run a tap.
This is an internal boiler component repair — only a Gas Safe registered engineer should open the boiler casing and replace parts.
Call a Gas Safe engineer and describe: "My combi boiler is heating the radiators fine but producing no hot water — I suspect the diverter valve." A new diverter valve typically costs £50–£150 for parts; labour brings the total to £150–£350.
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Frequently asked questions
Can I fix a broken diverter valve myself?
No — the diverter valve sits inside the boiler casing, and opening up a gas boiler is illegal unless you're Gas Safe registered. Attempting it will invalidate your warranty and create serious safety risks. Always call a qualified engineer.
How do I know it's the diverter valve and not something else?
The telltale sign is heating working perfectly whilst hot water is absent or lukewarm. You might also notice your radiators warming up when you run a hot tap, which suggests the valve is stuck and sending water the wrong way. A Gas Safe engineer can confirm the diagnosis with a quick inspection.
How much will it cost to replace a diverter valve?
Parts typically run £50–£150, but labour will add another £100–£200, bringing the total to roughly £150–£350 depending on your boiler model and your engineer's call-out fees. Some engineers may charge a fixed rate or hourly rate, so it's worth getting a quote beforehand.
How long does a diverter valve replacement take?
Usually between 1–2 hours for a straightforward replacement, though access and boiler design can affect this. A good engineer will also test the boiler afterwards to make sure everything's working correctly before they leave.
Will my boiler warranty cover the cost?
It depends on your warranty terms and the age of your boiler. If you're still under manufacturer's cover, a parts fault like a diverter valve may be covered — contact your supplier or check your documents. Extended warranties sometimes cover labour too, so it's worth checking before ringing the engineer.
What should I do for hot water in the meantime?
If you have an electric shower, use that for now. Alternatively, a portable electric heater or kettle can provide hot water for washing, and you can run the tap until it warms slightly from residual heat in the pipework. Keep windows open when using any temporary heating method.