It’s one of the most confusing and frustrating boiler problems: your radiators are piping hot, your house is toasty, but when you turn on the tap for a shower or to wash the dishes, all you get is cold water. It feels like a boiler mystery, but rest assured, this is a very common fault with a few key suspects.
Let’s dive into the most likely reasons why your boiler is prioritising your central heating over your hot water.
1. The Faulty Diverter Valve (The #1 Culprit for Combi Boilers)
If you have a combi boiler (the most common type of boiler in the UK, which provides heating and hot water on demand), a faulty diverter valve is almost certainly the cause.
Think of the diverter valve as a traffic controller inside your boiler. Its sole job is to decide where to send the hot water—either to your radiators for central heating or to your hot taps for domestic hot water. When it works correctly, it should prioritise the hot water request. When you turn on a tap, the valve closes the heating circuit and diverts the hot water to the taps.
If the diverter valve gets stuck, fails electrically, or has a mechanical fault, it may not switch over to the hot water circuit. Instead, it remains in the “heating” position, providing warmth to your radiators but nothing to your taps.
A tell-tale sign of this is that your radiators may feel warm even when the central heating is switched off, simply because the boiler is trying to produce hot water but is sending it to the wrong place.
2. A Faulty Flow Sensor or Microswitch
For your boiler to know you’ve requested hot water, it relies on a flow sensor or a microswitch. This small component detects when water is flowing through the domestic hot water circuit and tells the boiler to fire up the burner.
If this sensor or switch is faulty, it simply won’t register your request. The boiler will carry on believing there is no demand for hot water, even as you stand there waiting for the temperature to change. Your heating will work perfectly because it’s on a different circuit, but the hot water will stay cold.
3. Issues with the Plate Heat Exchanger
On a combi boiler, the plate heat exchanger is where the central heating water transfers its heat to the cold mains water to produce your instant hot water. Over time, this component can become blocked with sludge or limescale.
If the plate heat exchanger is partially blocked, you might get lukewarm water or a very low flow rate. If it’s completely blocked, no heat can be transferred at all, and your hot water will be stone cold, even though your boiler is working hard to produce hot water for the heating circuit.
4. The Zone Valve (For Conventional Systems)
If you have a conventional heating system with a separate hot water cylinder and a tank in the loft, the culprit may be a faulty zone valve. This electrical valve directs water to either the central heating circuit or the hot water cylinder. Just like a diverter valve, if it fails, it can get stuck on one setting (heating) and fail to switch over to the hot water side.
What to Do: When to Call a Professional
Diagnosing and fixing any of these issues requires specialist knowledge of gas appliances. Working on the internal components of a boiler should NEVER be a DIY task.
- Do a Quick Check: Ensure your boiler’s pressure is within the correct range (usually 1-1.5 bar). While low pressure is a different problem, it’s a good place to start.
- Call a Gas Safe Registered Engineer: The faults listed above require a professional to diagnose and repair them safely. An experienced engineer can quickly identify the faulty part and replace it.
If your boiler is providing heating but not hot water, don’t suffer through cold showers. For a fast and reliable repair, contact the experts at GMC Heating and Plumbing ltd. We have the tools and expertise to get your hot water back on. Call us today on 07736937010.
