If your radiators are warming up fine but nothing comes out of the taps except cold water, the fault almost always sits with one specific part rather than the whole system. It’s a strange one to diagnose from the outside, because the boiler is clearly firing and clearly working, which throws a lot of homeowners off.
We see this a fair bit on jobs across Southampton and Winchester, and it’s rarely as serious as it first feels. Here’s what’s actually going on, and what you can safely check yourself before calling an engineer out.
Why This Happens: The Short Answer
Most combi boilers use a single component, called a diverter valve, to switch between heating your radiators and heating your hot water. When that valve gets stuck, worn, or jammed in the “heating” position, the boiler keeps warming your radiators perfectly while sending nothing to the hot water side. Everything else in the system can be completely healthy, which is exactly why this fault confuses people.
If you’ve got a system boiler with a separate hot water cylinder, the cause is usually different again, more often an immersion heater fault, a faulty cylinder thermostat, or a stuck motorised valve controlling flow to the cylinder.
What You Can Safely Check Yourself
None of the checks below involve opening the boiler casing or touching anything gas-related, so they’re safe to do before booking an engineer.
- Check the boiler pressure gauge. It should typically read between 1 and 2 bar when the heating’s cold. Very low pressure can affect hot water delivery even if the heating still limps along.
- Check the programmer or timer. Some boilers have separate schedules for heating and hot water, and it’s an easy setting to knock by accident. Make sure hot water isn’t switched off or set to a schedule that’s currently inactive.
- Check the mode dial. Many combi boilers have a summer/winter or heating-only switch. If it’s set to heating only, hot water won’t come through at all, by design.
- Check nearby isolation valves. If any plumbing work has been done recently, a valve under a sink or near the cylinder may have been left partially closed.
If you’ve gone through all four and the taps are still running cold, the fault is very likely inside the boiler itself, and that’s where it needs to stop being a DIY job.
When to Call a Gas Safe Registered Engineer
A stuck diverter valve, a faulty printed circuit board, or an airlock inside the boiler’s internal pipework aren’t things to investigate yourself, whatever your general DIY confidence. These sit inside the appliance, and only a Gas Safe registered engineer should open the casing or work on the internals. Attempting it yourself risks making the fault worse, and in some cases risks safety.
The good news is that a diverter valve fault is usually a straightforward repair for an engineer who’s seen it before, rather than a sign you need a new boiler.
Does Hampshire’s Hard Water Make This Worse?
It can do. Much of Hampshire, including Southampton and Winchester, sits in a hard water area, and limescale build-up inside a diverter valve or heat exchanger is a genuinely common contributor to it sticking or jamming over time. It’s one of the reasons an annual service matters even when nothing seems obviously wrong, since a technician can spot early scale build-up before it causes a breakdown.
Frequently Asked Questions
Is no hot water an emergency? Not usually a gas safety emergency, since the heating still working shows the boiler itself is running safely. It’s an inconvenience rather than a danger, but it’s still worth booking a repair promptly rather than living with cold taps.
Can a boiler service prevent this fault? Often, yes. A service can catch early signs of a sticking diverter valve or limescale build-up before it fails completely, particularly in Hampshire’s harder water areas.
Will I need a whole new boiler if the diverter valve has failed? Rarely. A diverter valve is a replaceable part, not the whole appliance, so most of these faults are repaired rather than requiring a full boiler replacement.
Why does my boiler make a different noise when there’s no hot water? A stuck valve or airlock can sometimes cause a faint knocking or humming as the boiler tries to divert flow. It’s worth mentioning to your engineer, since it can help pinpoint exactly where the fault sits.
Still no hot water after checking the pressure, timer and mode dial? Our service and repairs team covers diverter valve faults, airlocks and cylinder issues across Southampton, Winchester and the surrounding areas. If this keeps happening, it’s worth reading up on our central heating page to understand how your system’s components work together, or comparing boiler types if yours is getting on in years and repeat faults are becoming a pattern. For homeowners who’d rather not think about it again, our care plans build an annual check into the cost, which is exactly the kind of visit that catches a sticking valve early. You can book an appointment directly when you’re ready.
For more on why Hampshire’s water causes extra wear on heating components, Southern Water’s hard water checker lets you look up hardness by postcode.