If your radiators are piping hot but nothing warm comes out of the taps, the boiler itself is usually fine. The fault almost always sits in the part of the system that switches between heating and hot water. Here is how to narrow it down before you pick up the phone.
The likely cause depends on your setup. A combi boiler heats water on demand with no cylinder, and most homes around Prescot and Merseyside built or refurbished in the last 20 years have one. If you have a hot water cylinder in an airing cupboard, you have a system or conventional boiler, and the diagnosis is different.
With a combi, heating and hot water share the same boiler but use an internal diverter valve to switch between them. With a cylinder setup, a motorised valve on the pipework near the cylinder does the same job, and the cylinder itself can also be the problem.
On combi boilers, a worn or stuck diverter valve is the single most frequent reason heating works while hot water does not. The valve can seize on the heating side, so the boiler fires happily for the radiators but never sends heat to the tap side. A telltale sign is water that runs briefly warm then goes cold, or hot water that only appears when the heating has been running.
This is not a DIY job because it means opening the boiler, which legally requires a Gas Safe registered engineer. Expect a replacement diverter valve to cost roughly 150 to 350 pounds fitted depending on the boiler make, though on older boilers it is worth weighing that against the age of the appliance. Sometimes only the rubber diaphragm inside needs replacing, which is cheaper.
Before calling anyone out, spend five minutes ruling out the simple stuff. Around a third of the calls we attend for this fault turn out to be settings or controls rather than a broken part.
On a system with a cylinder, a failed two-port or three-port motorised valve is the usual suspect. The heating circuit valve works, the hot water one does not, so the cylinder never gets charged. A faulty valve motor head can often be swapped without draining the system, typically 100 to 250 pounds fitted depending on access and the valve type.
One useful stopgap: most cylinders have an electric immersion heater as a backup. Switch it on at the wall and you should have hot water within an hour or so while you wait for a repair. It costs more to run than gas, so treat it as a temporary fix rather than a habit.
Hard water is not a big issue in most of Merseyside, as our supply is relatively soft, so limescale-clogged heat exchangers are less common here than in the south. If you have a combi and the hot water has been gradually getting cooler for months rather than failing suddenly, a partially blocked plate heat exchanger is still worth an engineer checking.
If the checks above have not fixed it, the fault is almost certainly internal to the boiler or a motorised valve, and both need a Gas Safe engineer. Anything involving the gas side or the sealed workings of the boiler is not something to attempt yourself, however many videos suggest otherwise.
For a diagnostic visit in the Prescot area you should expect a call-out and first hour somewhere in the region of 60 to 100 pounds, with parts on top if needed. A decent local firm will tell you honestly if a repair is not worth it on an ageing boiler rather than fitting parts to a dying appliance. If your boiler is over 12 to 15 years old and this is its second or third fault, ask for a repair versus replacement comparison before committing.
On a combi this usually points to a failing diverter valve or a worn diaphragm inside it, which stops the boiler holding on the hot water side. It needs a Gas Safe engineer, but it is normally a repairable fault rather than a new boiler.
No. Replacing it means opening the boiler casing, and work inside a gas boiler must legally be done by a Gas Safe registered engineer. You can safely check settings, timers and thermostats, but leave the internals alone.
Most diverter valve or motorised valve replacements take one to two hours once the part is in hand. If the part has to be ordered for your boiler model, allow an extra day or two, and use the immersion heater in the meantime if you have a cylinder.
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