First, work out which system you have
The whole diagnosis hinges on one question: combi or cylinder. A combi boiler is a single unit on the wall, usually in a kitchen or airing cupboard, with no separate hot water tank anywhere in the house. It makes hot water on demand the moment you open a tap. A cylinder system has a large insulated hot water tank, often in an airing cupboard or loft space, where hot water is stored ready for use.
If you are not sure, the giveaway is the tank. No tank means a combi. A big lagged cylinder means a stored hot water system. The cause of your cold taps lives in a different place depending on which you have, so it is worth being certain before reading on.
Combi: the diverter valve is the usual suspect
In a combi, a single component decides whether the boiler sends its heat to the radiators or to your hot taps: the diverter valve. When you turn on a hot tap, the valve is supposed to switch the boiler over to heating water for that tap. When it wears or sticks, that switch stops working properly, and the classic symptom is exactly what brought you here: full heating, but lukewarm or cold water at the taps. Some worn diverter valves give you hot water only when the heating happens to be running, which is the same fault showing a different face.
There is no safe DIY fix for a diverter valve, because it sits behind the boiler casing, which is Gas Safe registered territory. But it is a common, well-understood repair, and naming the symptom precisely helps: tell the engineer that the heating is fine and only the hot water has gone, and he often arrives knowing the likely part and carrying it in the van.
Combi: the few things worth checking yourself first
Before assuming the worst, rule out the simple causes. Check the boiler pressure: if the gauge is below about 1 bar the boiler may refuse to heat water at all, and topping it up to around 1.2 bar is normal homeowner territory, covered in the pressure guide. Check the display for an error code and note it down exactly, because that code often points straight at the cause. And confirm it really is every hot tap that is cold, not just one, because a single cold tap is a tap or a local problem, not a boiler one.
If the pressure is fine, there is no helpful error code, and every hot tap in the house is cold while the radiators are warm, the diverter valve becomes the likeliest answer and the next call is an engineer.
Cylinder systems: valves, the timer, and the immersion
A stored hot water system has more moving parts, so it has a few more suspects. The most common is a motorised valve, the component that opens to let hot water circulate to the cylinder. When it sticks shut, the radiators can still get their heat while the cylinder is left cold, which produces the exact split you are seeing. Like the combi diverter, that is an engineer job.
Two homeowner checks are worth doing first, though. The hot water and the heating on a cylinder system are usually on separate timer or programmer settings, so it is genuinely common for the hot water schedule to have been switched off or knocked out of sync while the heating stayed on. Check the programmer has hot water set to come on. And if your cylinder has an immersion heater as a backup, an electric element with its own switch, switching it on will tell you whether you can at least get hot water that way while the main fault is sorted.
When to stop and call a Gas Safe engineer
Call when the simple checks have not fixed it: pressure is fine, the timer is set correctly, and you still have warm radiators and cold taps. On a combi that points at the diverter valve, on a cylinder system at a motorised valve or related control, and both sit behind a casing or inside the system where the law says only a Gas Safe registered engineer should work. Do not keep resetting the boiler into the same state and do not open the case.
If you are in Bridgend county or the surrounding patch, the fastest route to an answer is to message Lloyd on WhatsApp saying the heating works but the hot water has gone, and whether you have a combi or a cylinder, or call 07712 646488. He answers his own phone in working hours and prices the visit before he sets off. If it turns out the boiler is genuinely on its way out rather than just needing a valve, you get the same honesty there, with a fixed replacement price from £1,900 on the table so you can weigh repair against replacement properly. The broader dead-boiler checklist, for when more than the hot water has stopped, is in the boiler-not-working guide.
