Radiators hot, taps cold: why a combi stops making hot water
Updated 1 October 2026 · 2 minute read · From our Gas Safe heating engineers
The radiators are warm, the boiler is clearly working, but the hot tap runs cold. It feels like the boiler is on its way out. Usually it is not.
The diverter valve
A combi boiler heats either the radiators or your hot water, not both at once. The part that decides which is the diverter valve. When you open a hot tap, it switches the heat over to the taps. When it sticks, you get one but not the other.
That makes it a common repair, not a reason for a new boiler.
Check these first
Before you ring a heating engineer, take two minutes to rule out the simple things.
- Is it every tap? If only one tap or the shower is cold, the problem is at that tap or the shower valve, not the boiler.
- The pressure gauge. Low pressure can stop some boilers firing. If it has dropped below about 1 bar, top it up and try again.
- A fault code. If there is one on the display, write it down before you reset anything. Codes narrow the fault quickly and often mean we bring the right part first time.
- The settings. A knocked dial or a changed timer catches more people out than you would think.
The opposite problem: hot water but no heating
If the taps are fine but the radiators stay cold, the diverter valve can be stuck the other way. It can also be the thermostat, the timer, a pump that has stopped, or a system so full of sludge the flow has given up. Tell us what the boiler does when the heating is called for and we will narrow it down on the phone.
System boilers with a cylinder
If you have a hot water cylinder in a cupboard rather than a combi, the hot water side works differently. The fault is more likely a motorised valve or the cylinder thermostat. We work on cylinders as well as boilers, including unvented cylinders.
What happens on the visit
We find the actual fault and show it to you, price the repair before doing anything, and test the boiler properly before we leave. The faults we see most often on the common makes are usually sorted on the first visit; where a part has to be ordered, you get a real timeframe, not a vague promise to chase.
Keep reading
- Cold radiators: how a heating engineer tells bleed, balance and flush apart Where a radiator is cold tells you the fix. Cold at the top, cold at the bottom, or whole rooms never warm: three symptoms, three different jobs, and only one of them needs a powerflush.
- Boiler pressure keeps dropping: how often is too often? Topping up once or twice a year is normal. Weekly is a fault. How to read your pressure gauge, the two usual causes, and why topping up every week costs more than the repair.
- When a powerflush is worth paying for, and when it is not A powerflush is the right answer for a genuinely dirty system and a waste of money for everything else. The signs that say flush, the ones that do not, and what has to happen afterwards.