No hot water but the heating works? Here's why
If your radiators get warm but the taps run cold, the most common cause on a combi boiler is a failed or stuck diverter valve. It is the part that decides whether hot water goes to your radiators or your taps. When it sticks towards heating, the central heating works but no hot water reaches the taps. It needs a Gas Safe engineer, but there are a couple of safe checks worth doing first.
That is the answer. Here is what is going on and what you can and cannot do yourself.
What the diverter valve does
A combi boiler has one heat exchanger doing two jobs: heating your radiators and heating water for the taps. The diverter valve switches the flow between the two. When you open a hot tap, it should send heat to the water; when you stop, it returns to heating. Over time it can stick or wear, and if it sticks on the heating side you get warm radiators and cold taps, exactly the symptom here.
Why it usually fails
Diverter valves wear with age and can get gummed up by sludge and debris in an older or poorly maintained system. It is one of the more common combi faults, which is why our engineers carry the common valves on the van, so most of these are fixed in a single visit.
The reverse problem: hot water but no heating
The same valve, or related controls, can cause the opposite: hot water at the taps but cold radiators. That can also be a thermostat, programmer or pump issue, or air and sludge in the radiators. The first check there is the same as below, then the thermostat and timer.
Safe checks before you call
There are a few things you can rule out without touching the boiler internals:
- Thermostat and timer. Make sure the hot water schedule (on a system boiler with a cylinder) is set and switched on. On a combi, hot water is on demand so this is less likely, but rule it out.
- Pressure. Check the pressure gauge is in its normal range, roughly 1 to 1.5 bar cold. Very low pressure can stop the boiler heating properly.
- One reset. If there is a reset button and a fault code, press it once. If it locks out again, stop and book a repair.
- Other hot taps. Confirm it is every hot tap, not one tap with a separate problem like a blocked aerator.
What you should not do is open the boiler casing or try to free the valve yourself. The diverter valve is inside the sealed boiler and is engineer-only work.
When it is urgent
No hot water is uncomfortable but not usually dangerous in itself. It becomes urgent if you also see a yellow rather than blue flame, smell gas, or see water leaking from the boiler. In those cases turn the boiler off, ventilate the room and treat it as an emergency. Our guide on 8 things to check when a boiler stops working covers the wider safety checks.
Getting it fixed
A diverter valve replacement is a routine job for a Gas Safe engineer and most are sorted in one visit because we stock the common parts. There is no separate call-out fee, it is £80 for the first hour plus any parts at trade price, quoted before we fit them.
If the safe checks have not restored your hot water, get a Gas Safe engineer out today and we will diagnose and, in most cases, fix it the same visit.