No Hot Water But the Heating Works? Here's Why
Six causes we find most often in Barnet homes — from diverter valves on combis to failed immersion heaters and scaled cylinders.
Hot water gone but the radiators still warm up? That combination narrows the fault down considerably, because it tells us the boiler is firing and the problem sits in the hot water side of the system. Here's how we work through it on a call-out in Barnet.
1. The diverter valve (combi boilers)
By far the most common. A combi diverts flow between heating and hot water; when the valve sticks in the heating position you get warm radiators and cold taps. Sometimes it's just the cartridge or a motor head, not the whole valve. This is a boiler repair job for a Gas Safe engineer.
2. Timer and programmer settings
Worth thirty seconds before anything else. Hot water schedules get knocked off after a power cut, a battery change or a new smart thermostat. Check the hot water is set to "on" or "constant", not just the heating.
3. Cylinder thermostat or motorised valve (system boilers)
On a vented or unvented cylinder setup, a failed cylinder thermostat or a seized two-port motorised valve stops the cylinder calling for heat while the heating circuit carries on as normal. Both are affordable parts.
4. Immersion heater failure
If you rely on an immersion — common in flats around N12 and Enfield — the element or its thermostat burns out, often accelerated by hard-water scale. Replacement is straightforward but the cylinder must be drained down.
5. Scaled plate heat exchanger
Where hot water is lukewarm rather than absent, and gets worse the longer you run the tap, scale in the combi's plate heat exchanger is the usual culprit. See our piece on hard water in North London for why this happens locally.
6. Airlock in the hot water supply
After a mains shutdown or cylinder work, air can lock the hot side. Sputtering, banging taps and no flow from the hot outlet only. Usually cleared quickly by a plumber cross-connecting the hot and cold at a mixer tap.
Getting it fixed
We carry common diverter cartridges, cylinder stats and immersion elements, so most no-hot-water calls across North London are resolved in one visit. Related: hot water cylinders, emergency boiler repairs.