Boiler Working but No Hot Water

On a combi boiler, a single heat source serves both your radiators and your hot water. A component called the diverter valve directs heat to whichever is needed. If the heating works but hot water doesn't, the boiler is still running — it's just failing to send heat to the hot-water side. The fault is nearly always mechanical (a valve, diaphragm, or microswitch) or a blocked plate heat exchanger, not the gas burner itself.

The most common causes, in order of how often we see them on call-outs:

• • Stuck or failed diverter valve — the part that switches between heating and hot water seizes or breaks.
• • Perforated diaphragm or cartridge — on older combis, a rubber diaphragm pushes the diverter; if it splits, it stops actuating.
• • Blocked plate heat exchanger — limescale or sludge narrows the domestic hot-water channels so little or no heat transfers.
• • Faulty flow sensor or microswitch — the boiler doesn't detect you've opened a tap, so it never fires for hot water.
• • Airlock in the hot-water pipework — common after a cylinder drain-down or recent plumbing work.
• • Y-plan / mid-position valve stuck (on systems with a hot-water cylinder) — the motorised valve stays in heating position.

A reset won't fix a mechanical diverter valve or a blocked heat exchanger, but it's worth one attempt if the boiler is showing a fault code. Press the reset button once, wait for the boiler to re-fire, then run a hot tap. If hot water returns but disappears again, the diverter is sticking intermittently — it still needs replacing. Don't reset repeatedly; if the fault returns, stop and book an engineer.

• • Run another hot tap (bath or kitchen). If that's also cold, the fault is system-wide, not the tap.
• • Check the boiler pressure — low pressure (below 0.6 bar) can stop hot-water production on some combis. Top up to 1.2 bar if needed.
• • Read the boiler display for a fault code. A code narrows the diagnosis and speeds up the repair.
• • If you have a hot-water cylinder, check the cylinder thermostat and the timer/programmer are set to 'on' for hot water.
• • If you've had recent plumbing work, an airlock may be trapped — but don't try to force it with mains pressure yourself; mention it to the engineer.

The diverter valve, diaphragm, and plate heat exchanger sit inside the boiler casing. Opening the casing, disconnecting water connections inside the boiler, or disturbing gas-side components is gas work and must be done by a Gas Safe Registered engineer. Even the plate heat exchanger — which looks like a simple plumbing part — requires the boiler to be correctly re-commissioned afterwards.

• • Confirm the boiler fires for heating but not hot water (or vice versa) to isolate the diverter circuit.
• • Test the diverter valve actuator and microswitch for correct operation.
• • Check the diaphragm or cartridge for splits and movement.
• • Measure flow rate and temperature rise across the plate heat exchanger for blockage.
• • Test the flow sensor / turbine and its signal to the PCB.
• • On cylinder systems, test the mid-position valve, cylinder stat, and programmer hot-water channel.
• • Re-commission the boiler and verify safe combustion after any internal work.

Our Gas Safe Registered (594547) engineers carry common diverter valves, diaphragms, and cartridges for Vaillant, Worcester Bosch, Ideal, and Baxi on the van, so most no-hot-water repairs are completed in a single visit across Surrey, Berkshire, Buckinghamshire, and West London. Book online in 60 seconds or call us — we'll diagnose, quote the part, and only fit it once you've agreed the price.