Hot water but no heating: what it usually means

Hot water from the taps while every radiator stays cold is usually a controls or valve fault, not a boiler breakdown, and it is worth five minutes of checking before anyone is booked out. On a combi boiler the most common cause is a stuck diverter valve, the part that switches the boiler's output between the taps and the heating circuit. On a system with a hot water cylinder, a faulty motorised zone valve does the same job and fails in a similar way.

How this is different from a breakdown

Our sheet on the first hour of a boiler breakdown covers the boiler that has stopped doing both jobs, no hot water and no heating together, which is a different fault with a different checklist. Hot water but no heating means the boiler is clearly firing and working, since it is heating water successfully; the fault sits in how that heat is being routed, not in whether the boiler runs at all. Keep that distinction in mind before you call anyone, because it changes what the engineer needs to bring.

The checks worth making first

1. The room thermostat and timer. Confirm the heating is actually calling for warmth: thermostat set above room temperature, timer or programmer on and on the right programme, and batteries fresh if it is a wireless model. A heating demand that never reaches the boiler looks identical to a part that has failed.
2. The thermostatic radiator valves. If every TRV in the house has been turned down or a smart TRV has lost its schedule, the system can read as "no heating" even though the boiler would respond correctly if asked.
3. The boiler's own mode switch or display. Many combi boilers have a separate setting for hot water only, heating only, or both; a switch left on the wrong setting after a reset or a power cut produces exactly this symptom and costs nothing to check.
4. The system pressure. Our sheet on reading the pressure dial explains the correct cold pressure for a sealed system; a pressure problem is worth ruling out before blaming a valve.

The diverter valve, in plain terms

Inside a combi boiler, the diverter valve is a small moving part that redirects the heated water either to the hot taps or into the central heating pipework, switching every time a tap or the heating calls for flow. It is mechanical, it moves often, and over years it can stick, especially if limescale or sludge from the heating system reaches it. A diverter that sticks in the hot water position gives exactly the pattern in this fault: taps work, radiators do not.

Zone valves, for cylinder-based systems

Houses with a hot water cylinder rather than a combi often use a two-port or three-port zone valve to send heat either to the cylinder or to the radiators, controlled by a small motor on top of the valve. A failed motor or a valve stuck in one position produces the same symptom from a different part. Our sheet on general maintenance for your water heater explains which of the three UK hot water systems a given house has, which is the first thing to establish before assuming either part is at fault.

What to tell the engineer before they arrive

Note which checks above you have already made, whether the fault is constant or comes and goes, and whether it followed a power cut, a boiler reset, or nothing you noticed. A diverter or zone valve replacement is routine work for a Gas Safe registered engineer on a gas-fired system, and knowing the symptom pattern in advance usually means the right part is already in the van rather than a second visit. The Gas Safe Register remains the only legal check worth making before anyone opens a gas appliance, exactly as for any other boiler work.

When it is more than the valve

If the radiators are warm at the bottom and cold at the top only, that is air in the system, covered in our sheet on bleeding radiators in the right order, not a valve fault, and bleeding should be tried first since it costs nothing. If the boiler has also started losing pressure gradually alongside the heating fault, read the pressure sheet before assuming the two faults are unrelated, since a leak feeding both symptoms is possible on an older system.

What this sheet does not cover

This sheet does not cover system boilers with a separate external pump and valves beyond the basic zone valve pattern above, oil-fired systems, which are an OFTEC matter, or specific fault codes, which differ by manufacturer and are covered in each boiler's own handbook. For a confirmed stuck valve or any work inside the boiler casing, book a Gas Safe registered engineer rather than continuing to force a reset.