Hot Water but No Heating: Causes and Fixes

Updated July 2026 · Emergency Plumber Chelsea

The reason hot water can work while heating fails is that most boilers treat the two as separate jobs. On a combi boiler, the unit heats water on demand for the taps through one path and sends water to the radiators through another, and a component called the diverter valve decides which path is open. When that valve, the pump or the heating controls fail, you keep instant hot water but the radiators get nothing. On a system or heat-only boiler with a cylinder, the hot water and heating are driven by separate motorised valves and controls, so one circuit can fail while the other carries on. Knowing which type you have tells you where to look first.

Start with the controls, because a surprising number of no-heating calls are simply a setting. Check the programmer or timer is actually calling for heat: confirm the heating channel is set to on or to a timed period that is running now, not just the hot water channel. Check the room thermostat is turned up above the current room temperature, since a thermostat set to 15C on a mild day will never fire the boiler. If you have a smart thermostat, confirm it has power, is connected, and has not dropped its schedule after a firmware update or a flat battery in the wall unit. Replace thermostat batteries if the display is blank or faint.

Check the boiler pressure next, because low pressure is one of the most common reasons heating stops while hot water continues. Look at the pressure gauge on the front of the boiler: cold, it should usually sit around 1 to 1.5 bar. If the needle is near zero or in the red, the boiler may lock out the heating to protect itself. You can often top up the pressure yourself using the filling loop, a small silver braided hose under the boiler, opening it slowly until the gauge reaches about 1.2 bar and then closing it fully. If the pressure drops again within hours or days, do not keep topping it up, because that points to a leak somewhere on the system that needs finding and fixing.

Confirm the boiler has power and note any fault code before you go further. Check the boiler is switched on at its own fused spur and that the display is lit, since a tripped switch or a blown fuse can stop the heating call getting through. If the boiler shows a fault or lockout code, write it down exactly as displayed, then reset the boiler once using its reset button and watch what happens. A boiler that fires and runs but leaves radiators cold points to the heating circuit itself, such as the pump, diverter or valves. A boiler that locks out again straight after a reset has an underlying fault that should not be repeatedly reset, because that can cause further damage. Either way the code gives the engineer a head start.

A stuck diverter valve is the classic combi fault behind hot water but no heating. The diverter is designed to send heat to the radiators when you have not asked for hot water, but over time it can seize in the hot water position, so every time the boiler fires it heats the taps and ignores the radiators. There is no safe DIY fix for this, since it means working inside a sealed gas appliance, but it is a well understood repair. A Gas Safe engineer can confirm the diagnosis, free or replace the valve, and have heating back the same visit in most cases.

A failed or airlocked pump will also leave radiators cold while hot water still works. The circulating pump pushes heated water around the radiator circuit, and if it fails electrically, seizes after a summer of standing still, or traps an airlock, the water simply does not move even though the boiler is firing. A common clue is a boiler that sounds like it is running normally while every radiator stays stone cold. Some pumps have a small bleed screw or can be freed by an engineer, but pump diagnosis and replacement is not a homeowner job on a live heating system.

Air in the system is a frequent and often fixable cause, especially at the start of the heating season. If most radiators warm up but one or two upstairs stay cold at the top, that is trapped air rather than a whole-system fault, and bleeding those radiators usually restores them. If the whole system will not circulate, an airlock near the pump may be the culprit and needs professional attention. After bleeding several radiators the boiler pressure often drops, so check the gauge afterwards and top up to about 1.2 bar if needed.

Seized or closed valves can quietly cut heating off too. Each radiator has valves at both ends, and if a lockshield or thermostatic radiator valve has been turned fully down or has seized shut, that radiator stays cold. More seriously, on a system with a cylinder, a failed motorised or zone valve can stop the whole heating circuit while leaving the hot water zone working. A thermostatic valve head that has stuck can sometimes be freed by pulling off the head and gently pressing the pin underneath, but a seized body or a failed motorised valve is an engineer repair.

On a boiler with a hot water cylinder rather than a combi, the split between working hot water and dead heating usually sits in the controls or valves. The cylinder can be heating happily from its own thermostat and motorised valve while the heating zone valve, the room thermostat, or the wiring centre that ties them together has failed. In many Chelsea mansion blocks the plant can be older and shared, and a fault on one flat's heating zone can leave that flat cold while the building's hot water is unaffected. This is diagnostic work best left to an engineer who can test the valves and controls in order.

Frozen condensate pipes cause sudden winter breakdowns, though these usually stop both heating and hot water rather than heating alone. A modern boiler drains acidic condensate through a plastic pipe, and if that pipe runs outside and freezes in a cold snap, the boiler locks out. If your boiler has stopped in freezing weather and shows a fault or gurgling near the flue, a frozen condensate is likely. Thawing it gently with warm, not boiling, water poured along an accessible external run can sometimes restart the boiler, but a pipe that freezes every winter should be re-routed or insulated properly.

Call a Gas Safe engineer when the safe checks above do not bring the heating back, or straight away if anything points to gas or a leak. Booking a professional makes sense once you have confirmed the controls are calling for heat, the pressure is correct, and bleeding has not helped, because the remaining causes such as diverter valves, pumps, motorised valves and control faults all need someone qualified working inside the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency Service on 0800 111 999 before anything else. For a heating fault that will not clear, Emergency Plumber Chelsea sends Gas Safe registered engineers across SW3, SW10 and nearby postcodes, diagnoses the cause and quotes before major work. Boiler fault diagnosis starts from around £89, confirmed on site, with repair costed once the fault is known. Call 020 3002 6667.

Quick tips

• Check the programmer and room thermostat first: heating channel on, thermostat turned up above room temperature, and batteries fresh in a wireless stat.
• Read the boiler pressure gauge cold. Around 1 to 1.5 bar is normal; near zero can stop heating while hot water still works.
• If pressure keeps dropping after you top it up, stop topping up and get it checked, because that usually means a leak on the system.
• One or two cold radiators upstairs points to trapped air you can bleed; a whole system that will not warm points to a pump, valve or diverter fault for an engineer.
• Never open a sealed gas appliance yourself. Diverter valves, pumps and zone valves are Gas Safe engineer work.

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