Hot water but no heating: why it happens and what to check

Start with the type of boiler you have

The same symptom has different likely causes depending on how your hot water is made. A combi boiler heats water on demand when a tap opens and has no hot water cylinder. A system or regular boiler heats a stored cylinder, usually in an airing cupboard, and splits the heat between the cylinder and the radiators with one or more valves.

If there is a cylinder in the property and the tap water is hot, the boiler is clearly firing and sending heat somewhere. The question becomes why the heating circuit is not taking its share. If you have a combi, hot taps working proves the burner and gas supply are fine, which points attention towards the controls, the pressure and the parts that switch the boiler between hot water and heating.

Checks you can safely make yourself

Before booking anyone, spend five minutes on the controls. Many call-outs for this symptom turn out to be a room thermostat set below the current room temperature, a programmer with the heating channel switched off or on the wrong schedule after a clock change, or a wireless thermostat with flat batteries that has lost contact with its receiver.

None of these checks involve opening the boiler casing. Anything behind the case, including the gas valve, burner, fan and internal electrics, is work for a Gas Safe registered engineer.

• Turn the room thermostat up a few degrees above the current temperature and listen for the boiler and pump starting.
• Check the programmer or smart app: the heating channel must be on or set to a timed period that includes now.
• Replace batteries in a wireless thermostat and check its link light or display.
• Look at the pressure gauge on a sealed system. Most read roughly 1 to 1.5 bar cold; check your boiler manual for its figure.
• Check that radiator valves are open, including any TRV heads turned right down.
• Look at the boiler display for a fault code and write it down exactly.

When low pressure is the reason

Some combi boilers will still run hot water at a lower pressure than they need to run the heating, or will lock out the heating first. If the gauge is below the manufacturer's minimum, the heating may not start at all. Topping the pressure back up using the filling loop, following the boiler manual, can restore heating.

The important part is what happens next. Pressure that falls again within days means water is leaving the system, through a leaking radiator valve, a weeping joint under a floor, the pressure relief valve or a failed expansion vessel. Repeatedly refilling a leaking system introduces fresh oxygenated water and can speed up internal corrosion. If you are topping up more than occasionally, the leak needs to be found, not managed.

If you smell gas at any point, turn off the gas at the meter if it is safe to do so, open windows, do not operate electrical switches and call the National Gas Emergency Service on 0800 111 999.

Diverter valves on combi boilers

A combi boiler uses a diverter valve to send heat either to the domestic hot water heat exchanger or around the radiators. When a tap opens, the valve switches to hot water. When the tap closes, it should return to heating. If the valve sticks in the hot water position, you get exactly this symptom: perfectly good hot taps and radiators that stay cold.

A tell-tale sign is radiators warming slightly while a hot tap is running, or the heating only working after the boiler has been reset. Diverter valve parts sit inside the boiler, so replacement is Gas Safe work and the part must match the boiler model. An engineer should confirm the valve is the fault rather than replacing it on suspicion, because a pump fault or controls fault can look very similar.

Zone valves on systems with a cylinder

On a system with a cylinder, a two-port motorised valve usually controls the heating circuit and another controls the cylinder. Older layouts may use a single three-port mid-position valve instead. When the heating valve's motor or actuator fails, the valve stays closed, so the boiler heats the cylinder happily but no water flows to the radiators.

Motorised valves have a small manual lever on the actuator head. Some people push it across to force the valve open as a stopgap, but that does not fix the motor and it can leave the heating running uncontrolled. The valve also switches the boiler and pump through a microswitch, so a fault can be mechanical, electrical or both. Our guide to zone valve faults explains the difference in more detail.

Pump faults and seized circulation

The central heating pump pushes water round the radiators. On many combis it is inside the boiler; on system and regular boilers it is often a separate unit near the cylinder or boiler. A pump that has seized after a summer of little use, or failed electrically, can stop the radiators heating while hot water still works on some layouts.

Signs include a boiler that fires briefly then shuts down, pipes near the boiler getting very hot while radiator pipes stay cold, or a pump that is warm but silent. Sludge and magnetite in older systems make pump failures more likely. If sludge is part of the picture, replacing the pump alone may not be enough; the system may also need cleaning and a magnetic filter.

What to tell us when you call

A clear description saves time on site and helps us bring the right parts. Tell us the boiler make and model if you can see it, whether you have a cylinder, what the pressure gauge reads, any fault code on the display and whether any radiators are warm at all. Mention whether the problem started after a power cut, a thermostat change, a boiler service or other building work.

Loyal Plumbers has 10 vans covering London and everywhere inside the M25, 24 hours a day. Gas work is carried out by Gas Safe registered engineers. Call us or send an enquiry with the details above and we will diagnose the fault before recommending a repair, so you are not paying for parts that were never the problem. With no heating in cold weather, tell us if anyone vulnerable is in the property so we can prioritise the visit.