Hot Water but No Central Heating

What working hot water already rules out

Treat the hot water as a clue, not a consolation. It proves the boiler has power, a gas supply, a working fan and a burner that lights. On a combi it usually proves the pressure is high enough too, because most combis refuse to run anything at low pressure. That removes most of the expensive causes before you start.

What is left sits in two places. Either nothing is asking the boiler for heating, or the boiler is trying to heat the radiators and the heat is not getting there. The quickest way to tell them apart is to turn the room thermostat right up and listen. If the boiler fires and the pipes leaving it get hot but the radiators stay cold, the heat is going nowhere. If the boiler stays silent, the call for heat is not reaching it.

Is anything actually asking for heat?

When the boiler stays silent, the fault is usually somewhere in the controls. Work through the chain from the wall to the boiler.

• Room thermostat: turn it at least three degrees above the room temperature. On a wireless thermostat, change the batteries and check the receiver box near the boiler still shows a link or signal light.
• Programmer or timer: check the heating channel (often marked CH) is on, not just the hot water channel. After a power cut, many programmers lose the clock or drop to off.
• Smart thermostats: check the app is not in away, holiday or frost mode, and that the hub still has power and Wi-Fi.
• Boiler heating setting: many boilers let you switch heating off while keeping hot water. On a Vaillant ecoTEC plus, the heating flow temperature can be set to Heating off from the boiler menu, which is exactly this symptom. On other makes, the heating temperature knob often has an off or summer position.
• Radiator valves: if only some radiators are cold, check the thermostatic valves are open and not stuck. See one radiator not working.

Leave the wiring alone. Thermostats, receivers and programmers run at or switch mains voltage, and the terminal strip behind the boiler is part of the appliance.

Combi boilers: the diverter stuck on hot water

Inside a combi, a diverter valve sends heat either to the radiators or to the hot water plate heat exchanger. Hot water always takes priority. If the valve seizes in its hot water position, or its motor or spring fails, the boiler fires quite happily for heating but the heat goes straight round the hot water side and back again. The radiators stay cold, and the boiler may short cycle and switch itself off because the water inside it heats up too fast.

A good sign of this is a boiler that clearly fires when the thermostat calls and warms up quickly, while the heating flow pipe to the radiators stays cool. On some models only the motor head fails and it can be changed without draining; on others the whole valve body has to come out. In the wider market, full diverter valve jobs usually land somewhere around £200 to £450.

Less often, a combi with sludge in the heating circuit will heat water but not radiators, because the radiator side is too restricted. A magnetic filter packed with black sludge under the boiler is the giveaway.

System and regular boilers: the zone valve or pump

If you have a hot water cylinder, the heating and hot water are separated by motorised valves outside the boiler, usually in the airing cupboard near the cylinder.

With an S-plan system there are two two-port valves, one for heating and one for hot water. If the heating valve has failed shut, hot water carries on perfectly and the radiators stay cold. With a Y-plan system, one three-port valve does both jobs, and a failed actuator can leave it parked on hot water. In both cases the usual failure is the powerhead, the motor box on top, not the valve body. A replacement head for a common Honeywell valve is an inexpensive part and can be swapped without draining the system.

On older regular boilers with gravity hot water and pumped heating, the pump is the first suspect. The cylinder heats by convection with no pump at all, so a dead pump leaves you with hot water and no heating. A pump that has seized may hum or feel very hot, or be completely silent. Market prices for a replacement pump fitted are commonly in the £200 to £400 range, and London labour tends to sit at the top of it.

Air is the other cause. If the radiators upstairs are cold and gurgle, and the system has recently been drained or topped up, an airlock in the heating circuit can stop the flow even though the pump runs. Bleeding radiators is a safe job for you to do, but on a sealed system check the boiler pressure afterwards, because bleeding lowers it. Our guide to radiators not heating up sets out the order.

Vaillant boilers with hot water but no heating

On a Vaillant the cause is often a setting in the boiler menu rather than a failed part. Before calling anyone, check the heating flow temperature on the display. If it shows Heating off, or a very low figure, set it back to a normal radiator temperature and turn the thermostat up.

Next, check the Vaillant controls. A sensoCOMFORT or vSMART that has lost its connection to the boiler will stop the heating call while the boiler still answers a hot tap. If there is a fault code, note it. An F75 relates to the pump and pressure sensor check when the pump starts, and points at the heating circuit rather than the controls. For anything beyond settings and controls, a Vaillant boiler repair visit covers the parts inside the casing.

When to call, and what happens on the visit

Hot water with no heating is not dangerous, but in cold weather it becomes urgent quickly for babies, older people and anyone unwell. Get an engineer out the same day if the controls are clearly calling and the radiators stay cold, if the boiler short cycles or shows a code, or if you can hear a pump buzzing without moving water.

The engineer confirms the call for heat at the boiler terminals, checks which way the diverter or zone valve is sending water, feels the flow and return pipes, and tests the pump. That order means you pay for the part that failed rather than a guess. Our central heating repair visit starts with the £60 call-out at any hour, and the repair price is agreed with you on site before any work starts. Gas work is carried out by Gas Safe registered engineers.