Why the hot water works but the heating does not
Hot water from the taps proves the boiler has gas, power and a working burner. That rules out a lot of expensive faults straight away. What is missing is the signal or the route that sends heat to the radiators.
On a combi boiler, a diverter valve inside the boiler switches hot water between the radiators and the taps. If it sticks in the hot water position, you get hot taps and cold radiators. On a system or regular boiler with a hot water cylinder, separate motorised zone valves open the heating and hot water circuits. If the heating valve fails to open, the cylinder still heats but the radiators do not.
This is the reverse of the problem covered in our guide to no hot water when the heating works, and several of the same parts are involved.
Checks to do before calling an engineer
- Turn the room thermostat up. Set it at least 3 or 4 degrees above the current room temperature. If the boiler fires within a minute or two, the thermostat was simply satisfied.
- Check the timer or programmer. Make sure the heating channel (often labelled CH) is set to on, auto within a timed period, or boost. After a power cut, many timers lose the time or reset to off.
- Check the boiler's own settings. Many combis have a winter and summer mode. In summer mode the boiler only provides hot water. Also check the heating temperature dial or setting is not turned right down.
- Check a wireless or smart thermostat. Replace the batteries and look at the receiver near the boiler for a fault or signal light. Our guide to a boiler not responding to the thermostat goes through this in detail.
- Check the pressure gauge. Some boilers will still give hot water with pressure on the low side but struggle with the heating. With the system cold, it should read 1 to 1.5 bar. If it is lower, follow our guide to repressurising a boiler. A low pressure code such as Ideal F1 or Baxi E119 confirms it.
- Look at the radiator valves. If only some radiators are cold, the fault is more likely at those radiators than at the boiler. See our guide to one radiator not heating.
Listen at the boiler when you turn the thermostat up. A click, then the fan and burner starting, means the call for heat is getting through. Silence usually points to the controls.
If the boiler fires when you turn the thermostat up but the radiators stay cold after 15 minutes or so, the call for heat is working and the problem lies further along, in the diverter valve, a zone valve or the pump. Carefully feel the two pipes under the boiler, as they can get very hot. If they get hot while the radiators stay cold, the heat is going somewhere other than the heating circuit.
The common causes in more detail
Room thermostat
A thermostat with flat batteries, a failed internal switch or a location next to a radiator, lamp or sunny window may never call for heat. Wired thermostats can also fail. If turning it right up does nothing, the thermostat or its wiring is the prime suspect.
Timer or programmer
Mechanical timers with push-in tappets can have a pin stuck in the off position, and digital programmers can lose their settings. Check the clock shows the right time and day, including after the clocks change.
Diverter valve (combi boilers)
The diverter valve has a motor or actuator on top of a valve body. When the motor fails or the valve seizes in the hot water position, the radiators get nothing. Sometimes the radiators warm slightly while a hot tap runs, which also points to a sticking diverter valve.
Zone valve (system and regular boilers)
Homes with a cylinder usually have a 2-port valve for the heating on an S-plan system, or a single 3-port mid-position valve on a Y-plan. Honeywell and Drayton valves are common. If the heating valve motor burns out, the valve stays shut and the boiler may not even be told to fire for heating.
Pump
On a combi, the same pump usually serves both heating and hot water, so a dead pump normally stops both. A weak or sticking pump can still manage the short route to the hot water heat exchanger but struggle to push water round every radiator. On older regular boilers with gravity-fed hot water, a failed pump can leave the cylinder heating while the radiators stay cold.
Symptoms and likely causes
| What you notice | Likely cause | Who fixes it |
|---|---|---|
| Boiler never fires for heating, fine for hot water | Thermostat, timer or summer mode | You can check settings |
| Radiators warm slightly when a hot tap runs | Diverter valve sticking | Gas Safe engineer |
| Cylinder hot, radiators cold, boiler silent on heating | Heating zone valve motor | Heating engineer |
| Boiler fires, radiators near the boiler warm, others cold | Weak pump, sludge or unbalanced system | Heating engineer |
| Fault code on the display | Depends on the code | Look it up, then engineer if needed |
Typical repair costs in the UK
These ranges include parts and labour. Prices are higher in London and the South East, for premium brands and for parts that are awkward to reach or need the system draining down.
| Repair | Typical UK cost |
|---|---|
| Diagnostic call-out | Typically £60 to £120 |
| Replacement wired room thermostat | Typically £100 to £250 |
| Zone valve motor head only | Typically £100 to £200 |
| Full zone valve (system drained) | Typically £200 to £400 |
| Diverter valve motor or full valve | Typically £150 to £350 |
| Circulating pump | Typically £200 to £400 |
A zone valve head can often be swapped without draining the system, which keeps the cost at the lower end. A full valve body means draining down, refilling and adding inhibitor, which takes longer.
When to call a Gas Safe registered engineer
If the thermostat and timer are calling for heat, the boiler is in winter mode and the pressure is correct, the fault is with a part you should not touch. Diverter valves and internal pumps sit inside the boiler casing, and only a Gas Safe registered engineer should work on them. Zone valves sit outside the boiler, often in the airing cupboard, but they are wired at mains voltage and replacing a valve body means draining the system, so leave them to a heating engineer too.
Do not remove the boiler casing to look at the diverter valve or pump. If you smell gas at any point, turn off the gas at the meter if you can and call the National Gas Emergency Service on 0800 111 999.
When you book, say what type of boiler you have, whether you have a hot water cylinder, whether the boiler fires when you turn the thermostat up and any fault code on the display. That helps the engineer bring the right part first time.