Why a combi can heat radiators but not taps
A combi boiler does two jobs with one burner. It heats water for the radiators, and it heats cold mains water on demand for your taps and shower. It cannot do both at once, so a part called the diverter valve switches the hot water from the boiler between the heating circuit and a small plate heat exchanger that warms your tap water.
When you turn on a hot tap, a flow sensor detects the water moving and tells the boiler to switch to hot water mode. The diverter valve moves across, the burner fires and hot water should reach the tap within a few seconds. If any part of that chain fails, you can end up with working radiators and cold taps.
If the boiler is not firing at all and shows a code such as Ideal L2 or Vaillant F28, you have an ignition problem instead, and the heating will not be working either.
Quick checks you can do first
- Check the hot water settings. Many combis have a hot water on or off button, a separate hot water temperature dial, or an eco or pre-heat mode. Make sure hot water is switched on and the temperature is not turned right down.
- Try more than one tap. If one tap runs hot and another does not, the problem is at that tap or its pipework rather than the boiler.
- Open the tap fully. A combi needs a minimum flow of water before it fires for hot water. A tap only slightly open may not trigger the flow sensor. The minimum flow figure is in your manual.
- Check the cold flow. Run the cold kitchen tap. If the flow is weak, the problem may be low mains pressure rather than the boiler. Check the stopcock is fully open and ask your neighbours if they have noticed a drop.
- Look for a fault code and check the pressure. Note any code on the display. If the pressure is below 1 bar, follow our guide to repressurising a combi boiler, then reset as described in the manual. Stop after 2 or 3 reset attempts.
- Clean the tap aerator. The small mesh at the end of many taps can clog with limescale, cutting the flow. Unscrew it by hand, rinse it and refit it.
Common causes of no hot water on a combi
Diverter valve stuck or faulty
This is one of the most common causes. If the valve sticks in the heating position, hot water goes to the radiators instead of the taps. A classic sign is radiators warming up when you run a hot tap, even with the heating switched off. The fix may be a new valve motor (sometimes called the actuator) or a full valve cartridge.
Flow sensor not detecting water
If the flow sensor fails or gets clogged with debris, the boiler never knows you have opened a tap. The water stays cold and the boiler stays idle, but the heating works as normal because it does not rely on that sensor. On some models there is also a small filter on the cold inlet to the boiler, which can block with debris and have the same effect.
Low flow at the tap
A partly closed stopcock, a clogged aerator, a blocked filter on the cold inlet or low mains pressure in your area can all stop the boiler reaching its minimum flow rate. If several cold taps are weak too, the issue is with the water supply rather than the boiler.
Scale in the plate heat exchanger
In hard water areas, limescale builds up inside the plate heat exchanger. The hot water may be lukewarm, go hot and cold, or take much longer to arrive. Heavy scale can stop it working altogether, while the heating carries on.
Thermostat and temperature settings
A hot water temperature set very low, or an eco mode that delays hot water, can make it seem like there is no hot water at all. It costs nothing to check the settings before calling anyone out.
Symptoms and likely causes
| Symptom | Likely cause | Who fixes it |
|---|---|---|
| Radiators get warm when you run a hot tap | Diverter valve stuck in heating position | Engineer |
| Boiler does not fire at all when a tap opens | Flow sensor or low flow at the tap | You can check flow, engineer for the sensor |
| Hot water lukewarm or goes hot and cold | Scale in the plate heat exchanger, or temperature set low | Check settings, engineer for descaling |
| Only one tap runs cold | Problem at that tap or aerator | You or a plumber |
| All cold taps also weak | Low mains pressure or stopcock partly closed | You, or your water supplier |
Typical repair costs in the UK
These are typical ranges including parts and labour. They vary by region, boiler model and how easy the part is to reach.
| Repair | Typical UK cost |
|---|---|
| Diagnostic call-out | Typically £60 to £120 |
| Diverter valve motor or actuator | Typically £100 to £200 |
| Full diverter valve replacement | Typically £150 to £350 |
| Flow sensor replacement | Typically £100 to £250 |
| Plate heat exchanger descale or replacement | Typically £150 to £450 |
When to call a Gas Safe registered engineer
If the settings are right, the flow is strong and there is still no hot water, the fault is inside the boiler. The diverter valve, flow sensor and plate heat exchanger all sit inside the casing and must only be replaced by a Gas Safe registered engineer.
Do not remove the boiler casing to reach the diverter valve or flow sensor. If you smell gas at any point, call the National Gas Emergency Service on 0800 111 999.
To reduce the chance of the problem returning, ask about a scale reducer on the cold supply if you live in a hard water area, and keep up with annual servicing. Our guide to boiler service costs covers what a service includes.
When you book, tell the engineer the make and model, any fault code on the display and exactly what happens when you open a hot tap. For example, say whether the boiler fires, whether the radiators get warm and whether the water turns lukewarm or stays cold. That helps them bring the right parts and can save a second visit.