How a combi heats your hot water
A combi does not store hot water. When you open a hot tap, it heats cold mains water as it passes through a plate heat exchanger. The boiler has a fixed maximum output, measured in kilowatts (kW), so there is a trade-off: the faster the water flows, the less the boiler can raise its temperature.
If you have no hot water at all while the heating works, that is a different fault. See our guide to no hot water when the heating works.
Flow rate vs temperature
Manufacturers usually quote a hot water flow rate at a temperature rise of 35°C, which is roughly the lift needed to turn winter mains water into a comfortable shower. The table shows typical figures by boiler size. Your boiler's exact figure is in its manual.
| Combi hot water output | Typical flow at a 35°C rise |
|---|---|
| 24 to 25 kW | About 10 litres per minute |
| 28 to 30 kW | About 12 litres per minute |
| 35 kW | About 14 litres per minute |
| 40 kW or above | About 16 litres per minute or more |
Two things follow from this. First, if you open the hot tap fully, or run a bath tap and a shower together, you may ask for more water than the boiler can heat, so it arrives cooler. Second, the mains is colder in winter, often below 10°C, so the same boiler gives noticeably cooler water in January than in August. Turning a tap down slightly often makes the water much hotter.
A power shower or large rainfall head can easily pull more water than a smaller combi can heat. If the water was fine until a new shower was fitted, the shower is likely the cause rather than a fault.
Check the settings first
- Hot water temperature. Most combis have a tap symbol dial or a hot water setting in the menu. Many are set somewhere between 40°C and 60°C. If it has been turned down, turn it up a step.
- Eco or pre-heat mode. Some boilers have an eco mode for hot water that saves gas but makes water take longer to arrive and may run slightly cooler. Check the manual for your model.
- Thermostatic shower or mixer taps. A thermostatic shower valve that is set low, or has its own scale problems, can make the water feel cool even when the boiler is working. Try a basic hot tap to compare.
- Cold mains flow. Oddly, weak mains flow can also cause problems, because some combis only fire at a minimum flow. Check the stopcock is fully open.
- Pressure and fault codes. Check the gauge reads 1 to 1.5 bar with the system cold and note any code. Low pressure can affect how some boilers behave. See our guide to repressurising a boiler.
Faults that make hot water lukewarm
Scale in the plate heat exchanger
In hard water areas, limescale builds up in the narrow channels of the plate heat exchanger. Heat does not pass across as well, so the water comes out cooler, can go hot and cold, and the boiler may make noises during hot water use. An engineer can descale it or replace it. Fitting a scale reducer on the cold supply helps prevent it coming back.
Diverter valve not moving fully
The diverter valve switches heat between radiators and taps. If it only moves part of the way, some heat goes to the radiators while you run a hot tap, and the water is lukewarm. A sign is radiators warming slightly when only the hot tap is on.
Flow sensor or temperature sensor
A faulty hot water temperature sensor can make the boiler think the water is hotter than it is and turn the burner down. A sticking flow sensor can make the boiler slow to respond.
Burner or gas supply
If the boiler cannot reach its full output, for example because of low gas pressure or a burner fault, hot water will be cooler. Check other gas appliances are working normally, but leave any testing of the gas supply to an engineer.
Getting the best hot water from your combi
If there is no fault and the boiler is simply working at its limit, a few changes can make a real difference:
- Run one outlet at a time. A combi splits its output between every hot tap that is open. Running the washing up while someone showers will cool both.
- Turn the flow down slightly. Reducing the flow from a mixer shower by a small amount often raises the temperature noticeably, because the boiler has less water to heat.
- Choose shower heads carefully. A water-saving or standard shower head suits a smaller combi better than a large drench head.
- Keep outlets clean. Descale shower heads and tap aerators in hard water areas so the flow stays even.
- Think about the next boiler. If the household has grown or you have added a bathroom, a larger combi or a system boiler with a cylinder may suit you better when the time comes to replace it.
It is also worth knowing that the temperature at the tap is lower than the boiler setting if the pipe run is long. Water cools on its way through pipes in a cold loft or under a floor, and it takes a little while for the pipe itself to warm up. A few seconds of lukewarm water before it runs hot is normal.
When to call a Gas Safe engineer, and typical costs
If the settings are right, the tap is not running faster than the boiler can manage and the water is still lukewarm, the cause is inside the boiler. Only a Gas Safe registered engineer should work on the heat exchanger, diverter valve, sensors or gas supply.
Do not remove the boiler casing or try to descale components yourself. If you smell gas, call the National Gas Emergency Service on 0800 111 999.
| Repair | Typical UK cost |
|---|---|
| Diagnostic call-out | Typically £60 to £120 |
| Plate heat exchanger descale or replacement | Typically £150 to £450 |
| Diverter valve motor or full valve | Typically £150 to £350 |
| Hot water temperature sensor | Typically £100 to £200 |
| Scale reducer fitted on the cold supply | Typically £80 to £200 |
Costs depend on the brand, the part price, access to the boiler and where you live. Many engineers check for scale at the annual service, so booking one can catch the problem early. Our guide to boiler service costs has more detail.