Radiators are warm but the tap runs cold or only lukewarm? Or no hot water comes out at all? When your combi boiler has no hot water, the cause is often simple – a changed setting, low pressure or an error on the display. This guide explains how a combi boiler heats water, what you can safely check yourself and when you need a boiler repair technician.

How a combi boiler heats your water

According to official advice from the boiler manufacturer Vaillant, a combi gas boiler gives you both heating and hot water from one appliance: water isn’t stored in a tank – it is heated the moment you turn on the tap.

Whether hot water goes to the radiators or to the tap is decided by the diverter valve inside the boiler. Vaillant explains that if the heating is on and someone opens a hot tap, the valve gives priority to the tap and sends hot water there until the tap is closed.

Combi boiler no hot water – wall-mounted combi boiler, bathroom and radiator

Combi boiler no hot water: first, are the radiators heating?

Vaillant recommends checking the radiators first. If they heat normally, the boiler is working and the problem is limited to domestic hot water.

According to boiler manufacturer Ideal Heating, if the heating won’t come on either, there is a general problem with the boiler – in that case read Boiler not turning on – what to check. If the heating works but there’s no hot water, Ideal says there’s a good chance the diverter valve is stuck.

What you can check yourself

The steps below are based on official manufacturer advice and don’t require opening the boiler casing:

1. The display and error codes

A modern boiler display shows its current status. If an error code appears, look up its meaning in your boiler manual. Baxi says most codes only require a boiler reset, but if the boiler locks out frequently, that indicates a fault and you should call a technician.

2. Mode: is the boiler set to heating only?

Vaillant advises making sure the boiler isn’t in “heating only” mode. The control panel usually lets you select a “heating + hot water” mode – your model’s manual explains exactly how. If the boiler is in standby or frost-protection mode, it is switched off and the heating won’t work either.

3. Hot water temperature

On many boilers the heating and hot water temperatures are set separately. Check that the hot water temperature hasn’t been turned down by accident. Ideal Heating notes that water below 30°C will feel quite cold. Vaillant generally recommends 50–55°C for hot water – a range that also helps prevent the growth of legionella bacteria.

4. System water pressure

Baxi says low pressure is the main reason hot water stops working, and it usually shows as an error on the control panel. According to Ideal Heating, the pressure in the boiler and heating system should be within 1–1.5 bar – outside this range the boiler may switch off for safety. Check the exact value in your boiler manual. Only top up following the manual; if you’re unsure, leave it to a technician. If the pressure keeps falling, read Boiler pressure dropping – causes.

5. Power and gas

Vaillant points out that a boiler needs water, gas and electricity to work. Check whether the boiler’s display and lights are on, whether there is a power cut and whether the boiler’s breaker in the fuse box has tripped.

6. Fully open a hot tap

Vaillant advises opening a hot water tap fully for a few seconds – the boiler should respond and fire up. If it stays silent, a part is probably faulty and you need a professional.

Ideal Heating notes that if no water at all comes out of the hot tap, the problem may be with the water supply (for example a supply cut or frozen pipes) rather than the boiler. If cold water comes through the hot tap, you can rule out a supply cut-off and the cause lies with the boiler.

7. Reset the boiler

According to Vaillant, a boiler sometimes detects a fault and shuts itself down for safety – you can restart it yourself. Reset the boiler following its manual. If the error keeps coming back, the boiler needs a technician.

Sources: Vaillant – Hot water not working (official page); Baxi – What to check if you have no hot water; Ideal Heating – No hot water? Here’s what to do.

Water is only lukewarm or only heats when the heating is on

If the radiators heat up but the tap only gives lukewarm water, the boiler is working but struggling to heat the water. According to Vaillant’s official pages, possible causes are:

  • A stuck diverter valve – some of the hot water passes to the radiators instead of the tap, so the tap runs lukewarm;
  • The valve no longer switches to the hot water circuit – if hot water only comes out when the central heating is on, the valve isn’t diverting properly. Vaillant says this often becomes apparent in the warmer months, when the heating isn’t needed;
  • Heating priority – temporarily delays or blocks hot water production;
  • The temperature sensor – wrongly indicates that the water is already hot enough, so the boiler stops heating it.

Ideal Heating says a stuck valve can sometimes be loosened by turning the hot taps on and off a few times while the central heating is on. If that doesn’t help, the valve needs repairing or replacing.

Source: Vaillant UK – How to fix a boiler diverter valve (official page)

What only a technician should do

Don’t open the boiler casing. Vaillant advises against trying to fix a diverter valve yourself: any work on a gas appliance is dangerous and can damage the boiler further. A technician will check whether the valve really is the cause and clean or replace it as needed. According to Vaillant, ignition faults and temperature sensor faults are also a job for an expert. If you smell gas, don’t use open flames or electrical switches, open the windows and contact the gas emergency service immediately.

When to call a boiler technician

  • The boiler doesn’t respond when you fully open a hot tap;
  • The water is lukewarm or only heats when the central heating is on;
  • An error code returns after a reset or the boiler locks out frequently;
  • The pressure keeps dropping after topping up;
  • There’s still no hot water after checking the mode, temperature, pressure and power.

Vaillant says regular maintenance is key to keeping your hot water flowing reliably – see Annual boiler service – what it includes. A MasterMan technician will find out on site why your boiler isn’t heating water and repair it – more on our boiler repair page.

Frequently asked questions

The boiler is working, and according to Ideal Heating there’s a good chance the diverter valve, which splits hot water between radiators and taps, is stuck.

Vaillant says this is a sign of a faulty diverter valve that isn’t diverting hot water to the taps properly.

Vaillant generally recommends 50–55°C, a range that also helps prevent the growth of legionella bacteria.

According to Ideal Heating, usually 1–1.5 bar, but check the exact value in your boiler manual.

No – Vaillant recommends calling a qualified engineer, because any work on a gas appliance is dangerous.

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