Vaillant boiler no hot water: causes and what to check

Combi or system boiler? Start here

Vaillant makes both combi boilers, which heat water instantly as you open a tap, and system or regular boilers, which heat a hot water cylinder. The checks are different, so it is worth knowing which you have. If there is a hot water cylinder in an airing cupboard, you have a system or regular boiler. If there is no cylinder and hot water comes straight from the boiler, it is a combi, usually an ecoTEC plus or ecoTEC pro.

With a cylinder, a Vaillant that is working perfectly can still leave you with cold taps if the programmer is not calling for hot water, the cylinder thermostat is set too low or a motorised valve is stuck. Zone-valve faults and cylinder thermostat faults are covered in their own guides. Most of this article is about combis, where the boiler itself makes the hot water.

Read the display before you do anything else

Open a hot tap and watch the boiler. On an ecoTEC you should hear the fan start and the burner light within a few seconds, and the display will show the boiler is heating water. Write down any code exactly as shown. The same letter and number can mean different things on different generations of boiler, so the model name from the front panel or data badge matters too.

Our guides on the Vaillant F28 ignition fault and Vaillant F75 explain those two codes in more detail. If a code comes back after one reset, stop resetting it. Repeated resets do not fix the cause and can make diagnosis harder.

Checks you can safely make yourself

None of these involves removing the boiler casing. They clear a fair share of no hot water calls, so they are worth two minutes before you book anyone.

• Pressure: the gauge should normally read about 1 to 1.5 bar when the system is cold. If it is below 1 bar, top up through the filling loop as your manual describes and close both taps on the loop afterwards. If it keeps dropping, see our guide to a boiler losing pressure.
• Gas: check that the hob lights and, on a prepayment meter, that there is credit. If nothing gas-fired works, the problem is the supply rather than the boiler.
• Hot water setting: make sure the hot water temperature on the control panel has not been turned down or switched off, and that any room controller or app is not set to a holiday or off mode.
• Power: if the display is blank, check the fused spur switch next to the boiler and your consumer unit.
• Taps: try more than one hot tap. If only one tap or one shower runs cold, the fault is probably at that outlet, and our guide to no hot water in shower may help.
• Reset: if the boiler has locked out, press reset once as your manual explains. Our guide on how to reset a boiler covers when not to.

Frozen condensate pipe in cold weather

ecoTEC boilers are condensing boilers. They drain acidic condensate through a plastic pipe, and where that pipe runs outside it can freeze in a hard frost. The water then backs up inside the boiler, which shuts down and usually shows an ignition or flame fault. You lose both heating and hot water, typically on the coldest morning of the winter, and sometimes the boiler makes a gurgling sound as it tries to start.

If the pipe is at ground level and safe to reach, pouring warm (not boiling) water along the outside section, or holding a warm pack against it, usually frees it, after which the boiler can be reset. If the pipe runs up a wall, or freezes every year, it probably needs rerouting or insulating, which boiler condensate pipe routing explains.

Heating works but the taps stay cold

When the radiators heat normally but a running hot tap is ignored, the burner, gas supply and fan are all working. The fault is in the parts that switch the boiler into hot water mode or pass the heat to the tap water.

The flow sensor tells the boiler a hot tap is open. If it sticks, often because of scale or debris, the boiler never knows you want hot water. The diverter valve then sends the hot water from the burner to the hot water side rather than the radiators. Vaillant publishes its own guidance on diverter valve faults, and a valve stuck in the heating position is one of the most common reasons for heating without hot water. Finally the plate heat exchanger passes that heat to the mains water. London water is hard, and scale inside the plate heat exchanger gradually reduces how hot the water gets and how much flow the boiler can heat.

All three are inside the sealed boiler case, and replacing them means working on a gas appliance, so this is a job for a Gas Safe registered engineer. Our broader guide on no hot water but heating works covers the same pattern on other makes.

Hot water that is only warm, or goes cold

If the water gets warm but never hot, or starts hot and then fades, the boiler is firing but not coping. Scale in the plate heat exchanger, a diverter valve that is partly stuck, a temperature sensor giving the wrong reading, or low gas pressure can all cause it. Running several taps at once can also ask more of a combi than it can deliver, particularly in a larger home.

It helps to notice whether the water is fine at a low flow and goes cold at a high flow, and whether the problem started suddenly or gradually. Gradual loss over months usually points to scale. A sudden change is more likely to be a component. Our guide on hot water runs cold explains how to describe the pattern.

When to call an engineer

Call if the same code returns after one reset, if the pressure keeps dropping, if there is no code and the heating works but the taps are cold, if you can see water leaking from the boiler, or if the boiler is making unusual noises. If you smell gas, turn off the gas at the meter if it is safe to do so, open the windows and call the National Gas Emergency Service on 0800 111 999.

Loyal Plumbers carries out boiler repair and servicing and hot water repair across London and everywhere inside the M25, with 10 vans on the road 24 hours a day. Gas work is carried out by Gas Safe registered engineers. When you get in touch, tell us the boiler model, the exact code on the display, the pressure reading and whether the heating still works. Our call-out charge is £60, and the engineer explains the fault and the repair before any work goes ahead.