Why is my Vaillant not heating water?
A Vaillant stops heating water because the boiler is not firing, because hot water has been switched off or set too low, or because a part that directs heat to the taps has failed. Whether the heating still works is the quickest way to tell which.
On an ecoTEC combi one burner serves both the radiators and the taps. When you open a hot tap, a sensor detects the flow and a diverter valve inside the boiler sends the heat to a plate heat exchanger, which warms the mains water on its way to the tap. If the boiler has locked out, you lose both heating and hot water. If only the hot water has gone, the problem is usually in that switching process or in the hot water setting.
The ecoTEC plus operating instructions include a short troubleshooting list for a boiler that gives no hot water. The table below combines it with the faults an engineer is most likely to find.
| What you notice | Likely cause | What to do |
|---|---|---|
| No hot water or heating, F.22 on the display | Water pressure too low | Top up the heating system |
| No hot water or heating, F.28 on the display | Ignition failure, often no gas | Check the gas supply, then press the fault clearance key |
| Display blank, nothing happens | No power or boiler switched off | Check the fused spur and the boiler's on switch |
| Heating works, taps cold, no code | Hot water switched off or set too low | Check the hot water setting |
| Boiler fires on a hot tap but water stays cold, radiators warm | Diverter valve stuck | Gas Safe registered engineer |
| Boiler silent when a hot tap runs, heating fine | Flow sensor or weak mains flow | Check the cold supply, then call an engineer |
Check the hot water setting on your ecoTEC
Start with the boiler's hot water setting, because it is easy to change by accident and it can be turned off completely. The ecoTEC plus operating instructions show the set hot water temperature on the display when you press the hot water button, and you change it with the arrow buttons and confirm it. On a combi, they give a range of 35 to 65 °C. They also describe turning hot water off from the same setting, so an ecoTEC showing off for hot water will heat the radiators and nothing else.
The ecoTEC pro works in a similar way. Its operating instructions add that if you have a Vaillant controller, you set the maximum hot water temperature on the boiler and then adjust it on the controller. If you have a room controller or app, check its hot water setting and schedule too, as well as the boiler.
The ecoTEC pro instructions also carry a warning that legionella bacteria multiply at temperatures below 60 °C, and they ask you to consult a competent person before setting the hot water lower. If you turned it down to save energy, that is worth discussing with your engineer at the next service.
Comfort mode
Both the ecoTEC plus and ecoTEC pro have a comfort mode. The operating instructions say it supplies hot water at the required temperature immediately, without you having to wait for the water to heat up. With comfort mode off, the first few seconds of water at the tap are cold while the boiler heats up. That is normal and does not mean there is a fault. The ecoTEC plus shows a symbol on the basic display when comfort mode is on.
Older ecoTEC plus models have rotary knobs for heating and hot water on the front. If yours has knobs, check the hot water knob has not been turned to its lowest position, and see the user manual for what that position does on your model.
Check the display for F codes
If the heating has stopped too, the boiler is not firing, and the ecoTEC usually shows a code. The two a homeowner sees most often are these:
- F.22 means low water pressure in the heating system. The boiler will not fire for heating or hot water until you top it up. Our guide to Vaillant boiler pressure shows where the pressure is shown and how to top up.
- F.28 means the boiler failed to light after several attempts. Check that the gas hob works, the gas meter is on and any prepayment meter has credit.
Other codes point to specific parts. F.75, for example, relates to the pump or pressure sensor. Look up any code before pressing reset.
Check the gas, power and cold water
The ecoTEC plus troubleshooting list starts with the basics: the gas isolator closed, the power off at the building or the boiler switched off. If someone has recently had work done on the gas or electrics, check the gas isolation valve under the boiler is open, with the handle in line with the pipe, and that the fused spur next to the boiler is switched on.
A combi also needs a good cold mains supply. Run a cold tap. If the flow is weak, check the stopcock is fully open. Open the hot tap fully when testing, because a combi needs a minimum flow before it switches to hot water. If no water at all comes from the hot tap, see our guide to no water coming out of the hot tap.
Stand by the boiler while someone opens a hot tap fully. If you hear the fan and burner start within a few seconds, the boiler is detecting the flow. If it stays silent, the flow sensor or the cold supply is the likely problem.
Diverter valve and flow sensor faults
If the settings are right, there is no code and the heating works, the fault is most likely inside the boiler. Two parts cause most of these cases.
Diverter valve
The diverter valve switches the heated water between the radiators and the hot water heat exchanger. When its actuator fails or the valve sticks, the heat keeps going to the radiators. The signs are the boiler firing when you run a hot tap but the water staying cold, and radiators warming up while the tap runs even with the heating off. The actuator can often be replaced on its own, which keeps the cost down. Our guide to diverter valve replacement cost covers typical prices and what drives them.
Hot water flow sensor
The flow sensor tells the boiler a hot tap is open. If scale or debris stops it working, or it fails electrically, the boiler never knows you want hot water and stays silent while the heating still works when called. An engineer can test it and clean or replace it.
Scale in the plate heat exchanger
In hard water areas, scale in the plate heat exchanger usually causes lukewarm or fluctuating hot water rather than none. Our guide to hot water that is not hot enough covers that problem.
How do I reset a Vaillant for hot water?
Reset a Vaillant only if it shows a fault code. The ecoTEC plus and pro operating instructions say to press the fault clearance key for one second, after which the boiler makes another attempt to light. If three attempts do not clear an ignition fault, they tell you to consult a competent person. A reset does not affect the hot water setting, and there is no separate reset for hot water. For more on resetting, see our guide on how to reset a boiler.
Do not open the boiler casing to reach the diverter valve or flow sensor. They sit beside the gas valve and electrical parts, and only a Gas Safe registered engineer should work on them. If you smell gas, turn off the gas at the meter, open windows and call the National Gas Emergency Service on 0800 111 999.
When to call an engineer
Book a Gas Safe registered engineer, or contact Vaillant if the boiler is under guarantee, when the hot water setting is on and sensible, the pressure is correct, there is no code and the cold supply is good but the taps still run cold. Also call if F.28 or F.22 keep coming back after you have dealt with the obvious cause.
Tell the engineer your model, whether the heating works, whether the boiler fires when you open a hot tap and whether the radiators warm up at the same time. For the same symptoms on any make of boiler, our guide to no hot water when the heating works covers combi and cylinder systems.