The house heating not coming on is usually a more common problem in climates like the UK where the average temperatures are not very high. But a Vaillant boiler that won’t turn off leaving you sweating in the summer heat can be an issue too, not only affecting comfort but wasted energy and increased fuel bills.
Boiler won’t turn off causes
The main cause of a boiler that won’t turn off is a room thermostat that is set too high, and this is the easiest to rectify. Other causes are faulty thermostats inside the boiler and in the room they are placed, programmer issues, boiler wiring and a problem with the motorised valve. Most of these will require a gas engineer to fix.
1. Faulty thermostat
This is the most common and easiest to fix of the issues. Replace the batteries on the room thermostat because if they are weak, the message may not be sent to the boiler to switch off.
Check the thermostat to see if it has a frost setting where if the temperature drops below the setting, the boiler will fire up. Make sure the thermostat isn’t in a cold place and sending a message to keep the boiler on.
If the system won’t turn off and heat keeps being delivered, set the thermostat to its lowest temperature setting and wait a few minutes.
With new smart thermostats, such as nest and hive, some of these can be set to override the wireless thermostats in the event that the batteries are running low on power or the thermostat fails. IF you have these set up, check the smart control settings. Vaillant has its vSmart control which is fully compatible with their boilers so you shouldn’t see this problem.
2. Boiler Programmer issue
A programmer allows you to set your heating times on and off during the day and week. You can pre set the times and the boiler will fire up and shut off at these times.
Once a thermostat fault has been discounted, this will probably be the next stage of investigation by the gas engineer who would also check the wiring and connections with the other components.
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3. Issue with Thermostatic Radiator Valves (TRV)
In the summer when you don’t need the heating on but still require the hot water, many people turn the TRV to the zero or off setting and this should shut the valve completely. But sometimes these valves don’t close properly and some heat can get into the radiator.
To fix this, you can turn off the lockshield valve at the other end of the radiator and this should stop residual heat seeping through. Remember to count how many turns you made to turn off the lockshield because when you want the heating back on in winter, the radiators will need to be balanced and each of these valves turned on as they were before.
4. Boiler wiring or printed circuit board
A wiring issue could be the cause where even when the programmer is off, power is going to the boiler through the microswitch instructing it to continue heating. Although not a common problem with Vaillant boilers, a gas engineer will be able to check this too once other causes have been discounted.
5. Problem with motorised valve (3 port valve)
A motorised valve is usually found near the hot water cylinder and controls what type of hot water is sent – either from the boiler to heat the radiators or from the cylinder to the hot taps.
If this valve is stuck and there is a demand for heating at the same time as the valve is in the hot water position, the valve won’t close completely and will allow some hot water to still pass through to the radiators and warm them up. The valve will usually need to be replaced.
Other valves inside the Vaillant boiler can also become stuck, which sends the wrong information to the control board to continue firing the boiler up.
6. Issue following a repair
If any changes have been made to the heating system an issue can arise, especially with system boilers that have a cylinder. After a repair, a plumber or even someone working on it themselves puts a pipe fitting on the incorrect part of the flow and return pipework coming from the cylinder.
So instead of the water going back to the return pipe of the boiler, it can sometimes circulate in the opposite direction to head towards the radiators. In this case, you will need to get a plumber to investigate and fix it.
Is it safe to turn the boiler off at the mains?
It is safe to turn your boiler off at the mains as long as there are no other issues with it. A combi boiler turns itself off anyway for various reasons like when there is no flame present or low water pressure. When you turn the boiler back on, it will go through its start up sequence which can take up to a minute or so.
The above issues are the most likely causes of heating not turning off and with most of them, you will need a qualified gas engineer to find and fix the fault. If your boiler isn’t in warranty, why not get home heating cover with Hometree?
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Summary
These are potential causes of a Vaillant boiler that won’t turn off:
- Thermostat faults.
- Boiler Programmer issues.
- TRV (Thermostatic Radiator Valve) problems.
- Boiler wiring faults.
- Motorised Valve
- Issue following a repair.
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*The information in this article should be used for general guidance only. Full details are on the link in the footer to our disclaimer page. Always discuss your requirements with a competent and suitably qualified professional before undertaking any work.