A Vaillant boiler that keeps turning off when the set temperature is reached is normal. But in some cases, a boiler switching itself off and on frequently indicates a problem. And there are a few reasons for that. Most of these will need to be looked at by a professional. Here, we look at what a normal on and off timeframe is and what isn’t.
Vaillant boiler keeps turning off
A Vaillant boiler that repeatedly shuts off before reaching the set temperature is likely malfunctioning. Faulty components like the thermostat or water pump can cause untimely boiler shutdowns. Low water pressure trips the safety cutoff switch. Too much limescale impedes water flow causing overheating. A worn-out ignition electrode may fail to spark properly leading to ignition lockout. Annual servicing and repairs by a Gas Safe engineer can diagnose and fix shutdown issues.
Maintaining system pressure and descaling prevents premature cutoffs. With some basic troubleshooting, Vaillant boiler owners can get their appliance running smoothly again during the colder months.
- Lack of water flow.
- Boiler short cycling.
- Faulty thermostat.
- Oversized boiler.
- High boiler pressure.
- Low water pressure.
- Faulty pressure relief valve.
Causes of a Vaillant boiler turning itself off
Faulty Thermostat
One of the most common reasons a Vaillant boiler may repeatedly turn off is a faulty thermostat. The thermostat controls the temperature of the water in the boiler system. If it is not working properly, the boiler may think the water is hot enough when it’s not, causing the boiler to shut off before reaching the proper temperature. A faulty thermostat can be caused by dead batteries, incorrect settings, or a broken internal component. Replacing the thermostat can often fix this issue.
Low Water Pressure
Boilers require adequate water pressure to operate safely and effectively. Over time, small leaks in the system can cause pressure drops. Vaillant boilers are programmed to automatically shut off when the water pressure drops below a certain threshold, usually around 1 bar of pressure. Frequent topping up of the system and checking for leaks is required to prevent low pressure boiler lockouts.
Boiler Short Cycling
If a boiler is oversized for the building it is heating, it will reach the set temperature quickly before shutting off, then repeat this cycle frequently. This short cycling can cause premature wear and tear over time. Short cycling can also be caused by restricted pipes, boiler pump issues, and poor system design. Consulting with a heating engineer to determine proper boiler sizing and identify any system design flaws is key to stopping short cycling issues.
Lack of water flow
A closed valve somewhere on the system, or trapped air will restrict the water flow to the boiler. If the water flow isn’t sufficient, the boiler will turn off. Check the stop tap and valves to ensure the ones that need to open, are fully open.
Vaillant Combi Boiler Hot Water Goes Cold: Problems And Solutions
Limescale Buildup
Hard water areas with high limescale content can lead to limescale accumulation in the heat exchanger and pipes of a Vaillant boiler. This buildup restricts water flow and causes the boiler to overheat. Vaillant boilers will shut down when the temperature climbs too high due to limescale. Preventing this requires annual descaling of the system by a qualified engineer. Chemical flushes can remove existing limescale. Water softeners also help reduce further buildup.
Ignition Lockout
All Vaillant boilers have ignition lockout safety to prevent dangerous gas leakage. Ignition failures can be caused by a lack of gas supply, a faulty thermocouple disrupting the pilot light, or problems with the gas valve or PCB. If ignition fails repeatedly, the boiler enters lockout mode. To restart it after an ignition lockout, the boiler system needs to be reset, which often requires an engineer visit. The underlying cause of ignition failure also needs to be diagnosed and repaired.
Lack of Gas Supply
If the gas supply to the boiler is disrupted, such as due to an empty propane tank or issues with the main line, the boiler cannot ignite and will shut down. Vaillant boilers do not have a standing pilot light, so any interruption of the gas supply will prevent ignition. Checking for gas supply issues and calling the utility company if the main line is disrupted can get the boiler back up and running. Keeping propane tanks full is also important in peak usage seasons.
Blocked Condensate Pipe
Vaillant condensing boilers have a plastic pipe to drain condensed water vapor. If this pipe gets obstructed, typically due to freezing in winter or debris buildup, condensed water will back up and cause the boiler to shut down as a safety precaution. Monitoring the pipe regularly to ensure it drains freely prevents this shutdown cause. Heat tape, pipe insulation, and clearing debris help maintain an unblocked condensate drainage pipe.
Vaillant Boiler Pressure Too High: How To Bring It Down
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Vaillant Boiler Won’t Turn Off: Common Problems And Fixes
High boiler pressure
If the water pressure is too high on the gauge, there could too much air in the system so the boiler is shutting down for safety reasons. Too much air in the system can reduce the water circulation so the required temperature is never reached, but the water temperature is very hot.
Check the pressure on the gauge isn’t too much above 1 bar, if it is, you will need to bleed a radiator until it comes down to around that level.
Low water pressure
This is a common cause of the Vaillant boiler turning off, but this is true for all boiler makes. When the water pressure gets too low, the boiler will turn itself off. If there is a leak in the system and the heating or hot water is turned on, the boiler may work for a while before the water pressure drops enough to cause it to stop the boiler working.
Why Does My Vaillant Boiler Keep Losing Pressure? Causes
Faulty pressure relief valve
If boiler pressure isn’t the problem, but the boiler keeps switching off, a faulty pressure relief valve may be the culprit. The prv works to shut the boiler down in cases where the pressure is too high, after3 bar. If the prv is faulty (usually because debris gets stuck), it may send a message for the boiler to switch off.
Combi boiler hot water goes cold
A common problem with combi boilers is hot water goes cold before getting hot again when showering. One reason is a sudden increase in demand when someone in the house turns a hot tap on. This puts extra demand on the combi to supply both outlets at the same time. Another reason could be an internal problem like a blocked heat exchanger, especially on old boilers.
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*The information in this article should be used for general guidance only and not as financial advice. 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.
