A boiler that switches itself off when the set temperature is reached is normal but in many cases, the boiler switches itself on and off frequently, 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.
Why does my boiler turn on and off frequently?
- The preheat function.
- Faulty thermostat.
- Oversized boiler.
- High boiler pressure.
The preheat function on a boiler
This is one reason why the combi boiler switches itself on and off and is normal. It helps deliver hot water to the tap quicker when you turn it on.
Boiler fires up randomly
The preheat function, when turned on, provides you with hot water quickly when turning on the hot tap. The boiler keeps the part of the boiler that heats tap water – the plate heat exchanger, hot so it’s ready to send hot water to the tap when needed. The boiler fires up for short periods to keep this part warm.
We can tell the preheat function is activated when looking at the display flashing independently without a demand. It is useful if the hot taps are turned on frequently but if the house is mostly empty during the working week, it doesn’t need to be on.
This preheat function can be deactivated by the user and doesn’t need an engineer to come and sort. Each appliance has a different way to activate and deactivate this function and either ask the installer or check the user manual for the boiler.
Short cycling of a boiler
If the boiler keeps on firing up and then turns off again quickly, it may be short cycling. A short cycling boiler is when the burner inside turns on and off quickly, within a minute. This means whenever heating demand is needed, like when the radiators come on, it is quickly met before the boiler shuts down again, then restarts shortly after. It can happen for a few reasons but the main one being an oversized boiler.
In this case, you may notice your heating bills becoming more expensive as well as it leading to the boiler getting more wear and tear over time, resulting in increased repairs.
If you want a professional to look at your boiler, Hometree can help:
Hometree offers customers peace of mind with home care packages for their boiler, central heating, plumbing, drains and home electrics all at an affordable monthly or annual cost. Hometree’s aim is to take the stress out of boiler servicing and home maintenance with unlimited call-outs and a 24/7 helpline, as well as a no price-hike promise.
Oversized Boiler
It is important to get the right size boiler for your property and the number of occupants. A boiler that’s too powerful heats up too quickly and then shuts off again quickly then turns back on as the room temperature room has not been met. It repeats this cycle frequently instead of heating up and running slowly.
Faulty Thermostat
A faulty thermostat that isn’t sensing the correct temperature in the room can cause short cycling by signalling the boiler to turn on when it’s not needed. Check if the batteries need replacing. Also, position the thermostat away from particularly cold spots where draughts can cause it to think the temperature is cold all over the house.
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.
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.
Why a Boiler short cycling is not good
Allowing the boiler to continue short cycling puts it under greater strain and reduces its lifespan due to wear and tear. It can also increase energy bills as it frequently turning on at full fire then shuts down quickly.
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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.
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