Q & A

Pressure issues

My boiler keeps losing pressure when the heating is on. Why is it doing this?

It is more than likely related to the ‘expansion vessel’. Check if there is water coming out of the overflow pipe to confirm this. Most modern central heating systems are a ‘sealed system’. Water expands by around 4% when it is heated, this extra expansion needs to go somewhere. On a sealed system you have a component which takes up the extra expansion called an expansion vessel. The expansion vessel is a cylinder which usually can hold between 8 – 12 litres of water on a domestic system. Inside the vessel there is a rubber tube which is generally inflated to between 0.75 – 1.5 bar.

Eventually the rubber tube inside the vessel goes flat, just like a car tyre. When this happens the expansion vessel can’t work properly and the pressure inside the system builds up to 3 bar and is released when the pressure relief valve opens up to relieve the pressure. This keeps happening until the problem is fixed. Usually the internal rubber tube can be inflated, but sometimes the tube pops and the vessel needs to be replaced.

I have a combi boiler, how much pressure should be in the system?

As a general guideline you should try to keep the pressure at around 1 bar when its cold, it doesn’t have to be exactly 1 bar, a little more or less is fine. Some boilers have a green band that indicates where the needle should be. If pressure needs to be added to the system then this should be done when the system is cold. The pressure will go up slightly when the central heating is on, this is normal.

Hot water

I have a Combi boiler and my central heating works but my hot water doesn’t.

If the heating works ok, but the boiler doesn’t fire up for domestic hot water then it is usually the mechanism that tells the boiler that a tap is running that is faulty. This is quite a common issue on Combination boilers.

My boiler has a ‘preheat’ function, what does this mean?

Some boilers have a preheat function which is a permanent feature and some boilers give you the option to turn it on or off. A preheat is a feature that keeps the water inside the heat exchanger(s) hot so that when you run a hot tap the hot water is delivered quicker. Often manufacturers create their own terms to describe this feature, like warmstart for example. Preheat is just the general term for it.


How do I know what type of boiler I have?

There are three main types of boiler, a Combi, heat-only and a system boiler. As a general rule if you live in a small house there is a good chance you will have a combi boiler installed. Combi is an abbreviation for combination, they are called this as they do both the heating and hot water(combined). You can tell if you have a Combi as the boiler will fire up every time you run a hot tap.

The other type of boiler you may have is a ‘heat only’ or ‘system’ boiler, which are similar and both work on the principle of stored hot water. You will know if you have one of these boilers if you have a means of storing the hot water i.e. a cylinder in your airing cupboard or loft space or somewhere else. These boilers don’t fire up every time you run the tap, the water inside the cylinder needs to be heated first.