Helpful Hints – Combi Boiler Top

It’s an all too common occurrence, particularly in rental properties (landlords take note).  Too many people have combi boilers and don’t know how to to ‘top up’ the pressure.

A little background: combi boilers (usually gas but can be oil powered) heat mains water ‘on demand’ so there’s no hot water storage cylinder or ‘immersion’ as they are sometimes known. ‘Combi’s’ heat the hot water for radiators in the same way as a regular boiler.  If you don’t have a  cylinder in an airing cupboard or the loft then the chances are you have a combi boiler. You won’t have a loft storage tank either as with a combi boiler system everything works at mains water pressure. This also means no shower pumps.

If the pressure in a combi boilers drops to a pressure the system considers to be too low then the boiler will shut down and provide neither hot water or heating. 

Don’t confuse the terms ‘condensing’ and combi – all boilers are now condensing as it makes them more energy efficient.

Why do combi boilers need topping up? The answer is simply that whilst the pressure will fluctuate a little as a matter of course any small leak, usually unnoticeable will allow the pressure to drop. The usual causes are radiator valves, pressure relief valves,  expansion vessels, leaking pipes or air in the system.

Where to check? Combi boilers have a pressure gauge,  for most modern boilers on the front panel but it ban be underneath or near the boiler. Most gauges are marked green and red.

How often? A combi boiler typically needs topping up a couple of times a year. If you are having to top up frequently there’s something wrong. 

How much? This can vary by model but for most when the heating system is cool, the pressure should be between 1 and 1.5 bar on the pressure gauge and the indicator needle would usually be in the green section.

How? Unfortunately this varies by manufacturer and installation but there are common factors. For useful guides see:

https://www.worcester-bosch.co.uk/files/news/System_filling_guide.pdf
http://www.vaillantservice.co.uk/Vaillant_Boiler_Self_Help.html

There’s plenty of YouTube videos available as well.

Can it be over pressure? Yes and it will show as such on the gauge. If this happens use a radiator valve bleed key to let out some water add be careful, it can be hot and never unscrew it until it comes out, take a slowly slowly approach. Note that if the pressure rises by more than 1 bar when the heating is up to temperature then the expansion vessel may require re-pressurising. This will need a service engineer.