A boiler that keeps dropping below 1 bar is one of the most common problems we get called out to across South West London. Sometimes it is a two minute fix you can do yourself with the filling loop. Sometimes it points to a leak or a failing part that will only get worse. Here is how to tell the difference.
Most modern combi and system boilers run happily between 1 and 1.5 bar when the heating is off and the system is cold. When the heating fires up and the water expands, it is normal for the needle to rise towards 2 bar, then settle back down as things cool.
If the gauge sits below about 0.8 bar, many boilers will lock out and refuse to fire, often showing a fault code such as F22 on a Vaillant, E119 on a Baxi or A281 on a Worcester Bosch. Losing a little pressure over several months is normal. Needing to top up every week, or every day, is not.
In our experience across homes in Wandsworth, Putney, Wimbledon and the surrounding areas, pressure loss almost always comes down to one of a handful of culprits. The age of the housing stock matters here: plenty of South West London properties have older radiators and pipework buried under floors, which is where slow leaks like to hide.
Topping up is safe to do yourself and takes a couple of minutes. Find the filling loop, which is usually a braided silver hose with one or two small valves underneath the boiler. Some boilers, such as newer Worcester and Ideal models, have a built in filling key or lever instead, so check the manual if you cannot see a hose.
With the boiler switched off and cold, open the filling valve slowly and watch the gauge climb. Close it at around 1.2 to 1.5 bar. Do not overfill: above 2.5 to 3 bar the relief valve will open and dump the excess outside. If the boiler shows a fault code, reset it once the pressure is back up. Then keep an eye on the gauge over the next week.
If you are topping up more than once every few months, the water is going somewhere, and it is worth finding out where before it damages a ceiling or floor. A pressure drop of 0.5 bar or more inside a week is a clear sign something needs attention.
Diagnosis usually starts with a visual check of every radiator and valve, then the expansion vessel and relief valve. Recharging or replacing an expansion vessel is typically a job of one to two hours. Costs depend on the boiler and what we find: a valve or vessel repair is normally in the low hundreds of pounds, while tracing a leak under a floor takes longer and is priced on inspection. Anyone working on the gas boiler itself must be Gas Safe registered, so always ask to see the card.
It is not usually dangerous, as the boiler will simply lock out at low pressure. The real risk is the leak behind it causing water damage, and constant topping up with fresh water can corrode the system over time.
On a healthy system, no more than once or twice a year, typically after bleeding radiators. If you are reaching for the filling loop monthly or more, there is a fault worth investigating.
You can in the short term, but each top up adds fresh oxygenated water that encourages rust and sludge inside radiators. It is cheaper to fix a weeping valve now than to replace corroded radiators or a heat exchanger later.
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