Derek and I have been very busy educating people about expansion vessels and fixing boilers.
If your boiler is loosing pressure on a regular basis and you have to keep topping it up via the filling loop it’s MOST likely that there is a problem with your expansion vessel.
In a pressurised heating system such as a modern combi, the water in the radiators is pressurised in a sealed system. Like all water, when the circulating water is heated, to provide the heat in your radiators, it expands. When it expands it needs somewhere to expand to and this is where the expansion vessel comes in. The expansion vessel is filled with pressurised air at the same pressure as the system so that it retains a certain volume. Once the boiler starts heating the circulating water the expanding water is forced into the expansion vessel and compresses the air in the vessel to thus allows vessel to accommodate the expanding water. When the system cools down the reverse occurs.
Problems occur when there is no or very little compressed air in the expansion vessel. When it doesn’t have any air in it is when it fills with water and water is NOT compressible and so doesn’t provide a space for the circulating water to expand.
What happens next is key; the pressure increases in the system and the pressure relief valve opens as a safety device to expel some of the excess volume. This can be seen as water dripping from the small copper pipe at the back of the boiler, usually in the back yard.
When the system cools and the circulating water retracts to its original volume there is less water in the system because some of this water has drained out of the pressure release valve. This results in lowering the pressure and sometimes the boiler will shut down as a safety measure because of low pressure. The user then adds water via the filling loop and the whole process begins again.
Some “plumbers” will say there is a small leak or that its normal to have to keep topping up your boiler – THEY ARE WRONG!!
The expansion vessel is full of water and needs to be serviced.
Most manufacturers service manuals state that the expansion vessels is serviced on a regular basis.
So if you are finding that your system water constantly needs replenishing then it most likely your expansion vessel, which is a relatively straight forward job for a busy bee 🐝.