Boiler losing water pressure – needs regular topping up

If yours is a sealed heating system and you find any of the following:

  • The boiler pressure gauge reading is low and you need to top it up fairly frequently using the filling loop, causing you to suspect leaking pipework.
  • The boiler’s pressure gauge reading increases dramatically when the boiler is firing.
  • Water is being lost from the discharge pipe.

It’s quite likely that the system’s expansion vessel needs recharging with air. On small systems, the expansion vessel is often housed within the boiler – see the images below – the expansion vessel is the grey item next to the right hand side of the case:

Why does it happen? Well, it’s common for this to be excluded from a boiler service. It’s not just service engineers who are to blame – some manufacturers don’t provide definite (or in some cases, any) guidance on the subject, others specify that the expansion vessel should be attended to at least once every three years.

I’ve yet to check an expansion vessel and find the air pressure anywhere near the specified value.

If you keep having to use the filling loop, don’t let this persist – get it checked and re-charged with air if necessary – it’s not a long job. The boiler’s pressure (safety) relief valve is likely to be operating frequently, weakening it, so that it lets water out at too low a pressure, and it may need to be replaced.

If indeed the problem has been present for some time, expect two visits – one to sort out the expansion vessel pressure, and a second to replace the valve.

At worst, the expansion vessel membrane may be split and the vessel have to be replaced. Some manufacturers make this easy, others not so, in some cases requiring the entire boiler to be taken off the wall.

I’ve covered expansion vessels in more detail in a Tech Note on my website.