Central heating system pressure: combi boiler gauge and filling loop gauge say different things

[Participant]

Hi all

We have a combi boiler in the loft (with a pressure gauge) and the filling loop (also with a pressure gauge) is in the airing cupboard underneath.

Both gauges show significantly different pressures, with the filling loop reading 1.3 bar but the combi reading 0.7 bar.

Which do I go by when filling the system please folks?

 

[Participant]

Hi both, thanks for replying and sorry for the delay in coming back to you:

Hmm.. Well the filling loop is just that.. It's not part of the system per se, and won't nesesarily have the same pressure.

Is the filling loop just a simple hose with a tap on each end?

If so i'd just go off the boiler gauge when topping up, and just slowly turn the tap off on the loop as the main gauge approaches the correct pressure.


I'm guessing the loop gauge might be reading slightly higher as it's more exposed to mains pressure when open?

Yes just a braided hose with two valves and a pressure gauge. Here's the filling loop and gauge, showing 1.5 bar of heating system pressure:

The filling loop should be turned off at both ends when not in use, or to do it properly to code, removed entirely so it can't over pressurise the system if the taps fail.
But in reality most people just leave the loop installed as it's unlikely both taps will fail at once.

Yes I keep both taps closed.

Our boiler is in the loft as well and we have the filling loop in the airing cupboard along with a second pressure gauge for convenience. The second gauge is within the system though, not isolated to the filling loop.

Do both gauges move when you fill the system?

Yes both gauges show increased pressure when I've filled up the system, but they don't show the same readings so I'm not sure which to go by. Boiler gauge, taken at the same time as the above photo, showing only 1 bar:



What I've done is hedge my bets by filling until the airing cupboard gauge says 1.5 bar and the boiler gauge says 1 bar, which should hopefully mean I'm OK no matter which is correct. Would be nice to understand it though.