Slightly raised boiler pressure

[Poisonata]

Recently drained the CH system completely to replace numerous faulty TRV and several radiators in our Victorian property. Nine radiators of varying sizes powered by a Worcester Greenstar 24i Junior (open vented system). Refilled the system (included significantly more inhibitor than recommended), painstakingly purged the air from every radiator / boiler. Fired up the boiler, ran it for an hour then bled everything again to remove final traces of air. Once cold the pressure sat at 1.25 bar

Following day repeated the process, fired up, bled but ultimately almost no air was discharged from any radiator. Air in the system naturally collects in either two highest radiators and / or magnetic filter assembly (fitted directly under the boiler on the pipe flowing in). Back to cold, pressure sitting at 1.2 bar.

Filling loupe permanently disconnected. Boiler auto bleed value appears to be working correctly.

Not had the heating on since. Fast forward two weeks go to pop the heating, surprised to see the boiler pressure is now sitting at over 1.5 bar. Initial thoughts were fresh water that was introduced (only introduced first time refilled & added inhibitor, no extra water been added since) remaining dissolved air had now discharged from system liquid increasing pressure. Checked every radiator and the usual suspects had some air albeit small amount (half a cup). Pressure reduced slightly but still over 1.5 bar. Released about a cup full of liquid from the magnetic filter assembly to reduce pressure further back to 1.25 bar.

Just wondering why this has happened. My thoughts on the matter were

1. Pressure gauge isn't totally accurate

2. Warm day when work was done. Cold this evening. Slight contraction of system due to cold caused this increase in pressure.

3. When refilling a system it's normal to make further slight pressure adjustments until things settle down / inhibitor takes effect, that sort of thing.

Am I missing something. Is this fairly normal.

Considering overall discharge of air / liquid from system consisted about 1 1/2 cups to return gauge to 1.2 bar is fractional considering total volume of liquid I'm not overly concerned.

Did consider checking the cold pressure of the expansion vessel as my next 'rule it out' way of thinking. Although didn't see how this could be the fault. Once heating is on pressure rises to just over 2 bar. Returns to original 1.2 when cold. Previous faulty expansion vessel (old house) caused pressure to drop to under 1 bar when cold and sufficiently high when on to engage the PRV at 3 bar.

Just wanted to be sure I'm not missing something obvious.

Thanks in advance for anyone's thoughts on the matter.

 

[Mottie]

Worcester Greenstar 24i Junior (open vented system).

Once cold the pressure sat at 1.25 bar

I’m not a plumber but should there be any pressure in an open vented system? That’s the type with a F&E tank isn’t it?

 

[Madrab]

If you have a pressure gauge on your boiler then normally it won't be open vented. The greenstar junior 24i is typically a sealed system combi boiler.

A rise of >0.8 bar on a system ove hot, with 9 rads, is a little excessive, so the EV should be checked for correct pre-charge pressure. A rise of up to 0.5 bar would be normal. system pressure needs to be released and a drain point left open if you need to re-charge the vessel.

When the system was drained, was it clean?

If the filling loop is disconnected and there are no other connections that could add water into the system then there are typically only 2 other ways the pressure could rise - hydrogen (typically) being created by a reaction in the system or a pin holed plate HEX leaking mains water into the system.

 

[Poisonata]

Apologies, hot water system the boiler feeds is open vented. Heating system is closed loop.

 

[Poisonata]

@Madrab thank you, extremely helpful