Pressure after expansion vessel replacement

[JamiePattison]

We had the expansion vessel replaced as it was filled with water.

A few hours later I noticed the pressure dropped so I topped it up. Next day exactly the same where I'm topping the pressure up every few hours.

I spoke to the plumber who replaced the vessel and he said this is normal and to give it 2 weeks for it to adjust.... Does this sound correct?

 

[Fitzroy]

Not in my experience, system should hold stable pressure equal to the charge pressure of the expansion vessel, assuming filed and well bled to get all the air out.

 

[Bingy man]

I’m going to say was the correct fault diagnosed, so many vessels get replaced for incorrect diagnosis . What was the original problem ? Once the vessel was replaced , filled , and air removed and ran to test - the pressure should not drop . As the system ( rads ) warm up the system pressure will rise slightly but should not go into the red section. In heating mode keep your eyes on the system pressure ( 1.2 1.5 from cold . ) If everything is set up correctly then the pressure could rise to around 2 bar give or take. If the pressure exceeds this eg ( 2.5 -3.0 bar or above ) it’s possible that the capillary tube or hose connecting the vessel to the boiler heating side is blocked or severely restricted. Your pressure safety valve opens at 3 bar and dumps the excess pressure outside. Your system pressure has now fallen and the whole process starts again as you will naturally top the pressure back up when you notice it’s fallen . Another possibility is a leak from the radiator pipework or an internal leak from the main heat exchanger which is unseen as the escape of water drains via the condensce pipework. It only takes a small amount of water to leak out and drop the pressure. Is your filling loop via a small flexible tube and a tap/ valve ? Or is it built into the boiler ? If the valve is not turned off correctly or the valve is passing then this will increase the system pressure which will activate the safety valve if it pushes the system pressure above 3 bar .. long post but a few of the possible causes of pressure loss ..

 

[Sideways]

I agree with the guys above but the one caveat, that the system was properly bled after the expansion vessel was swapped, is important.
If the plumber didn't or wasn't able to get all the air bubbles out of the system at the time, then these should be swept around the system and may eventually be released at an automatic air vent if you have one.

If enough (a lot) of trapped air was involved, this might end up affecting the pressure but repeated pressure drops and top ups in under 24 hrs sounds like they did something wrong. Call them back.

I have a late 70's house with solid concrete ground floors and all the downstairs radiators are fed by drops from above. Most of the plumbing is under the upstairs floors. These are notorious for getting air locks and bubbles whenever the system is partially drained so I've fitted 4 automatic air vents in addition to the one in the boiler and self bleed valves in all of the radiators. These get rid of most of the air bubbles within a few minutes but small ones can still take days to get rid of. These small bubbles don't cause a drop in pressure because they are small compared to the expansion vessel.

 

[Bingy man]

The but that makes me dubious is - “ give it 2 weeks to adjust “ rather than - I’ll call back round check everything is ok ( giving the cust piece of mind and reassurance- whilst at the same time checking you haven’t done anything daft like left the filling loop partially open or the drain off isn’t fully closed ( easy to do after a long day )