Boiler increasing pressure with filling loop disconnected, new boiler same problem.

[Ltnman]

In 2009 I posted this thread about my Worcester Bosch boiler, that was 9 months old and was increasing pressure with the filling loop disconnected.

https://www.diynot.com/diy/threads/pressure-increasing-on-a-worcester-combi-boiler.166884/

Opinions ranged from internal holes, corrosion between the dhw and chw waterways and corrosion due to a lack of inhibitor and to bleed the radiators for gas and add more inhibitor despite me already putting a nail in the Magnaclean, which had already came out rust free. The system clearly settled down, as I stopped posting to the thread and in fact forgot all about it.

Forward to 2023, when my one year old Ideal Vogue had its first service. I noticed that after the service the pressure was creeping up by half a bar with the filling loop disconnected. After testing the inhibitor using a Sentinel test kit my X100 was under strength so I added a bottle via the Magnaclean and drained water from the rad to reduce the pressure by half a bar. This I had to do more than once, as the pressure kept creeping up. It then settled down.

Last month my boiler had its second service where I asked the engineer not to increase the pressure, as I run the boiler at one bar cold so he didn’t touch it. After testing the inhibitor again after he had left I found it was again under strength so I added another bottle again via the Magnaclean without touching the filling loop and then run the system for 30 minutes to circulate. The boiler then settled back to 1 bar when cold. 3 weeks later it is at 1.4bar. The heating hasn’t been on since running the boiler for 30 minutes. Yes well within limits but there is a trend here where on two boilers the pressure increases for no reason, this time after 2 services. What is going on, as this is a repeat of 2009 and last year?

 

[Johntheo5]

Knowing the precharge pressure is vital.

Is/has the filling loop disconnected and is it disconnected since it was last topped up, what is the vol and precharge pressure of the EV.
A 10L EV with a precharge pressure of 0.75bar will have a reserve vol of 1.25L at 1.0bar charging pressure, at 1.4bar the reserve vol will be 2.71L.
What was the pressure with a hot system?.

Is this a combi boiler?

 

[Ltnman]

Note that the heating hasn’t been on since a 30 minute run 3 weeks ago.

One leg of the filling loop is disconnected and is just hanging.

The system hasn’t been topped up since the first service, as I forgot to mention to the engineer that I kept the system at 1 bar due to a previous leak in 2010 that needed a bottle of rad seal to stop a hidden leak.

The Manual shows the system pre-charge in 2 columns as none and 1bar. Vessel charge as 0.5 to 0.75. A system volume of 100 litres shows the expansion vessel as 6.3 litres that is associated with the none column and 7.4 litres that is associated with the 1 bar column, while 75 litres is 4.7 litres and 5.5 litres.

The system is a 32kW Combi feeding 2 towel rails and 11 rads, 5 of which are doubles in a small 4 bedroom 1990 built detached.

I bled the system but it didn’t change the pressure and next to no air came out.

When hot the pressure increases to just below 1.5 bar from 1 bar. Hot being 50C as I run the heating long and low.

 

[Ltnman]

EV is 10 litres

 

[Johntheo5]

OK, its unlikely that a precharge pressure of 0bar was used so a few numbers below at 0.5bar and 0.75bar precharge with 1.0bar charging pressure for both. (10L EV) system hot temp 50C
precharge/filling/final pressures & reserve vol.
0.5bar/1.0bar/1.35bar/2.5L. If the filling pressure increases to 1.4bar then the vol rises to 3.75L, a increase of 1.25L.
0.75bar/1.0bar/1.29bar/1.25L. If the filling pressure increases to 1.4bar then the vol rises to 2.71L, a increase of 1.46L.

If this increase in pressure is due to water ingress then the only logical explanation is a leak in the plate heat exchanger since the filling loop is disconnected?

 

[Johntheo5]

I see this boiler has a DHW preheat function, most boiler makers now stipulate that a EV is installed at the boiler cold mains to avoid PHEX failure.

 

[dilalio]

Put an external gauge on the system... Radiator vent tapping is the easiest place and see if you get that rising.

You could also do this with flow and return valves from boiler off.

This would be a simple experiment to implement to see if you can repeat the anomaly elsewhere!

 

[kidgreen61]

Turn off the cold water supply to boiler underneath overnight, if the pressure remains constant, then you will almost certainly have a pin holed plate heat exchanger allowing mains cold water to enter heating system.

 

[Ltnman]

If I reduced the pressure on the central heating back to 1 bar by bleeding a radiator and then isolating the CH by turning off the CH boiler valves from underneath the boiler, would this not highlight any fault quicker as the built in gauge would only be monitoring a build up in pressure of the boiler and not the whole system?

 

[dilalio]

If I reduced the pressure on the central heating back to 1 bar by bleeding a radiator and then isolating the CH by turning off the CH boiler valves from underneath the boiler, would this not highlight any fault quicker as the built in gauge would only be monitoring a build up in pressure of the boiler and not the whole system?

Yes. But be aware valves on some boilers can fail when used.

 

[Johntheo5]

It took 3 weeks for the pressure to increase from 1.0 to 1.4 bar and ~ 1.4L of water so pretty difficult to see any meaningful rise even over a 24 hr period, ~ 0.02bar/24hrs.

 

[Ltnman]

I have removed 1.4 litres of water to get the boiler down to 1 bar again and have isolated the CH by turning the valve off allowing me to keep the hot water on. I will now play the waiting game and will report back if the pressure changes in the coming days.

 

[Johntheo5]

Have you used the DHW preheat function and is there a PRV on the cold supply to the boiler?, if no PRV then any expanded water can flow back to the mains so no excess pressure in the PHEX, of course it could still be pinholed.

 

[dilalio]

You sure there's not another filling loop somewhere??

 

[Ltnman]

Pre heat is switched off and there is no other filling loop, as the Bosch had a filling loop above the boiler that was removed when the boiler was replaced.

What I am struggling to get my head around is why did I have the same issue last year after the first service that eventually cured itself and why did the same thing happen in 2009 with my Worcester boiler. Maybe a coincidence? The difference now is that for the first time I have isolated the CH today. My assumption is that if the boiler is faulty the pressure will now increase quickly as it is only pressurising the boiler and not the whole system.