Vizo 24 Boiler - Losing/Gaining Pressure - Some tests done

darren72

We have a Heatline Vizo 24 condensing combination boiler which is about 3 years old.

For the last few weeks the pressure has been dropping slowly (when not being used) from an initial pressure of 1.5 bars, to around 0.8 bars - resulting in the boiler not coming on. When it is on, the pressure increases to over 3 bars over a 15-30 minute period, and therefore cuts out.

We also tried testing the pressure in the expansion vessel with some difficulties, but this was done about a week ago and the pressure was reset to 1.5 bars (the manufacturers recommended amount).

We have isolated the radiators by turning off the two flow/return valves on the boiler. That has resulted in the pressure still dropping (by around 0.2 bars) over a 12 hour period. (The boiler wasn't on at all during this time).

We have checked the plastic condensing pipe on the boiler for water by placing the end of the pipe into a saucer, and leaving the boiler off for several hours. During this time nothing came from the pipe.

The copper overflow pipe (the one outside) has been checked by attaching a plastic bag to it for a few days. The result has been a very small amount of water coming from it - but it is difficult to tell, as it may just be a little condensation.

Does anyone have any other ideas on what may be causing this, or any other tests we can do to try and narrow down the cause of the issue ?

Thanks in advance for any assistance you are able to provide.

southcoastrgi

who tested the expansion vessel pressure & how did they test it ?

macman

Nothing is going to flow out of the condensate pipe unless the boiler is running and condensing. That has nothing to do with the pressurised part of the system. With the CH circuit isolated and tested, it's ether the EV or the PRV. To test if it's the PRV, you need to run the boiler to see if it vents when the pressure increases, as it will when the system is hot.
Once a PRV has lifted it will often not reseat fully and has to be replaced.

southcoastrgi

mm, not strictly true you could get water out the condensate if there was a fault, also the boiler doesn't have to be running for the prv to let by

macman

Point taken, but given the depressurisation issue, testing it when hot/firing will give a more immediate indication of a PRV problem.