Combi boiler pressure building up steadily
Looking for some opinions from heating engineers/plumbers.
I have a 15-year-old Vaillant ecoTEC Plus 824 combi. For a long time I’ve noticed the system pressure slowly increases over time. Not quickly, we’re talking weeks rather than days.
Originally I’d top the system up to around 1.3-1.5 bar cold, and over a couple of weeks it would gradually climb towards 3 bar. I’d then drain a bit of water and repeat.
I had an engineer out recently. He found the expansion vessel had lost its charge and was waterlogged. He drained the system, recharged the vessel and refilled the system to about 1.3 bar cold. He also tightened the filling loop valves.
Since then:
* Cold pressure is now around 1.8 bar
* Hot pressure is around 2.1-2.3 bar
* Pressure rise when heating is only about 0.4-0.5 bar
What I’m struggling to understand is this: before the expansion vessel was recharged, where was the extra water coming from? If I was periodically draining water from the system but the pressure kept recovering over time, surely water had to be entering the sealed system from somewhere?
One thing that makes me suspicious is that over the last few weeks the heating hasn’t really been used, only hot water for showers, washing up etc, and I still thought I was seeing pressure creep upwards.
The engineer mentioned the possibility of an internal leak through the plate heat exchanger letting mains water into the heating circuit, but wasn’t convinced enough to start replacing parts.
Does this sound like a filling loop issue, plate heat exchanger issue, expansion vessel issue, or am I misunderstanding what can happen after an expansion vessel loses its charge?
Interested in what you’d check next before spending money on manufacturer diagnostics.