How can I find out why water pressure decreases over days in my UK flat?

Question

Intro:

I have a flat in Scotland, UK. We live on the 2nd floor. We have been here for about 18 months and we have a (new-ish) Combi boiler holding a static pressure measurement of around 1.6 bar.

Boiler water pressure indicator increases by about 0.2 bar when central heating is on.

First Notice:

We usually have no need to check the pressure and we found to our surprise a few days ago that the hot water (washing) was not heating, checking the boiler it had a stated pressure of only 0.3 bar, and so the boiler was not firing.

The water flowed out the taps are at a suitable speed.

I hooked up the repressuring inlet tube and raised the boiler pressure to 1.9bar. This is the first time we needed to do this in 18 months.

Ongoing Concern:

From that day, I have been keeping an eye on the stated pressure of the boiler -- and the pressure is decreasing by about 0.2bar a day, right now it is 1.1bar . There has been a little central heating usage in the last few days (1-2 hours only) but I'm curious as to why the pressure seems to be dropping so persistently and if this cause would bring it down to under 0.5bar as before.

Observations:

We live in Scotland. The water does not contain limescale.
I have extensively checked all visible pipes and no indication of any leak was found.
Tap pressure hot/cold are roughly equal, cold tap pressure is not an issue.
We have a new (18 months ago) Combi gas fired boiler fueling hot water and central heating.
My only point of pressure measurement is the boiler display which is dial rather than digital.
Radiator heating circuit is set to be turned on manually so we never have the heating on without doing so ourselves.
Boiler is "Logic Combi ESP1 30". Boiler was installed new at property purchase.

Other Q&A on here:

I have read Low water pressure which sounds similar but the question deals with a lot of (technical?) stuff I don't recognise (their pressure guage for instance). I also viewed and a few similar posts but which seem to deal with fast/sudden pressure drops rather than gradual persistent pressure drops over time.

Question:

It feels like something's changed because previously this has never been a notable issue and certainly pressure has never fallen below 1bar in the last 18 months.

What else could/should I be looking at/for in order to try and establish the cause of this?

How can I fix this if needsbe?

Ecnerwal · Accepted Answer · 2022-12-06T15:15:30.057

If the "sealed loop" portion of a hydronic boiler system is losing pressure, there's a leak. There's typically a small variance between cold and hot, (so it goes up a bit as it heats and drops a bit as it cools) but any drop in pressure at the same temperature is an indication of a leak.

If the pressure loss is slow, the leak is small/slow.

The leak can be located anywhere on the sealed loop. Hydronic radiators go "on" and "off" by whether a pump is running, and perhaps also a single valve on the radiator itself blocking or allowing flow, but they are connected the loop at all times unless there are valves completely closing off both supply and return paths, which would be unusual. So a leak can be in that portion without running the heat.

I would hope that at merely 18 months of age since newly installed your heating system supplier/installer would sort this out without additional charge, if its a defect in their work or the boiler itself.

score 3 · Answer 2 · answered Dec 06 '22 at 15:54

Presumably your boiler has a bladder-type expansion tank. If so, the bladder may have failed. The gas pressure in the bladder would force bubbles into the hydronic system, where they would be accumulated and eliminated by an automatic air vent. This may be the cause of your slowly falling pressure.

If this is the case, your expansion tank will eventually lose all of its air and it will be completely full of water. The next time the heating system warms up after that, there will be a pressure spike because the warmed, expanded water will have no place to go, and (hopefully) your safety pressure relief valve will spill some water on the floor to reduce system pressure.

If the Schrader valve on your expansion tank points downward, release a tiny bit of air from it by pressing the center pin. If water comes shooting out instead of air, the tank is failed and it must be replaced.