Water – Pressure level in central heating system

When the boiler is running (especially in heating rather than hot water mode) watch the pressure gauge: does it rise almost all the way to maximum? And do you then get a sudden discharge of hot water through the pressure relief discharge (PRD) valve and pipe to the outside of the building? (If the boiler is mounted on an external wall the PRD pipe usually goes directly through the wall behind the boiler.)

If so this is where you're losing water (and hence pressure). It indicates that the expansion vessel (probably built-into the boiler) is faulty. It may just have lost pressure (there's a Schraeder(sp?) car-tyre-style valve you can use to top it up) but could have failed entirely (e.g. if water comes out of the air valve!). For expansion vessels built into boilers it's often more trouble and expense than it's worth to replace the fitted one and quite acceptable to fit a vessel externally.

The uk.d-i-y usenet newsgroup is generally a good place to discuss this sort of problem. There is also a wiki/faq at http://wiki.diyfaq.org.uk/

Your diagnostics so far point to the indirect hot water heater coil leaking potable water into the furnace loop as the most likely issue. You could perhaps verify if you can shut your hot water down for a few days (not convenient, I know.)

Depending on the overall condition of the indirect water heater tank, the coil can often be replaced as a part, if the indirect hot water tank is otherwise in good shape. Some are not designed this way, however.

While your test might leave room for a failed expansion tank, if you did not cool the boiler down too much (so the cool water then expands as it's heated; which with a failed expansion tank would raise the pressure significantly, and with a functioning one will raise the pressure slightly) it's probably valid, especially if the pressure rise took days, rather than shooting up the first time the boiler fired to temperature (a sure sign of expansion tank troubles.)

For a typical "modern" bladder-type expansion tank, there's an easy, quick test of the tank integrity - drop the pressure to zero on the system again and check the pressure on the air fill valve for the expansion tank. If it's also zero, the tank (diaphragm/bladder) has failed. The even quicker (if crude) test is to "burp" the air fill valve very slightly (don't want to release significant air or you need to drop the system pressure and refill it) to see if air or water comes out - if it's water, the tank has failed. I would expect your service technicians to have already done one or both of these tests, given your symptoms.