In message , John
Rumm writes
On 10/02/2019 19:33, Tim Lamb wrote:
In have been running the cottage heating on tick-over (all the
roomstats set to 15 deg. C)
Yesterday I found the system had depressurised!
Apart from the panic about screws through underfloor heating
pipes:-) what would happen to a W plan system during a power outage?
The Veissman boiler has several minutes of circulation on the
internal pump and the system is happy with DHW thermostat reaching
set temperature and there being no call for room heating.
Based on the thermal expansion figures and discussion in here I
calculated that the* vessel fitted in the boiler would do the job.
There was a fair bit of teeth sucking from the boiler installers but
they didn't offer any alternative calculations.
Looking at the sizing tables from the vessel manufacturers, I am a
fair bit under the recommendations. Could this be why?

The way to tell would be to use a rubber band and a sandwich bag tied
over the end of the pressure relief pipe. If at any point it does vent,
you would see some water collected in the bag. (normally it would
remain empty).

This has happened once in about 4 months! The consumer unit is next to
the boiler so I can knock the power off while the boiler is running full
chat and watch the pressure gauge.

I'm just gathering the intestinal fortitude to try it:-)

--
Tim Lamb