Boiler overrun

[Tim Lamb]

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?

[tabbypurr]

everything stops & goes out, except the pilot light if a boiler has one. What is the unknown there?

You can always leave some air in the top of a large enough radiator.

NT

[Tim Lamb]

Er.. whether the thermal energy in the heat exchanger is sufficient to overpressure the system with no circulation.

I could if there were any:-) Fully underfloor system. Curiously, I am almost certain there is a second pressure vessel inside the thermal store DHW tank.

[John Rumm]

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).

[ARW]

The pump overrun is there to remove the excess heat from the boiler.

The worst a powercut can do on a W plan is cause an overheat trip on the boiler stat.

[Tim Lamb]

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]

Hmm.. the only symptom of an issue was the unexpected zero pressure reading. Fired up and operated normally without any other adjustments.