On Tue, 28 Jun 2011 19:14:57 +0100, Vortex10 wrote:

re the XCel you get no significant hot water with no pump operation.

Ta, I didn't think you'd get much considering how much energy it
takes to heat water. IIRC the normal PHE with the XCel system is over
100kW for 45l/m water... (Now we know why combies are so crap at 40kW
for a big one...)

In my case the heatbank has 3 identical pumps with isolator valves, so I
purchased a spare. Pump failure is a 15 minute problem.

I don't envisage pump failure being a problem. I've spent a day
changing a pump in the past, flange nuts siezed onto pump and bloody
gate valves... Think they are screwdriver ball valves on the XCel
rather than gate valves though.

Don't forget there is also the option of a passive (90C???) relief valve
that would release water (from the top) to drain as the ultimate
failover.

It doesn't release the primary water. Mains water goes through a coil
at the top and thence to a drain. The coil is only 12kW so not enough
to use for normal DHW. But if all you wanted was a slow flow for
basin/sink filling under power fail conditions it would do...
Hummmm....

If you have no electricity at all then that's a different matter, so
either you could have a UPS backing up the whole heating system (Sub
£200 for a quality 500VA APC UPS). Where I live that would simply be
completely neurotic.

It is loss of mains power that I'm concerned about. It does happen
about once a year to 18 months on average. If the power does go it
will quite often be gone for up to 12 hours which is long enough for
things to get rather too cold in the middle of winter. It's only in
the last week that the heating hasn't been kicking first thing in the
morning and that's with the room stat set for 18.5C.

Trouble is with a UPS is the size of batteries required to provide
and sensible amount of up time. The XCel system I'm looking at has 5
pumps and the oil boiler, guesstimate that lot to be not far short of
500W. Computer type UPS's generally only have uptimes of 10 minutes
or so at full load.
That's why I have 2kVA genset...

I would stick with indoor rads for a heat dump. Antifreezing a 500+
litre primary circuit would be ridiculously expensive

Oh yes, indoor heat dump without a doubt I've paid for it I'm not
going to throw it away unless I *really* have to. B-) If it gets
too hot inside that's what windows are for!

--
Cheers
Dave.