"John" wrote in message
...
The recent very cold weather has thrown up a problem I hadn't noticed
until now.

Boiler is a Grant 70/90 Euroflame and it's 25 kW output should be
more
than enough to heat the house through about 20kW rated radiators.
And indeed it did when the weather wasn't so cold outside. But now
it's colder it can't keep up. Have traced the problem to the boiler
output
temperature rising up to the cut-off limit (just under 90 degC) then
turning
off until the pump circulates the water enough for it to drop to the
turn-on
value (appears to be around 75 degC).

With the boiler in it's on or off state, the return temperature is
about 65 degC plus or minus a degree or two.


The hysteresis your stat exhibits is perhaps a bit excessive and a
suspect
stat may be involved here, however your return water temperature
appears
to
show little variation between cut out and cut in on the boiler
temperature.
This does suggest the rate of flow through the radiator system to be
too
low.

What should the stat limits be. The upper limit is programmable via a
knob
on front of boiler but manual doesn't say if the lower limit is fixed or
moves with it.


You will need to ask Grant for this info but some boiler stats have a
small
differential adjustment screw concealed about the body somewhere. I'd try
to
be certain that the flow rates through the boiler are adequate before
upsetting the factory adjustment of this though

They're "on holiday" until Jan 4th :-(


There is provision for a hot water feed but it is currently not
activated
as
I am going to be using a small hydroelectric generator to drive the
immersion heater so hopefully the boiler won't be called upon for this.

Sounds good - do you live near a fast flowing stream? I have a hankering
to
move somewhere hilly with a rushing torrent when I retire and do something
similar. If the water is pure I'll install a sandfilter and UV unit to
become more or less self sufficient energy and water wise at least

Yeah - we have a farm in the Peak District. There is a spring at the top of
the hill which needs a 10micron, 0.5 micron, then UV filter to make it
anything like safe to drink. But we could probably put it in a bottle as is
and sell it as mineral water :-)

As for stream, we have a good flowing stream running along the side of us.
Total drop on our land is 10m but that would rather a major dam so will try
to achieve about 5 feet which will yield around 2 to 3kW for the hot water
and charging batteries when the mains goes (every few weeks).

Hope you find something similar.