"Mike" 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.

Grant were one of the early entries into the Condensing oil boiler market
but are not one of the frequently encountered manufacturers up North at
least. Is your model a condenser or a non condenser?


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.


Have tried the pump (Grundfos Alpha) in every position on it and all give
the same effect, though some vary the time taken that the boiler spends on
or off. But typically it is 2-3 mins on, 3-4 mins off. Obviously if I
could get it to be on a bit longer and off less the house would be warm
again.

I think you should get hold of a differential thermometer with sensetive
clip on probes or a non contact IR thermometer and check the differential
temperature across each radiator under steady-state conditions. If your
boiler is non-condensing the differential should be approximately
10/11degreesC.

If you study the FAQ on balancing a system it will help you to understand
the aims of balancing a system correctly and if your radiator load is as you
mention the performance should be closer to the expectations. Does your
assumed load include the hot water demand also or is that on top of your
figures?
You will find that the boiler will operate cyclically but with a higher flow
rate you may find the thermostat responds better because the water moves
more "actively" within the boiler, if not consider replacing the boiler
thermostat


Whereas in warmer weather I didn't really notice the radiators warming up
and cooling so much now it is obvious that they are continually heating up
and cooling down with the boiler.

This can be minimised by matching the boiler output to the load (trivial but
needs calibration and analysis equipment) but you will probably find that
balancing the system will give better comfort levels and once the house is
warmed up you will not be so conscious of variations