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.

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.

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.


Arrangement is


|------------------------ pump----4 zone valves to HW and 3 CH zones
Boiler
| | | |

|--------------------------------------------------------|----------|---|--
-|

Only unusual thing is the length of pipe between the boiler and pump - about
40 metres due to position of boiler in an outbuilding. Obviously it's 40m
back as well.
But temperature drop across this very well lagged pipe appears to be a
degree at most.

My guess is the pump doesn't circulate enough water and was thinking of
putting another pump in the return :

40m
|------------------------ pump----4 zone valves to HW and 3 CH zones
Boiler
| | | |
|------------------------
ump ------------------------|----------|---|---|
40m

Should this help ?

Any alternative suggestions or routes to investigate gratefully received.

Many thanks

Mike