"Andy Hall" wrote in message
...
On Wed, 29 Dec 2004 20:16:35 -0000, "Mike" wrote:

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


Basically the flow rate is too low, as is the return temperature. You
have about 25 degrees temperature drop on a design that should be
about 12 degrees.

I turned off the upstairs system and with the Alpha near max on the variable
pot settings, the return temperature went up to 76 deg. So it would appear
the pipe cannot source upstairs and downstairs so I'll need another parallel
pump and set of pipes from the boiler to upstairs.

However, even with the return temp correct, the thermostat still has a huge
hysteresis. It turns off at about 90 deg C but doesn't restart until the
output temperature drops to about 75 degC. At this point the input
temperature is still the same, i.e. a degree higher going into the boiler
than coming out, presumably due to the large thermal mass of the water in
the system.

Grant are "on holiday" until the New Year. Any idea what hysteresis an oil
boiler control thermostat should have ?

Thanks

Mike