"Mike" wrote in message
...


SNIP
.. 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 :-(


A thought occurs to me regarding temperature gradients through the boiler
shell. Most boilers have tappings for flow and return at either side of the
boiler. Some boilers are not happy if flow and return are connected to the
same side of the boiler as it "can" lead to still water (or at least slow to
move around) conditions surrounding the thermostat phial. What piping
arrangement do you have and does it agree with reccomendations in the
Installation and /service booklet for your boiler?
I would expect a replacement thermostat to be in the region of £20.00 or so
unless its a special. Most boilers seem to use a Ranco model stat