In article ,
fred writes:

No great insights I'm afraid, at least no more than I am sure you have
gleaned from just watching the boiler in operation, as you have described
above. I've guessed that the pump speed increases in stages to match the
power output but that is just based on what would seem sensible to get
the increasing heat away.

Andy Gabriel seems further ahead with his control system than I am and I
am hoping he will publish new insights as he comes across them, much
as he did for his recent slime beast solution ;-). I've picked up a lot from his
posts on this boiler and his system, also quite a bit from yours, thanks.

Cheers. Some things I have gleened -- may not all be true...

I think the knob sets the flow temperature -- using a clip-on pipe
thermometer (not accurate), I calibrated it from 30C at the lowest
setting through to 85C (IIRC) at the highest.

The pump seems to run at full speed until the temperature differential
across the flow and return starts getting small, at which point it
slows down in a number of steps. This is presumably to achieve a better
temperature differential.

The gas valve electronic control seems to be a straight on/off affair.
The gas rate seems to be varied by the fan speed, presumably by using
the pressure drop in the gas/air mixer to control the gas flow -- there
is a small hose from the mixer back to the gas valve which looks like
it might be providing pressure feedback for this.

If there are not enough radiators operating to absorb the 7kW minimum
boiler output but there is still a call for heat, the boiler seems to
let the water temperature creep up by about an extra 5C before cutting
off the burner completely. There is then quite a large hysteresis
before the burner fires up again.

Several people here have noted the temperature control is not
adiquate for ideal heating and hot water in a condensing system.
Another thing the Keston is missing is provision for connecting
remote frost stats. I believe it has an internal one which will bring
the boiler on until the return water is 30C. However, in my case the
boiler is in a room where this is unlikely to trigger, but much of
the pipework is in an unheated area under a suspended floor where
it's more likely to go below freezing. Ideally, I should be able to
trigger the boiler's frost protection operation remotely from this
location too. I have temperature monitoring in this location, and
intend to do this by causing it to run at lowest temperature setting
(30C) when this area goes below zero, with no other call for heat.

--
Andrew Gabriel