On Thu, 30 Mar 2006 22:05:22 +0100, Roger Mills wrote:



OK - that tells us *how* it does it - but what's actually driving it? In
other words what feedback parameter(s) - e.g. water outlet temperature,
return temperature, temperature differential, etc. - is it setting out to
control by modulating the burner?

Most boilers seem to govern the flow temperature. This is usually where
the NTC is placed.

A few seem to monitor both flow and return temps and some alter the
pump settings as well.
I strongly suspect that on my Keston C25 the front knob controls the flow
temp and that affects the burner power but the boiler senses the return
temp to adjust the pump in an attempt to get the differential higher.

Quite a few models seem to have a "cruise-before-decision" approach, in
which they operate at middling lowish power after ignition for around a
minute or so then ramp up (usually) to the required power.

In almost every boiler I've fiddle with, the algorithm/electronics is
usually sensitive to the rate of change of temperature as well as the
temperature. For instance if you move the control quickly down the burner
may cut out but if you move it slowly down the flames just reduce.


--
Ed Sirett - Property maintainer and registered gas fitter.
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