In an earlier contribution to this discussion,
Ed Sirett wrote:


I seriously suggest you read all the FAQs available on heating,
plumbing and boilers. (Sorry if you have done that already).

The mechanism for modulation is essentially one of two forms:

On conventional burners the gas pressure is varied typically over a
range of about 5:1. Giving a power range of about 2.5:1

On a premix burner the fan speed is adjusted thus changing the flow
rate through the burner. Typically the fan speed would alter over a
range of 2:1 or even as much as 3:1 giving a power range of 4:1 or
even as much as 9:1.

The gas valves for conventional modulation usually are fed with 230V
for one/off and a signal of 0-9V DC which alters between the lowest
and highest rates.


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?
--
Cheers,
Roger
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