On Thu, 30 Mar 2006 17:52:05 +0100, Jeff wrote:


"Tony Bryer" wrote in message
...
On Wed, 29 Mar 2006 18:23:53 +0100 Jeff wrote :
Can anyone explain how boiler modulation works, its benefits etc
and the same for weather compensation

In brief (as I'm just going home) modulation means that the boiler
includes a facility for varying the fuel being burnt (and thus the
output) - for example my Glow-worm 24Cxi combi will modulate between
5 and 18kW. When the temperature of the water coming back to the
boiler starts to rise (because the radiators cannot get rid of all
the heat being produced) the burn rate is reduced accordingly.

Think of it as driving a car: if you only had an on/off switch for
the engine you'd have to fire up, then switch off and coast for a
bit, start up again .... instead you ease off the throttle until the
engine is producing just enough power to maintain the desired speed.
Likewise a modulating boiler.

Weather compensation adjusts the boiler's output temperature to suit
the current outside temperature, so the rads will be hotter on cold
days, less hot on milder ones. This makes for greater comfort and
also enhances the boiler efficiency.


So in the above analogy... weather comp varies the 'desired speed' in effect
turning up or down the boiler temp knob ?

The control for the modulation must work on a fixed differential from flow
to return ?

Tony you said 'in brief' - i would love the long and detailed version as i
am trying to get my head around this for a future project.

Regards Jeff


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.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html