On Fri, 17 Nov 2006 20:59:59 +0000, Doctor Drivel wrote:

"Ed Sirett" wrote in message
...
On Fri, 17 Nov 2006 09:34:49 +0000, Fred wrote:

"Ed Sirett" wrote in message
...
Hi all,

This week I put a 31kW Vaillant Condensing Combi in. The other month I
put
in a 24kW unit. As I was doing the job, I started to think is there
_anything_ in the boiler that's different between the two units? I had
sort of assumed they would be _almost_ identical but with perhaps a
bigger
fan, gas valve or burner/heatX.

Well I was showing my apprentice how you could find out about all the
different settings.
We looked up the fan speed when it was doing HW and it was 499 rpm, I
was
fairly sure when I commissioned the 24kW unit the fan speed was 395 on
HW.
Then the penny dropped, they are physically identical units to the last
nut
and bolt, except for a couple of bits in the firmware! The power is
proportional to the air flow rate which is more or less proportional to
the fan revs. 499:395 is very near 31:24.

I strongly suspect the 37kW unit is also the same (the fan will just go
yet faster). I did a 28kW unit about 12 month ago and that ran to the
mid
400s on HW.

Part of me is truly impressed with the design which makes a whole range
of
models from the same production line. Part of me is wondering by
what mechanism can a the huge price differentials be maintained ( £815
versus £1009) for essentially a firmware difference.

I suppose some people would say buy a 30kW boiler from someone else who
could potentially be cheaper for the higher powered models.

In Hong Kong there is probably someone who sells grey market chip
upgrades...



I would think it will be a combination of fan speed and burner pressure.
Is
everything else identical? One way of checking would be to look at the
spares list - I assume they have one.

What sort of fan motor is it? What determines fan speed? I presume it's
not a modulating boiler.

The manual and spares list is common to all models.

The gas valve acts more like a carburettor on this sort of boiler the
faster the fan moves the air the more gas that is injected.

How about injector size? same?

They use a "zero-governor" type gas valve. No injector in the usual sense,


The fan speed is altered by the PCB changing the voltage to the fan motor.
It is checked and fed-back to the PCB by a tachometer on the fan shaft.

The units modulate over a wide range by altering the fan speed.

Ideal do the same.


....
and Keston and Worcester and all the other that use forced premix burners.


--
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
Gas Fitting Standards Docs he http://www.makewrite.demon.co.uk/GasFittingStandards