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.

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.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
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