"Doctor Drivel" wrote in message
reenews.net...

"Andy" wrote in message
...

"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...


Surely the heater exchangers must be different from model to model,

Matt, why should they be? Motor makers have done this years. The BMC A
series engine block was used in 700cc to 1300cc engines, although with
heavier cranks and different cly' heads. Same block though. heat exch =
block. Some mods here and there and a new model with different
performance to add to the range.


You're actually partly right here. The Mini (and variants) used 2 blocks
which were rather different. The lighter one was used for the 850 to 1100
cc varieties and the other for 1275 and Cooper engines.

There were other A series engines such as 803cc but they were inline engines
and slightly different again.