A commenter on this blog has helpfully noted that he has found a way to
avoid cycling, by setting the controls to "return regulation". He has
a 392 controller rather than a VRC430:
"The 392 is also missing a "minimum flow temp" setting which makes masking the short/micro cycling all the more difficult. The only way I found was to set d.17 to return regulation which has the effect of increasing the minimum flow temperature and stopping the micro cycling."
In our hands, this change - on its own at least, and in the context of a
VRC430f - does not help our systems, but perhaps it will help others.
To explain, the default state of the boiler is flow sensing, which is to
say that the boiler (and the controls) monitor the temperature of the water
leaving the boiler (the 'flow'), and uses this as the basis for controlling the
boiler. This can be changed so that instead the temperature of the returning
water (the 'return', unsurprisingly) is used. The change can be made by
altering parameter d.17 on the boiler from its default state of zero to one.
This change was made on Vaillant's first visit to the other author of
this blog, who has a 428 boiler, in June 2011. The Vaillant employee (Mick
Shorter, who is the relevant product manager - presumably for the weather
compensation controls) did not explain why he thought that might improve
things. Anyway, he changed d.17 from 0 (flow sensing) to 1 (return sensing) and
left it that way. Subsequent charts showed no difference.
I also tried this change on my boiler (a 438), with the help of my
installer. The result is shown in this graph, where the behaviour both before
and after the change is short firing (ignore the >70C peak, which is hot
water demand) (pdf of graph here):
I left it on return sensing for about three days, and the behaviour
stayed the same. I did not observe micro firing in this period, but that is not
to say that micro firing was abolished, since it is not always observed.
However, the effect on short firing - as shown in the graph - was
decidedly detrimental. Instead of the boiler staying ignited for maybe 20min at
a time, it only stayed on for about 3-5min before going into anti-cycling
mode. I therefore reverted to flow sensing.
Another poster on diynot.com also found that return sensing did not help:
"I'd be seriously miffed if there was a magic fix. I had hopes for d17 but
experiments showed it to be unhelpful."
However, the graph described above was before I knew how to rid myself
of short firing by using the Pump Delay Time setting (see page entitled
"What works and what doesn't"). I shall therefore repeat the d.17
experiment when the opportunity presents itself.
