boiler cycling on & off on DHW

message from John Stumbles on 16 May 2004

Somebody has asked me about a problem with the hot water from their combi -
a 4 year old Worcester Bosch 24Cdi - which alternates hot & cold, with the
boiler switching on for about 20 secs then off for about the same time. The
initial problem they asked me about was with the shower but they say it does
this with demand from other hot taps. From the Cdis I've seen I thought this
was how they normally worked :-) but they say it used not to behave like
this.

Any clues what's amiss? Obviously the diverter valve is a potential suspect
but when I googled somebody (Ed I think) said these machines also have a
separate flow sensor. Not sure how that works, and I can't find the paper
copy of the manual I thought I had and Woosh Boster's web site seems to be
on strike atm. :-(

Another previous thread suggested a scaled up DHW heat exchanger or dodgy
temperature sensing thermistor might account for it. I've asked them about
the flow and temperature but I won't hear for a day or two...

BigWallop replied to John Stumbles on 16 May 2004

It could be a component problem inside the boiler, but check that the water flow isn't
to high as it enters the boiler, as a first test. I've known a few systems where this
has caused the same problem you describe.

Ed Sirett replied to BigWallop on 16 May 2004

I can't off hand see why a high flow of water would cause this problem so
if you have any more info I'd be interested.

Cycling is nearly always caused because the boiler can't get the heat into
the water fast enough or because a sensor is misreading.

Every combination boiler needs someway to switch on for water heating
there a number of different methods used by different models.

However if the boiler makes some HW the flow sensor is not the culprit.

Archived message: boiler cycling on & off on DHW (UK DIY Home Repairs)