"Ed Sirett" wrote in message
news
On Mon, 18 Oct 2004 17:06:43 +0100, Wainscotting wrote:


"Ed Sirett" wrote in message
news
1) From completely cold with the instant switch off does the boiler
fire
when you run a tap? If no then the problem is with the DHW flow sensor
(diaphragm) or more likely with the micro switch this sensor triggers
or
its operating mechanism.


Just tried this and no, with the instant hot water switch in the "off"
position and hot tap running, the boiler will not fire up and there is
no
evidence of the pump running. If I switch to instant hot water whilst
the
tap is running the boiler will fire up no problem.

OK. The problem is definitely with the DHW flow sensor.
This is a differential pressure type with a rubber diaphragm.
1) This could have split.
2) The pin it pushes could be stuck
3) the microswitch it moves could be duff.
4) the wires to the switch are broken.

When you put the instant (hot standby) switch on the boiler fires because
it is trying to preheat the innards, as you draw HW the preheating is
thwarted and the boiler delivers HW after a fashion.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at 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


Thanks for all your help and advice Ed. I will investigate the DHW flow
sensor. Much appreciated.