In an earlier contribution to this discussion,
Duncan wrote:

Hi all,

I have a standard y-plan gas heating/hot water system. The hot water
works fine, but for the last six months or so whenever I switch on the
central heating it works for around ten seconds and then cuts out,
with quite a loud "clunk" seemingly eminating from the Honeywell
V4073a mid-position valve. After another ten seconds or so, the boiler
reignites and the same thing happens again, and so on...

I've discovered that if I slide the slider on the Honeywell valve to
the "Open" position, then the heating works OK. But I noticed on the
pdf data sheet for the valve

(http://content.honeywell.com/uk/home...trols/2.21%20V
4073.pdf)
that "Continuous operation of the valve motor at the fully Open
position is not recommended".

So my questions are these:

1. What is most likely to be causing the problem?
2. Is it safe/sensible to leave the valve in the Open position until
this is fixed?

I'd be hugely grateful for any help.

Duncan Clark

It sounds as if the actuator (the electrical bit, or "power head") has
become deranged. This can fairly easily be replaced without affecting the
wet bit of the valve.

The valve has 3 positions - HW-only (un-powered/spring return position),
CH-only (powered - fully open) and HW+CH (powered - mid-position). Operating
the manual lever moves the valve to the mid-position - *not* to the fully
open position. I've no idea what the sheet means about not running
continuously at the fully open position. In very cold weather, but with the
HW demand satisfied, the valve may *need* to be at the CH-only position for
relatively long periods - and has to be able to cope with it. [The motor is
running stalled - but that's what it's *designed* to do!]

There's no reason why you can't run it in the manual position in the short
term (like a few days). BUT be aware that, in this position, you'll get CH
*and* HW - even if the HW demand is satisfied. The HW will potentially get
*too* hot, and you may have to turn the boiler stat down a bit - and not
have the radiators quite as hot as usual - in order to prevent this.
--
Cheers,
Set Square
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