On 12/12/2019 09:44, Harry Bloomfield wrote:
MM used his keyboard to write :
Even if
one has the central heating on and it is then turned off, it's a good
idea to switch on the hot water briefly, even for only 30 seconds, so
as to reposition the valve beneficially.

I don't get it, or the logic?

Unless there is a call for CH, then a spring return 3-port valve will
always return back to the HW position, which is the reason they suffer
so much wear - constant back and fourth motion. They need to be powered
by the call for CH, to be moved away from the default HW position.

One common cause of failure is that of the motor - and that can be a
separate issue from wear on the mechanics.

If you look at the "normal" Y-Plan circuit:

http://wiki.diyfaq.org.uk/index.php/..._return_valves

Imagine the the programmer has DHW "off" (i.e. the "HW off" output is
live and the "HW on" one is not), and it and the stat are both calling
for heat on the CH. The valve will move to position A or "CH only", and
both microswitches will be closed (they are both open in the port B or
spring return default position). The room stat now, reaches set point
and opens. So there is no call for heat from either CH or DHW. However
because SW1 is in the position where the grey wire is connected to the
motor, and the wire is live because "HW off" is live, the motor will
remain powered, and the valve will stay in position A.

Should the programmer demand DHW now (even if the cylinder stat is still
satisfied), "HW Off" will go low, and power will be removed from the
motor - and that will allow it to spring return to the B position.


--
Cheers,

John.

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