Pump over-run is working, but ...

[ramchip]

...the zone valves are shut so the hot water in the heat exchanger is not circulating as it ought to.
Once heating demand is satisfied the 2-port spring return motorized valves (MVs) close, hence no circulation.
The wiring diagram in the boiler manual doesn't make any allowance for this, so at best it's inaccurate and at worst just plain wrong.
Reading the data sheet on the V4043 MV it states: "continuous operation of the valve motor at the fully energised position is not recommended."
In this situation what is the best way to open a zone valve on a Honeywell S Plan installation to allow the pump over-run feature to actually circulate the residual hot water?

 

[lightning]

What boiler do you have??? some have built in bypass,if not then your need a auto bypass just after pump

 

[ramchip]

The boiler is a very old but still capable Potterton Netaheat Mk 2F (16-22).
No sign of any bypass pipework. The boiler flow goes directly to the pump.
The output from the pump goes to the rads and the cylinder coil via MVs.
(There’s a T-junction in the pipe from the pump)
One solution would be to include a mains voltage AC relay that switches
on one of the MVs when pump over-run is in operation.
I have a suitable relay to hand. Could this be the answer?
I don’t know about auto-bypass, please explain.

 

[ChrisR]

You can do it with one relay but you'll find one of the valves would be held open all the time. Unless you can come up with a cleverer circuit than me! You can do it with two relays though.

Or you can put an auto bypass valve across the 2 port that goes to the cylinder, or back to the boiler.
They have a sping inside to only open on pump pressure, when other paths are shut.

Your boiler's 25 years old though, and it ain't broke...

 

[ramchip]

Thanks ChrisR. I appreciate what you say about one of the MVs being constantly held open using one relay. I’ll have a think tonight to see if I can get round it using two as you suggest. True the boiler ain’t broke … so no need to fix it!
Back in Feb. it was a different story and the old Potterton was having a rough year – I was getting explosive ignition and kettling. To top it all the cold feed furred up and I experienced ‘pump-over’ for the first time. When it happened I thought it was a jet aircraft over head, but no, it was hot water filling the header tank very noisily above my head! It was no joke having all these problems at the coldest time of the year. A new electrode and lead fixed the ignition problem. A system drain and flush fixed the kettling. A new inlet pipe to replace the furred-up one brought the system back to normal.
I plan to replace the old Potterton with a Worchester Bosch soon but I wanted to get the wiring sorted out in advance, hence the original post. I have a BEM5000 to go on this system so I need to be completely familiar with all the wiring. Actually, the pump over-run connection was not being used until I started re-wiring the heating controls.
It looks like the old Potterton is able to function well enough without removing the excess heat from the heat exchanger, but I’ll persevere for my own peace of mind, not to mention longevity of the replacement boiler.

 

[mrgas]

I thought the Netaheat MK2F 16-22 doesnt have pump overrun unless my mind has gone! (again)

 

[ramchip]

Hi mrgas, I think you must be confusing it with lower output models (6-10 and 10-16), which don’t have the pump over-run feature. The 16-22 certainly does; in the original installation manual it’s called ‘pump delay’ on terminal block connection 4. Later versions of the 16-22 have this on terminal block connection 6.