CH System - By Pass Valve Alternative

[Davidm]

My CH system has a by pass loop with a Honeywell DU145 Auto Bypass Valve. I'm now on my 2nd valve, and I think that has also failed open like the first did - it seems a common problem with these valves.

I'm thinking of replacing it with a Honeywell D4043 2 port valve, plus a relay wired such that the valve gets it's ON supply from the CH pump feed, but ONLY when both the CH and HW zone valves are OFF (i.e. it only opens when the pump is in overrun mode). I have one radiator with no thermostatic valve, so there's always one "open path" through the system when the CH zone valve is open. The wiring is pretty simple, I've not bothered to try and draw it here, might even add an indicator light.

Anyone see any "no no's" with this?

[Roger Mills]

I'm assuming that you have an S-Plan system with two or more 2-port valves - one for each zone. In that case, why not use a normally open valve[1] which has to be powered shut. Connect its motor to the paralleled output from the zone valves' micro-switches so that the valve closes when one or more zone valve is open, and opens when they are all closed. That way, you won't need a relay.

[1] These are mainly used in solid fuel systems, and are designed to fail safe - i.e. open - in the event of a power failure. They may only be available in 28mm - but you can use reducers.

[Cynic]

Considering that some boiler manufacturers simply need a fixed small "bleed" circulation I've never been convinced that the inclusion of one of these devices bore scrutiny. I simply fitted a lockshield bypass loop and set it to a very low flow