Bypass valve setup

2 Aug 2010
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United Kingdom
I have a Honeywell Bypass Valve DU145

Need some info from the experts about how to set it up on a S plan.
Is it supposed to be totally closed when both CH and HW valves are open and then only bypass when both the valves are closed
or is it supposed to bypass a little even when both valves are open to maintain a higher temp in the boiler .
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It's meant to maintain minimum flow rate through the boiler, check the boiler's instructions for that figure. So it should be closed when there's a demand and open when the zone valves are closed.

It's difficult to check the flow rate so I usually just catch it on pump overrun with the valves closed and open the bypass until I can hear a reasonable flow. Not 100% accurate but it does me!
Just set it on dhw only, and close the balance valve.
Do remember to open the valve after you finished setting up the abv
I was using the same tune by ear method :LOL:
If it was set too low the rads didn't work so I ended up on about 4
The original heat dump on the system was unwittingly removed(not by me)
and I can't rember exactly how it was plumbed but I think it had the flow on the valve 22mm outlet pipe and the return on the 28mm boiler return although the rad used to work fine when the CH was on
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I think for some reason that the large kitchen rad worked when the heating was on but like I say that circuit was removed by mistake when I had the pipes re arranged. It either worked by convection or it was getting flow from the bypass valve outlet.
Not sure it would flow very well as both flow and return in effect are just connected to the main return so no pressure differential
Maybe when the DHW got up to temp and its valve shut the bypass opened up a bit more

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