In article , Paul
Roman writes
SNIP
Looking at the BES site (thanks Ed and why isn't it in the FAQ?), is the
Differential By-Pass Valve (12161) the approiate part?

Looking at the diagrams the bypass valve should be fitted from the flow
to the return between the boiler and the zone valves. But according to
the Keston installation instructions the flow/return temperature
differential should be 10 to 15 C. In the layout I was planning the zone
valves are only about a meter from the boiler. I can't see that short
run of pipework producing that much of a differential. Any thoughts?

Any other thoughts/observations would be welcome.

Thanks

Paul

Thanks for your reply , Fred.

I've now mailed and spoken to Keston and I'm still not clear about how
to deal with this issue. What is normal practice since Part L came in?
Are installers simply ignoring the boilers requirement for a temperature
drop across the by-pass circuit? Or are they creating an (artificially)
larger loop for the by-pass circuit? Or are they still using a rad as a
by-pass?

Thanks for your responses.

Paul
It is actually a lot easier than you think. Just think of the (automatic)
bypass not coming into operation at all when the heating is at full demand.
Its purpose is to provide an uninterrupted water path _only_ when the TRVs
on the rads are shutting down, which is a transient state just before the
boiler says 'right, the house is hot enough, I'm shutting down', so don't
worry about it. The absence of a temp drop across the bypass will tell the
boiler that it is time to shut down.

It is actually more complex than that as at first the bypass will come on a
little and the return flow will be a mixture of rad return water (at low temp)
and flow water ((hot) from the bypass). This will have the effect of
increasing the return temp a little, which will cause the boiler to throttle
back a bit. This is not a fault or a contravention of regs 'cos it is the normal
control loop coming into play and (eventually) shutting down the boiler
when the house is up to temp.

Hope that makes it a little clearer.
--
fred