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 main radiators in any house are in the hallways or reception rooms
(front lounge) so this radiator should, in actual fact, be your by-pass
loop. The proper thing to do with any heating system, is to place a
thermostat in the same room or area as the by-pass radiator. This way the
system is actually controlled by the loop and not from any radiator willy
nilly telling the system that it is to hot and should close down
everything.


I'm a bit confused by this.
If the bypass valve is normally shut your primary rad will be mostly out of
action until such time as all the other rooms are up to temp as specified by
the TRV's. Because the room that the primary rad is in is still cold the
boiler still has a heat demand which then flows through the bypass to heat
that area until the room stat says ok and shuts the boiler down?

Correct?
What actually causes the bypass valve to function?
The Heating and Hot Water Guide to Part L states that:

"If the boiler manufacturers instructions advise that a bypass is to be
fitted, as a requirement ot the new boiler, then an automatic bypass valve
must be used in conjunction with any requirements for a minimum pipe length
specified in the manufacturers instructions."

Now if Keston specify an explicit temperature gradient for the loop then it
implies that whatever length of pipe for thet to happen is necessary. It
also doesn't specify that a rad cannot be used to achieve this.
However, if the bypass isn't normally an open loop now then there is a
potentially large functional loss to the system

I'm concerned by this as I am about to replace my boiler with a Celcius 25
and convert to a fully pumped system probably W plan with an open loop to
feed towel rads as a bypass.
It seems I may not effectively be able to do this any more and will be in
the same boat as the otehr Paul.

cheers

Paul