I'm in the throes of (slowly) fitting a new central heating system. The
house is a 2 bed terrace in North London. The boiler is a Keston Celsius
with the system kit and is located in the first floor bathroom. I've
removed the middle section of the chimmey breast to about head height to
create a cupboard and run the flue and intake up the chimmey. I've got
seven rads sized according to the Myson Heatloss calculator (although
I've reduced the size of the kitchen rad as the program can't take
account of all the cupboard volume). The two bedroom rads will have
thermostatic valves but the main living area (living room/dining room)
will be controlled by a room thermostat. I'm setting it up as an S-plan
with a Landis & Staefa RWB9 programmer set for 5+2 operation.

In any case I was looking around the Honeywell site as a result of
another thread (Central Heating - Controls) and I noticed this:-

"If a bypass circuit is fitted, an automatic bypass valve must be used.
(Slumber or Bypass radiators are only acceptable on solid fuel
systems)."

on this page:- http://content.honeywell.com/uk/homes/Regulations.htm

I was intending to use the bathroom rad with a couple of lockshield
valves as the bypass but it now seems I'll have to scrub that. :-(
Is that correct?

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