In article ,
Paul Roman writes
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?
The idea is to set the bypass pressure on the automatic bypass such that
it will only open when all (or most of) the TRVs on the system rads have
closed and are up to temperature. When the bypass operates, it stops the
boiler seeing and 'open circuit' giving rise to pump and boiling problems.
When the boiler sees the return water rising in temperature, it will
modulate down and eventually turn off, which is what we want as the house
has reached the desired temperature. It will restart again when the temp
drops.

There is nothing to stop you having a towel rail on bypass and an
automatic bypass in parallel.

I experienced a problem (with a near identical system to yours) where the
bypass was close to the boiler and would tend to 'pop off' rather than
gently increase in flow as it was coming into operation. This resulted in a
nasty, but slow, pressure oscillation when the system was running on
bypass. To cut a long story short, I chose to limit the capacity of the
bypass by placing a 15mm lockshield valve in series with it. This was
enough to damp the oscillation.

HTH
--
fred