In an earlier contribution to this discussion,
Doctor Drivel wrote:


If the temp drops below 71C the system requires more heat and the
pipe stat brings on the burner. The design is 10C across boiler and
rads with 80C flow temp. If the 10C differential narrows (higher
return temp than 71C) the house has warmed up, so the burner is
switched off. An interlock that detects when the house has reached
temperature.
If the TRVs close up and the by-pass valve opens the return temp will
rise above 71C, then the burner will be switched off. The house
cools and some TRVs open and a flow of water goes to the rads
dropping the return below 71C, then the burner switches in.

If the design is 70C flow with 22C across the rads and and boiler
then set the pipe stat to 49 or 50C. It then ensures condensing at
all times, and no by-pass returning 75-80C water back through the
return and lowering efficiency, and ensures the flow temp to the rads
is low.

But as someone else has pointed out, there is a major falicy here - and the
boiler will cycle. As soon as the return *pipe* falls below 71C, the boiler
will fire again. It doesn't require the *house* to cool and a TRV to open.
So it won't satisfy Part L. Or are you suggesting keeping the pump running
all the time? - in which case you'll waste a lot of electrical energy. Even
so, the boiler will still needlessly heat the water to 71C in mid summer!
--
Cheers,
Set Square
______
Please reply to newsgroup. Reply address is invalid.