"Set Square" wrote in message
...
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.

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?

That is what I said

- in which case you'll waste a lot of electrical energy.

Not much at all. You could have another stat set 3 degrees above to switch
out the pump.

Even so, the boiler will still needlessly heat the water to 71C in mid
summer!

Nope. If the return cools the house has dropped in temp and/or the TVRs
have opened.