In article ws.net,
"Doctor Drivel" writes:
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.

This fails to comply with Part L.
Such a scheme would need to use a flow switch to
detected all zone valves closed and keep the boiler
off.

--
Andrew Gabriel