"Christian McArdle" wrote in message
...

To have TVRs on all rads one method is
have an auto by-pass valve fitted
between the flow and return. Have a boiler
which can maintain a constant
flow temp. Have a it set to 80C. Balance
the system to 10C across the rads
and boiler, if an existing system. Install
a pipe stat on the boiler
return, so that the burner is off at 71C.

How is interlock achieved?

Via the pipe stat.

You don't have a flow switch in this design.
Explain how the burner never fires if the
outside temp is, say, 30C.

The pump is always on. Lag the return pipe around the pipe stat too.

As it stands, this system will cycle,
as when the water drops below 71C, it
will fire up again even if there is no call for heat.

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.