On Thu, 10 Jan 2008 17:46:11 -0000, "Roger Mills"
wrote:

Most modern-ish boilers - which hold relatively small quantities of water -
require it. Old heavy cast iron boilers usually don't.
[snip]
If your boiler needs it, there should be something about it in the
installation manual. Also, if you look at the boiler's wiring connections,
there will be a permanently live input - in addition to the switched live -
and there will be pump connection terminals.

My boiler looks ancient and only has live and neutral, so I guess I'm
ok? It's also oil rather than gas; does that make a difference?

In situations that require it, how is it achieved?

For pump over-run to work, there must always be a flow path open for the
water to follow. A 3-port valve always has at least one port open. A 2-port
valve doesn't - and all (2 or 3) could be closed when all demands are
satisfied. You would then need a by-pass circuit - preferably using an
automatic by-pass valve - to provide that flow path.

I've heard of these bypass valves. Do you also use them if TRVs are on
all radiators? I've always left TRVs off the hall rad (thermostat) and
bathroom (towel rad) hoping that would be sufficient for whent he
other TRVs close. Do I need a bypass in addition or is that enough?

Thanks again.