"John Aston" wrote in message t...

Doesn't an automatic bypass valve stop this problem?

No. I don't think I was clear.

The first paragraph referred to a heating system with a mixing valve,
but only one pump, i.e., the way NOT to do it. The mixing valve was
between the pump and the boiler. At low loads, most of the flow
produced by the pump was re-circulated around the heating circuit
leaving only a small amount to return to the boiler. The boiler did
not like this.

The second paragraph ( "In the above case you might have a primary
pump………etc..") described how the system should have been set up, with
primary boiler pump and a secondary (variable temperature) pump
downstream of the mixing valve.

On a conventional 1-pump system, an automatic by-pass valve might be
connected between the pump's flow and return pipes, to maintain a
minimum flow through the boiler if TRVs are mostly shut down. It's
operated by the increase in the pump delivery pressure as the flow
rate reduces.




With the DHWS controlled by a zone valve, I guess.

Yes. You'd probably need a regulating valve in the heating return to
the boiler, which would be a straight pipe with little hydraulic
resistance. The water would otherwise by-pass the DHWS branch, which
would have a higher resistance. It could also be controlled by
another secondary pump but generally the domestic DHWS flow rate is
too small. My one's got a 3-port mid-position valve.

That sounds so reasonable. Unfortunately, none of the domestic
heating
design books I've read refer to anything other than one primary pump
plus
zone valves. Being a beginner, I don't know if I'd have the confidence
to go
for pumps instead of valves.

It (primary/secondary pumping) is very rare on UK domestic
installations, mainly because of the costs. IMM has mentioned a £250
controller; most domestic customers cringe at £50 for a replacement
heating timeswitch/on-off controller. Whether IMM's recommended
controller is applicable to this is another convoluted question, but
£250 is probably on the low side. With commercial stuff it's
tax-deductible, so they're less price-sensitive and are prepared to
pay for effective control.

You would be very fortunate to find a UK ‘heating engineer' who
understood such a system.

Also, the Americans generally have bigger houses, so it's more common
in the US. See ‘ Primary-Secondary Pumping made Easy' by Dan Holohan,
available from HeatingHelp.com. It's written for home owners, so it's
fairly free from techno-jargon. It's in US/Imperial units (gpm,
BTU/hr, degF) but it's easy to grasp.