"Colin M" wrote in message
...
Standard stored hot water gas boiler CH system:

Following the balancing of my system, I decided to upgrade my pump (all
done) but notice a gate valve in a funny place. Any idea what it's for? My
ASCII art is rubbish so I'll describe:

Hot feed from boiler to pump, from pump to diverter valve. Diverter valve
to
HWC and CH system.

Now, from the pumped output prior to the diverter valve there is a 15mm T
off through a gate valve to the return pipe of the HWC which presumably
returns to the boiler. The gate valve when opened would then feed pumped
hot
water directly back to the boiler bypassing diverter and HWC coil.

The system was originally installed with a condensing boiler (not combi)
which I since replaced with a "standard" boiler. Is the valve to balance
the
hot water side of things or to allow the pump to work OK on the overun
when
the CH and HW are satisfied? Surely the diverter valve will just open to
the
HW side when CH is satisfied and tank stat simply switch off boiler when
satis?

What is the gate valve for and how should it be set?

Colin



It sounds like a classical by-pass circuit - which you may or may not need.

Whenever the boiler stops firing (either on its own stat, or as the result
of a room stat, cylinder stat or timer going off) it needs to keep the pump
running for a little while until the boiler's internals have cooled down.
[If you look at your wiring, you'll probably find that the pump is
controlled by the boiler].

The water pumped during this "pump over-run" period has to have somewhere to
go - even if all the zone valves are closed - hence a by-pass circuit.

In your case, if you have a mid-position diverter valve, there *will* always
be somewhere for the water to go - UNLESS you have TRVs on all your
radiators AND they are all shut OR all the rads are turned off manually - so
you probably don't need a by-pass. [My system is like that, and I *haven't*
got one]. If you're brave enough, try completely closing the gate valve. If
the boiler trips on its over-heat stat, and won't re-start until it's been
manually re-set, that will show that you *do* need a by-pass - in which case
you will need to partially open the gate valve again. Have it open as little
as possible, consistent with preventing boiler trips. Better still, replace
it with a spring-loaded automatic by-pass valve which *only* opens when
necessary.

HTH,
Roger