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