My recently upgraded CH system uses 1" pipe flow and return to DHW cylinder.
I think the 1" pipes were part of the original gravity fed primary.
System now fully pumped with zone valves for water and radiators.
There is a full bore valve on the return just after the cylinder. How should
this be set?
Fully open I would say. You probably don`t want to be restricting the flow
to and from the HW tank as this will slow recovery times (reheating the
If you have a tank stat, that will regulate the temp in the HW tank via the
Actually, it depends on both the boiler and the cylinder; by leaving the
valve fully open you can actually increase the recovery time.
Treat the cylinder like a huge radiator and set the valve to give a 10 degree
drop when the cylinder is cold. After all you wouldn't run a radiator with the
lockshield fully open . . .
If you have a high output modulating boiler and leave the cyl valve fully
open, the cyl is unlikely to sink all the heat in the flow as it whizzes
through the coil with the result that the boiler sees a high return temp and
modulates down, giving a longer recovery. Balance the cylinder like a
radiator and you will see maximum power transfer with the boiler at max
power for longer.
Also, if you leave the valve open, the loop pressure will fall to near zero
meaning no flow in the rads & no heating for the reheat time. Ok no bigee,
but my boiler is big enough to give a 20min recovery _and_ heat the house
at the same time so why not tune the system to take advantage of it.
It might easily rob the rads of almost all the flow if left full wide.
I'd say the best way would be to balance the HW coil temperature drop
against the radiator drop.
The reheat time is a function of the cylinder unless the flow rate to it
is really small.
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Have read the balancing faqs.
Your "I'd say the best way would be to balance the HW coil temperature drop
against the radiator drop" has me a littled puzzled.
I balanced the radiators with the hot water zone off.
Do I now open the hot water zone and balance that like a radiator or what?
I know you asked this of Ed, but here's my thoughts too.
I think it was just tune the cyl to drop the same as the rads eg.
conventional boiler, with say 10 degree drop then go for 10 degree drop on
cyl but if you have chosen 15 on the rads then go for 15 on the cyl. I differ
a bit in that I have a condenser and chose a 10-15 degree on the cyl vs
15-20 on the rads, so the cyl tends to steal a bit from the heating when on,
a sort of HW priority by default.
As I would/did.
I would say no, the H/W cyl is such a big 'rad' that it could change the
balance of the system. CH + HW demand is such a transient condition
that you don't want to risk compromising the operation of CH only by
tuning it for CH + HW. I'd say balance CH and set cyl drop separately.
Good advice is acceptable from all.
With info from the group I upgraded my system from a one zone system with no
controls other than a timer.
Now have Heating 1, heating 2 and water. Also laid in pipe work for kitchen
with two valves so I don't have to drain whole system to add new rad that
Santy might be bringing.
Didn't have the option to go to sealed system. No over-heat thermostat on
back boiler. Recent service showed "Heat exchanger rusting. Might go now or
could last couple of years". So now researching for new boilers.
Got an overheat thermostat kit from UK for half the price including delivery
than it would have cost over the counter here in Ireland. Thinking of going
to UK for boiler. ("But Mr Ryanair, it's hand luggage")
This is just in the return line and across the flow and return?
If it is just in the return line, it is probably for balancing the system.
How to set it depends on your zoning arrangements. However, if the circuit
is properly zoned, thermostated and pumped, I'd set the valve wide open.
This will cause more rapid recovery of the cylinder. The radiators can
normally be made to wait until the cylinder stat is satisfied. Set the hot
water to be on 24 hours a day, or an hour before the heating.
If it is across flow and return, it is probably an old style permanent
bypass, the setting of which is a black art that can be avoided by replacing
the valve with an automatic bypass valve.
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