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 water) down. If you have a tank stat, that will regulate the temp in the HW tank via the boiler/valves.
John
Hi All,
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?
Francis.
In article , John writes
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.
HTH
Yeah, there is a cylinder stat
Francis
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.
Christian.
It's a lever operated 28 mm full bore valve in the return line just after the cylinder.
Treating it as a big rad seems to make sense.
Francis.
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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.
Thanks Ed,
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?
Francis.
In article , Francis writes
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.
Thanks fred,
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")
Francis
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