Am I missing something

[Mike2007]

Honeywell three port mid position valve. Having just worked on one again (not done for some time, as I usually fit two port zone valves).
If the spring inside holds it in the central heating closed position (unpowered), then the hotwater is always open after the boiler has stopped.
Thus the boiler + cylinder has the opportunity to thermo syphon the remaining hot water from the boiler jacket, creating more cylinder heat?
This is on an oil boiler where the jacket water contents is much greater than a gas boiler.

 

[Gasguru]

Yes indeed the 3 port sits in DHW mode under spring pressure unless you've had a previous heating call where the valve will sit in heating mode until another DHW call.

Unless you have a particularly old inefficient calorifier (say a single coil of 1" pipe) in the cylinder or a modern purpose designed gravity cylinder I doubt you'll get any syphoning especially given the head loss of the pump/3 port/likely 22mm pipework/route etc.

 

[fixitflav]

Honeywell three port mid position valve. Having just worked on one again (not done for some time, as I usually fit two port zone valves).
If the spring inside holds it in the central heating closed position (unpowered), then the hotwater is always open after the boiler has stopped.
Thus the boiler + cylinder has the opportunity to thermo syphon the remaining hot water from the boiler jacket, creating more cylinder heat?
This is on an oil boiler where the jacket water contents is much greater than a gas boiler.

It differs from an old-style gravity HW system in that the boiler only fires when one or both of the thermostats are calling for heat. So the problem doesn't arise.

 

[picasso]

just for you flav

 

[fixitflav]

just for you flav

View attachment 168869

OK point taken! Reading it again I see he's talking about the remaining heat in the boiler after firing stops.
But apart from Gasguru's comments, it won't be an issue. Although the oil boiler jacket contains much more water than a gas boiler, it's still likely to be a lot less than the HW cylinder.

 

[Mike2007]

Yes indeed the 3 port sits in DHW mode under spring pressure unless you've had a previous heating call where the valve will sit in heating mode until another DHW call.

Unless you have a particularly old inefficient calorifier (say a single coil of 1" pipe) in the cylinder or a modern purpose designed gravity cylinder I doubt you'll get any syphoning especially given the head loss of the pump/3 port/likely 22mm pipework/route etc.

It's not my work, but it's been partially upgraded.
It still has 28mm primaries to the cylinder, which by its age I would say it's got gravity coil from the previous old boiler.