Gravity hot water, pumped heating conversion

[MethodPH]

Hi have been asked to fit an unvented cylinder (I’m G3) but the heating system is as follows.

ideal Mexico 2 boiler

gravity hot water pumped heating

28mm flow and return from boiler to cylinder with 22mm flow and return to boiler with pump for heating.

is it possible to put an unvented cylinder on the gravity 28mm’s? Thinking of capping the 22mm heating flow and return from the boiler moving the pump onto the 28mm return then a splitting into 2 x 2 port valves one on the 28mm hot water return one back to onto the heating return. As the system fills from loft tank into 28mm flow on cylinder. Any thoughts? Should I just suggest upgrading whole system boiler as well?

 

[Chuck]

is it possible to put an unvented cylinder on the gravity 28mm’s?

Unless the MI's say otherwise explicitly, I think the answer is a big 'no'. All the unvented cylinders I'm aware of require pumped circulation and the manufacturers say that 'Gravity circulation will not work.'.

This is because the heat exchanger in modern unvented cylinder is a relatively long narrow-bore tube, which maximises surface area and hence heat transfer but requires a bigger circulating pressure than a gravity system can deliver.

 

[EvilDrPorkChop]

Cap the 28mm Gravity Circs at the boiler.

Convert the system to a S plan using the 22mm flow and returns from boiler. Seal central heating system when fitting the Unvented cylinder.

Least then when boiler goes it'll be a straight swap for a heat only boiler.

Either that or talk them into a new system boiler as well.

 

[MethodPH]

Unless the MI's say otherwise explicitly, I think the answer is a big 'no'. All the unvented cylinders I'm aware of require pumped circulation and the manufacturers say that 'Gravity circulation will not work.'.

This is because the heat exchanger in modern unvented cylinder is a relatively long narrow-bore tube, which maximises surface area and hence heat transfer but requires a bigger circulating pressure than a gravity system can deliver.

Thanks for your reply chuck. That’s why I was thinking of converting it to a fully pumped system. Just wondering if I could use the existing 28mms as the new primaries and cap off the existing heating flow and returns from the boiler and then re-pick them up from the 28’s. Adding a couple of 2 ports on the returns, new pump and new wiring. Or whether a boiler change with a conventional single flow and return was the only option for an unvented and couldn’t utilise the old boiler that has two flows and two returns

 

[SimonG]

Where does the cold feed and open vent connect into the heating circuit.

 

[EvilDrPorkChop]

Where does the cold feed and open vent connect into the heating circuit.

On the gravity circs by the sounds of it

 

[SimonG]

On the gravity circs by the sounds of it

That's what I would expect, just needs to be careful it doesn't pump over.

 

[Chuck]

That’s why I was thinking of converting it to a fully pumped system.

My mistake, I read your post as asking whether you could stick with gravity circulation or needed to convert to fully pumped. Re-reading it, my answer would be the same as @EvilDrPorkChop.