Dunsley removal and unvented conversion to sealed.

[gingalig]

Need to pick a couple of brains whilst exposing my chasm of limited understanding.

Before I qualified, about 10 years ago we had a Dunsley neutraliser fitted, combining a Firebird oil boiler and a Hunter multifuel. Over time we've never really been happy with the set up and have a shower pump which obviously makes a hell of a racket.
I managed to get a pre plumbed 170ltr heatrae Sadia unvented for a very good price from city in a fit of pique, and plan to relocate the cylinder to downstairs close to boiler, ( I have my G3 ) disconnect and replace the multifuel with a standard stove as it's seen better days, The existing pipework for the flow and returns is in 28mm with 15mm drops to the rads, there are 15 rads it's a very old house. The boiler has a pump connected to return on top of the boiler and another on the Dunsley all 28mm. I'm confident I can make the conversion but the connections on the unvented are in 22mm. My questions are is this going to have a negative effect on the flow rate and if so would I be better to keep the pump on the boiler or have I brought a lemon and need 28mm from the unvented or can I make the connection and scale up to 28mm immediately. Repiping isn't an option and all the 15mm was replaced with install, alternatively would cct,s be an option.

Thanks for reading, any input would be appreciated.

 

[SJB060685]

Whats the output of the boiler? Can you not keep primaries in 28 and drop down to 22 after zone valve?
If its a simple s plan system then I see no need for CCT'S or am I misreading the above?

 

[SimonG]

Keep at 28 and then drop down as SJB said.