Boiler upgrade for extension

[onjangiu]

We are currently part way through a part single storey part double storey extension which is going to involve removing the existing unvented hot water tank from the first floor airing cupboard. We had originally planned on replacing the existing boiler (a Worcester Greenstar 24Ri) with some kind of storage combi boiler (maybe a Viessmann Vitodens 222-F). However we've been advised that this might not be such a great idea and we'd be better sticking with an unvented tank. We do have space downstairs for a tank although it might need to go into the garage. Once extended the house will have two ensuite bathrooms and a third family bathroom - we wouldn't expect to run all three showers at once but definitely two at the same time would be a requirement. There will also be 17 or 18 radiators in the house once finished.

So is the combi a bad idea? If we went for an unvented cylinder would 250L be an appropriate size for a family of 4 and would the existing boiler be up to the job?

Thanks in advice for any advice!

 

[cross thread]

Combi is a poor choice, existing boiler ok ,cylinder may be ok ,performance will be governed by water inlet supply

 

[onjangiu]

What's the alternative if cylinder not ok?

 

[muggles]

What's the alternative if cylinder not ok?

A bigger one. 250 should be fine though

 

[onjangiu]

Just to add a bit more detail. I've just measured the water pressure and it's just under 3 bar with flow rate of ~20 litres per minute. Does this sound good enough?

 

[onjangiu]

Actually flow rate is maybe more like 17 litres per minute, would adding an accumulator make sense if more than one shower is required to run at the same time?

 

[ianmcd]

You could do what somepeople do depending on your layout and have a combi and an unvented Cyl, with the combi feeding one shower and the Unvented feeding the other outlets

 

[Gasguru]

Combi and Viessmann

 

[onjangiu]

After reading around the forum I've now learned that 3 bar is the static pressure, which is not the same as the dynamic pressure and it's this that might be more relevant?

One post seemed to suggest measuring the pressure and then opening enough taps until the pressure drops to 1.5 bar, then adding up the flow rates from all the open taps. Not sure what I'm meant to do with this number though!!?

Just trying to establish what I might need to add to the system to cope with additional load. I'll be getting a pro in to do the work, I just prefer to be able to have an informed view of what's required.