Boiler upgrade for extension?

[stemill]

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 happen on occasion. 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?

Static water pressure is just under 3 bar with flow rate of ~17 litres per minute. Does this sound good enough or would adding an accumulator make sense if more than one shower is required to run at the same time?

Thanks in advice for any advice!

 

[ShaunCorbs]

What’s your dynamic flow rate and pressure like

Eg with an outlet open

 

[Gasmk1]

the 222 has a 100 litre store. have never fitted one but fitted a couple of 111 and they were superb coped with 3 showers running at once. so i would imagine the 222 is even better. they are a great alternative to unvented.

 

[stemill]

Can i measure dynamic flow and pressure anywhere (i'm using the outside tap) and just open any other outlet?

I was originally actually thinking of the 111 but decided to up it after being warned it was unlikely to cope with the load. Lots of conflicting opinions about combis!

 

[ShaunCorbs]

yes open the kitchen tap/ any when you do the test and test from the outside tap

if you have the space a 222 will easily do 18lpm its more down to incoming water pressure

View: https://www.youtube.com/watch?v=076eZlBA15k

 

[stemill]

I will test the dynamic pressure and flow tomorrow and report back. Thanks.

 

[centralheatking]

I will test the dynamic pressure and flow tomorrow and report back. Thanks.

I would test at specific times 7.00am before the school run, 7.00pm when little ones get bathed
And when its really quiet 5am or 6 on a Sunday or weekday if you are that motivated CHking
Then let us know how many litres per min for each time. Do the incoming cold main nothing else.

 

[stemill]

Sods law the builders have disconnected the water today so I can't test it!