Sizing and unvented?

[Scott_D]

I can’t find my unvented book, will have to get a new one.

Been asked to quote for some plumbing in a house that will become a HMO (they want to spend as little as possible)
3 en-suites and one communal bathroom

I’m thinking keep the combi to supply the kitchen sink and communal bathroom
Then fit a direct unvented for the 3 en-suite

They will need a new mains supply
I’m thinking 300litres cylinder
With flow restrictor on all hot outlets

 

[SimonG]

Maybe bigger on the unvented if it was me. Peak use somebody could be disappointed.

 

[Scott_D]

Maybe bigger on the unvented if it was me. Peak use somebody could be disappointed.

Just looking at prices and they almost double from 300 to 400 litres cylinders

 

[townfanjon]

You could pair a couple of 250’s if theres room , however a 300 litre takes some emptying, especially if as you say , you fit some restrictors.

 

[ShaunCorbs]

250-300 with fast recovery

 

[Scott_D]

250-300 with fast recovery

Will have to be a direct cylinder

 

[ShaunCorbs]

Will have to be a direct cylinder

Can’t you get a set from the boiler ?

400l then

 

[Welder]

Can’t you get a set from the boiler ?

400l then

IMHO, the sensible option as Shaun indicates, is an indirect using the combi when its not being used for heating. When it's used for heating and HW temp drops to where input is required then enable the immersion. That's a doddle to do control wise.

At least that way they can't really run out. In addition, they have the option to turn off the immersion to reduce running costs because, quite probably, the immersion will be running outside economy 7 (or whatever its called theses days) timings.

Using this control regime I feel 300L would suffice - assuming the input is sufficient.

 

[chris watkins]

300L should be fine thats 100L @ 60deg C for each en-suit.
If you want to increase the recovery then go for a DD or even the DDD. (2 or 3 x 3kW immersions)