Renovation advice: unvented cylinder or combi, + u/f heating

[converse]

We are just starting a large renovation and extension project on our 4 bed house: 2 bathrooms, 1 downstairs WC. Extending the kitchen/diner (to 5x6m), living (to 5x6m) and adding a utility (there are also 2 more reception rooms downstairs)

Water is currently a vented system, HW cylinder taking space in bedroom cupboard which we want to move out completely for space.

3 of us living there now, max family size would be 4.

Builders and plumbers saying we should either go for megaflow/unvented cylinder (architect says upstairs in new airing cupboard, I say in garage or utility), or just a big combi boiler (builder recommendation).

We'd like decent pressure showers (currently we have power shower on the unvented) and good hot water baths for our young family.

Builder suggests than unvented are pressure limited, so we wouldn't get anything like power shower flow at the showers.

We haven't considered just having a new vented system, as thought unvented gave better performance...

Regardless of choice, it's likely the boiler will be replaced anyway.

Finally we are thinking of underfloor heating in the two new rooms, possibly wet as I hear it's more efficient...

Thoughts and opinions would be great, thanks all.

PS I've read the older threads on similar topics but most seem to date 2-4 years ago, so wanted up-to-date advice.

 

[Richardthe3rd]

I never like the generic term 'Builder'. If he's the Main Contractor, with his name on the Small Works contract, then he's the Main Contractor. Take what he says with a pinch of salt; for example both a combi boiler AND an unvented cylinder requires both a good flow rate & incoming mains water pressure.

A combi - regardless of size, will not supply two hot taps simultaneous.

You'll love UFH in any room, live with it you'll never go back to any other type of c/heating.
HTH

 

[bazdaman]

Unvented and wet ufh, defo not a combi the new boiler and cylinder can go in the garage out of the way most probably.

 

[Dan Robinson]

As above.

Also be wary of builder's/main contractor's in house plumbers.

Most don't know their arse from their elbow when it comes to system design and instalation.

I'm working on putting right a real doosey at the moment.

Interior designers, consultants and "AV specialists" along with kitchen fitters should all be banned from any involvement in any building project.

 

[Agile]

I always wonder what bullshit answer the builders give when the owners discover his recommended combi will not supply two hot outlets at the same time.

 

[converse]

Terrific thanks everyone - sage advice from those in the know! Just as I thought combi isn't the way to go. He will be the main contractor for the build and has his own plumber and sparky, but we are keen to talk to our plumber, one of many years experience and one who suggested unvented in the first place.

Will unvented HW coupled with mains cold give a decent shower pressure upstairs without the need for a pump? (We have an aqualisa power shower on vented system at mo)

 

[Dan Robinson]

Depends on your locality. Where I am the pressure is great.

 

[D_Hailsham]

Measure the incoming cold water flow rate using a marked bucket and a watch. The best tap to use is the one in the garden as it will have an unrestricted flow. Then repeat with the kitchen cold tap full on at the same time.

For an unvented, you need at least 20 litres/min from the garden tap and no less than 10 lpm when both taps are on

 

[converse]

Measure the incoming cold water flow rate using a marked bucket and a watch. The best tap to use is the one in the garden as it will have an unrestricted flow. Then repeat with the kitchen cold tap full on at the same time.

For an unvented, you need at least 20 litres/min from the garden tap and no less than 10 lpm when both taps are on

Looks like we are getting around 24 l/min from the main tap, which is through a very knackered old looking water meter under the sink. Will try with a couple on the go next.

 

[Norcon]

I always wonder what bullshit answer the builders give when the owners discover his recommended combi will not supply two hot outlets at the same time.

Turn one tap off.