Combi or Unvented

[azreal]

What did you go for in the end.
Would be keen to understand your real world experience.
I am looking at a combi vs unvented. Leaning towards the WB 550cdi. Or the atag q51c

Plumber advised me today that both unvented and combi rely on mains pressure. So as I have good pressure (and fairly high flow) the combi would be the easier Choice to fit.

The atag has a 14l internal tank and 23l/min flow and the Worcester Bosch 51l/min.

Looking around and using
Watts = 70 x L/min x degree C rise

So for a 25l flow
Watts = 70 x 25 x 35 = 61.25 KW

The Worcester is 41kw so cannot do 25l indefinitely. So uses the tank to top up.
When the tank is drained
41k/(70x35)=16.7l/min

The atag is 51kw. So when its internal tank drains
51k/(70x35) =20l/min
So realistically the atag can still supply at 20l/m

Any thoughts

 

[Agile]

Easier to fit for the installer!

A proper heat only boiler and an unvented cylinder are the best solution if you have more than one bathroom and more than two people living there.

Tony

 

[azreal]

Easier to fit for the installer!

A proper heat only boiler and an unvented cylinder are the best solution if you have more than one bathroom and more than two people living there.

Tony

Hi
Can you give some more detail?
From what I understand both use the mains to provide pressure and flow. So the unvented cylinder will only put out what goes in. I agree have 150l plus of hot water ready to go is good, but if a combi can output the same isn't that a more efficient way of working?

 

[Agile]

No!

Please search the forum this same problem is discussed every week.

See minimum boiler heating output too.

Tony

 

[Anteaus]

From what I understand both use the mains to provide pressure and flow. So the unvented cylinder will only put out what goes in. I agree have 150l plus of hot water ready to go is good, but if a combi can output the same isn't that a more efficient way of working?

There are pro's and con's - The advantage of the tank is that it can cope better with sudden heavy demands for hot water such as running a bath or several hot taps on. It should also give a more consistent temperature of water, I find that the combi output temp is very dependent on flow rate. Plus you can run a trickle of hot water for washing purposes - the combi cuts the heat if the flow is too slow. As mentioned you can have an electrical backup in case of boiler failure if you have a tank, but not without.

Combi advantages are energy efficiency - no tank wasting heat into the loft at your expense. Plus, no waiting time for hot water (well, about 60s in reality) if the system's been off. That, and it's a 'bottomless' resource so you don't run out during a big cleaning job.

Though I'd be inclined to agree that in a busy household the tanked system is probably the better option, as it can cope better with several hot taps on at once.

 

[Agile]

Combi advantages are energy efficiency - no tank wasting heat into the loft at your expense. Plus, no waiting time for hot water (well, about 60s in reality) if the system's been off. That, and it's a 'bottomless' resource so you don't run out during a big cleaning job.

Though I'd be inclined to agree that in a busy household the tanked system is probably the better option, as it can cope better with several hot taps on at once.

Modern unventeds are very energy efficient and only lose about 1-2 kWh per day.

I don't see a combi being more efficient than a smaller rated heat only boiler. In fact rather the reverse.

Efficiency figures are very confusing anyway. There are too many ways to quote and measure them.

Tony