Unvented Cylinder vs. Combi

Bodd · 27 Oct 2020

Different houses, plumbers Boilers and water pressure will all see performances differ. Bernard hasn't said to much that is wrong..

Ash D · 27 Oct 2020

Could I ask the experts on here about a Combi Boilers, as I’d love to know more.

Is there much difference between brands? As always, I want to stay with the known brands, but is an Ideal any different to a Baxi, for example? Is there anything I should look out for or avoid?

Likewise, is there any rational behind oversizing the power of the boiler. If I needed a 24KW unit and installed a 36KW unit, does it just mean I get a warmer house quicker?

Any guidance would be appreciated.

Ash D · 27 Oct 2020

Could I ask the experts on here about a Combi Boilers, as I’d love to know more.

Is there much difference between brands? As always, I want to stay with the known brands, but is an Ideal any different to a Baxi, for example? Is there anything I should look out for or avoid?

Likewise, is there any rational behind oversizing the power of the boiler. If I needed a 24KW unit and installed a 36KW unit, does it just mean I get a warmer house quicker?

Any guidance would be appreciated.

Ash D · 27 Oct 2020

Apologies, no idea why my posts keep appearing twice!

Bodd · 27 Oct 2020

If you go for a 36KW you will need good water pressure. Also depending how far the meter is from the boiler you would more than likely need a 28mm gas supply.

Most of my installations I run 28mm gas (even for 24KW)

denso13 · 27 Oct 2020

Ash D said:
Is there much difference between brands?

A bit but it is more to do with the quality of the installation than the make of the boiler in my opinion.

Ash D said:
If I needed a 24KW unit and installed a 36KW unit, does it just mean I get a warmer house quicker?

The output on boilers is more to do with the hot water requirement. They are all generally oversized for the heating requirement which is usually much less than the boiler can deliver.

bernardgreen · 27 Oct 2020

denso13 said:
A combi is, generally, pressurised to between 1 and 1.5 bar.

Isn't that the sealed Central Heating system ?

Ash D · 27 Oct 2020

Thanks @Bodd. (Again!). As before, changing the gas pipe would not be a big job, as the boiler is close to the meter.

I must confess, I just made up the 36KW figure (as one of the larger sizes I found) for example. Just wondered if “bigger was better”?

denso13 · 27 Oct 2020

bernardgreen said:
Isn't that the sealed Central Heating system ?

Yes

Bodd · 27 Oct 2020

Ash D said:
Thanks @Bodd. (Again!). As before, changing the gas pipe would not be a big job, as the boiler is close to the meter.

I must confess, I just made up the 36KW figure (as one of the larger sizes I found) for example. Just wondered if “bigger was better”?

How many bathrooms/ shower rooms do you have

Ash D · 27 Oct 2020

denso13 said:
The output on boilers is more to do with the hot water requirement. They are all generally oversized for the heating requirement which is usually much less than the boiler can deliver.

Thanks, that’s really interesting. As this thread is very much about my hot water performance, I assume a large/oversized Combi boiler will give me the best chance of improved showering?

Are Combis smart, in that they throttle back the output? Or do they run at full (36KW, for example) all the time?

Ash D · 27 Oct 2020

Bodd said:
How many bathrooms/ shower rooms do you have

We have two upstairs bathrooms (1 x family with a bath and 1 x en-suite with only a shower).

bernardgreen · 27 Oct 2020

denso13 said:
Yes

It is when the mains pressure drops (*) too low to force enough water through the DHW circuit to trigger the flow sensor to get the boiler to fire up for DHW

(*) due to cold tap(s) being turned ON.

denso13 · 27 Oct 2020

Ash D said:
Or do they run at full (36KW, for example) all the time?

No, some modulate and are now pretty sophisticated if the latest controls are used.

denso13 · 27 Oct 2020

bernardgreen said:
It is when the mains pressure drops (*) too low to force enough water through the DHW circuit to trigger the flow sensor to get the boiler to fire up for DHW

(*) due to cold tap(s) being turned ON.

How often in the real world does that happen or is it more theoretical combi bashing?

My point is that if you have 1-1.5 bar pressure (which is what you need as a minimum to pressurise the boiler) the flushing the toilet won't have any effect on the boiler.

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Unvented Cylinder vs. Combi

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