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Frictional losses in a Combi and the effect on HW flowrate?


 
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powerbooksub

from United Kingdom

Joined: 03 Jul 2011
Posts: 40
Location: Surrey,
United Kingdom

PostPosted: Sun Dec 04, 2011 2:49 pm Reply with quote

Hi

I would like some help please choosing between 2 boilers, Baxi Neta-Tec 28 HE and the 33 HE.

Max flowrate is 13.5 LPM for the 33 and 11.5 LPM for the 28. The difference between the two models is only 70 but I will only spend it if I will actually achieve the flow at the showerhead. It's going in to a 2-bed mid-terrace and either model is fine is terms of KW for the CH.

I currently get 14.5 LPM out of the filling loop directly underneath my existing boiler which is a 24KW which gives 9.5 LPM at the shower. Will I get 13.5 LPM if I install the 33 model?

The bath/shower is located around 2 metres from the boiler.

By the way I'm going for the Baxi as it's got a 5year warranty and has a high 13.5 LPM for low price. Of course if it turns out that I will
never achieve 13.5 LPM I will either get the 28 model or switch completey to a glow worm Flexicom which is also doing a 5-yr warranty until the end of the year.

Thanks!

Info here:

http://www.boilers2go.co.uk/Baxi-Neta-Tec-33-HE-Gas-Combination-Boiler.html

http://www.boilers2go.co.uk/Baxi-Neta-Tec-28-HE-Gas-Combination-Boiler.html

Technical specs here: http://www.baxi.co.uk/docs/baxi-think-guide-final-SPS.pdf
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manofgas

from United Kingdom

Joined: 22 Jul 2010
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PostPosted: Sun Dec 04, 2011 7:23 pm Reply with quote

if you are obtaining 14.5lpm below the boiler you should in theory obtain 13.5 lpm raised through 35 degrees from the boiler to the hot outlets.
However you would have to ensure that you have a sufficiency good and consistent inlet pressure from your cold water mains.
In addition you would have to ensure that your supply pipework is correctly sized and installed correctly, ensure that your shower head can pass the volume of water required .

IF YOU HAVE 14.5 LPM ON THE INLET THE BOILER SHOULD PASS THE MAX 13,5LPM IF THE CORRECT SIZE OF RESTRICTORS HAVE BEEN FITTED (IF THERE ARE ANY).
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powerbooksub

from United Kingdom

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Location: Surrey,
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PostPosted: Sun Dec 04, 2011 7:48 pm Reply with quote

Thanks.

I think I therefore get the larger boiler.
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Dan_Robinson

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PostPosted: Sun Dec 04, 2011 7:53 pm Reply with quote

Of course if you haver slightly less l/min, it will be raise to a higher temperature - which has its own benefits.
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powerbooksub

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Location: Surrey,
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PostPosted: Sun Dec 04, 2011 8:08 pm Reply with quote

good point, thanks
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dreadnoughtheating

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Location: Norfolk,
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PostPosted: Sun Dec 04, 2011 8:37 pm Reply with quote

PBS, 35 degrees temp rise is the standard quoted by most manufacturers.

I would ask you to note that if we have another cold spell like last year, an incoming cold water temp of 2-3 degrees will generate an output temp of say 38 degrees at the quoted flow rate. That is blood temp (give or take).

A comfortable shower temp would be way above that temp, so please do not expect 13.5 l/min all year round at the shower head if you enjoy a hot one.

Combi's are not the answer to every hot-water/heating 'upgrade'

DH
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Dan_Robinson

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PostPosted: Sun Dec 04, 2011 8:39 pm Reply with quote

Very true.
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mysteryman

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PostPosted: Sun Dec 04, 2011 9:26 pm Reply with quote

Make sure the water main can push the water through the boiler against the back pressure of the shower. If it can't, there's no point in having the extra power.
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DP

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PostPosted: Sun Dec 04, 2011 10:07 pm Reply with quote

If you are getting 14.5l of water at the filling loop, what do you think will happen to the flow rate when cold tap is run AS WELL as hot water delivered by the boiler. What if someone flushes the WC, you certainly will not get delivered hot water as per the data badge nor adequate cold water..
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