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boiler output: how many btu's do I need?



 
 
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  #1  
Old August 10th 08, 04:54 PM posted to uk.d-i-y
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Posts: 157
Default boiler output: how many btu's do I need?

Hello,

When fitting a boiler, how do I work out what output the boiler should
have? I can add up the rating of the radiators to give the power
needed for the central heating but what do I add for the hot water?
Also do I add a fraction to compensate for losses?

Thanks.
Stephen.
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  #2  
Old August 10th 08, 08:13 PM posted to uk.d-i-y
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Posts: 2,490
Default boiler output: how many btu's do I need?

In an earlier contribution to this discussion,
Stephen wrote:

Hello,

When fitting a boiler, how do I work out what output the boiler should
have? I can add up the rating of the radiators to give the power
needed for the central heating but what do I add for the hot water?
Also do I add a fraction to compensate for losses?

Thanks.
Stephen.



The boilers in Noah's Ark were rated in btu's - *Kilowatts* is what you
need!

Adding up the ratings of the radiators is putting the cart before the horse.
You need to start by working out the heat losses for each room, and *then*
choose suitable radiators for each room. The boiler needs to have sufficient
capacity to meet the total heat loss, plus some for pipe losses, plus some
for transient conditions (getting the house up to temperature from cold,
fairly quickly) plus some for hot water.

It's best to use one of the free Heatloss Calculation programs from the
likes of Barlo or Myson - they will add all the necessary 'bunce' factors
for you.
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!


  #3  
Old August 10th 08, 11:38 PM posted to uk.d-i-y
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Posts: 1,983
Default boiler output: how many btu's do I need?

On Sun, 10 Aug 2008 14:54:47 +0000, Stephen wrote:

Hello,

When fitting a boiler, how do I work out what output the boiler should
have? I can add up the rating of the radiators to give the power
needed for the central heating but what do I add for the hot water?
Also do I add a fraction to compensate for losses?


Current practice for sizing boilers is the whole-house calculation method
which you'll find on the diy wiki under central heating design.

--
John Stumbles

My karma ran over my dogma
  #4  
Old August 11th 08, 11:35 AM posted to uk.d-i-y
external usenet poster
 
Posts: 157
Default boiler output: how many btu's do I need?

On Sun, 10 Aug 2008 19:13:37 +0100, "Roger Mills"
wrote:

Adding up the ratings of the radiators is putting the cart before the horse.
You need to start by working out the heat losses for each room, and *then*
choose suitable radiators for each room. The boiler needs to have sufficient
capacity to meet the total heat loss, plus some for pipe losses, plus some
for transient conditions (getting the house up to temperature from cold,
fairly quickly) plus some for hot water.


Sorry, perhaps I should have made myself clearer. I used one of the
online calculators which asks for room size, whether it's north
facing, if it is double glazed, etc. and told me the radiator size
that room requires. It was those figures I have added up. Whether the
radiators that are in the rooms match those figures is another matter!
They were here when I moved in.

I'll check out the web sites you suggested. Thank you.
  #5  
Old August 11th 08, 11:58 AM posted to uk.d-i-y
external usenet poster
 
Posts: 19,948
Default boiler output: how many btu's do I need?

In article ,
Stephen wrote:
Sorry, perhaps I should have made myself clearer. I used one of the
online calculators which asks for room size, whether it's north
facing, if it is double glazed, etc. and told me the radiator size
that room requires. It was those figures I have added up. Whether the
radiators that are in the rooms match those figures is another matter!
They were here when I moved in.


Well, did the original boiler cope? If so just replace it with one of the
same size. If it didn't - was it the boiler or wrong sized rads?

--
*Why is it that most nudists are people you don't want to see naked?*

Dave Plowman London SW
To e-mail, change noise into sound.
  #6  
Old August 11th 08, 02:22 PM posted to uk.d-i-y
external usenet poster
 
Posts: 9,056
Default boiler output: how many btu's do I need?

John Stumbles wrote:
On Sun, 10 Aug 2008 14:54:47 +0000, Stephen wrote:

Hello,

When fitting a boiler, how do I work out what output the boiler should
have? I can add up the rating of the radiators to give the power
needed for the central heating but what do I add for the hot water?
Also do I add a fraction to compensate for losses?


Current practice for sizing boilers is the whole-house calculation method
which you'll find on the diy wiki under central heating design.

quick'n'dirty is between 50W/sq meter (well insulated house to modern
standards) through 100W/sq meter (DG, cavity walls, but no cavity
insulation, insulated roof, decent carpets on slab floors) through
200W/sq meter (single brick, but in good order otherwise) to 400W (old
house, no insulation at all)

boiler an rad sizing is not critical as long ass there is ENOUGH the
thermostat will modulate down to what is required.

when in doubt go bigger.
  #7  
Old August 11th 08, 03:49 PM posted to uk.d-i-y
external usenet poster
 
Posts: 2,490
Default boiler output: how many btu's do I need?

In an earlier contribution to this discussion,
Stephen wrote:

On Sun, 10 Aug 2008 19:13:37 +0100, "Roger Mills"
wrote:

Adding up the ratings of the radiators is putting the cart before
the horse. You need to start by working out the heat losses for each
room, and *then* choose suitable radiators for each room. The boiler
needs to have sufficient capacity to meet the total heat loss, plus
some for pipe losses, plus some for transient conditions (getting
the house up to temperature from cold, fairly quickly) plus some for
hot water.


Sorry, perhaps I should have made myself clearer. I used one of the
online calculators which asks for room size, whether it's north
facing, if it is double glazed, etc. and told me the radiator size
that room requires. It was those figures I have added up. Whether the
radiators that are in the rooms match those figures is another matter!
They were here when I moved in.

I'll check out the web sites you suggested. Thank you.


The on-line calculators I've seen are pretty crude, and only give ball-park
figures. You need one of the programs which you can download and run on your
own computer. These take proper account of wall construction, glass areas,
floor and ceiling losses, temperature in adjoining rooms, etc., air changes,
etc. - and give a much more accurate heat-loss figure.

You can download the Barlo program from
http://www.heatweb.com/programs/heatloss.html

The link which I have for the Myson program appears to be broken but, if you
ask nicely, Andy Hall will probably email a copy to you.
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!


  #8  
Old August 11th 08, 09:45 PM posted to uk.d-i-y
external usenet poster
 
Posts: 2,231
Default boiler output: how many btu's do I need?

On Sun, 10 Aug 2008 14:54:47 +0000, Stephen wrote:

Hello,

When fitting a boiler, how do I work out what output the boiler should
have? I can add up the rating of the radiators to give the power needed
for the central heating but what do I add for the hot water? Also do I
add a fraction to compensate for losses?

Thanks.
Stephen.


Check out the Boiler Choice FAQ.




--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

  #9  
Old August 18th 08, 04:00 PM posted to uk.d-i-y
external usenet poster
 
Posts: 157
Default boiler output: how many btu's do I need?

On Mon, 11 Aug 2008 14:49:56 +0100, "Roger Mills"
wrote:

You can download the Barlo program from
http://www.heatweb.com/programs/heatloss.html

The link which I have for the Myson program appears to be broken but, if you
ask nicely, Andy Hall will probably email a copy to you.


Thanks. I downloaded the Barlo program. Interestingly for my dining
room it recommends just over 5000 BTU whereas the calculator I used on
some web site (can't remember which, sorry) recommended just under
4000 BTU. Sorry to use such old units but Tool station's catalogue
works in BTU hence I am calculating in BTU. I think Screwfix sell the
same rads, only last time I looked, the figures were different because
one used delta T = 50C and the other used 60C. I do have a patio door
which perhaps explains why the program, which knew about this, gave me
a higher value that the online calculator which did not.

Andy, if you are listening, can you provide the other calculator?

Thanks.
 




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