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.

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!

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

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.

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        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

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.

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!

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

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.