Online boiler sizing calculators / sizing my boiler

mark.carline · 4 Jul 2007

I am now wondering what size boiler to get and who I should belive when it comes to what size to get. I have seen these online calculators :

http://www.energysavingtrust.org.uk/housingbuildings/calculators/boilersizing/

http://www.idhee.org.uk/calculator.html

http://www.sedbuk.com/whole_house.htm

and I wondered if they are any good ?

I am looking to get a Baxi Platinum combi boiler and so I have three models to choose from which are 24, 28 and 33kW models :

http://www.baxi.co.uk/products/productDetails.php?idx=1128&cat=2

I dont have the size of our house to hand but its a 3 bedroom detached house with cavity wall insulation, 8 radiators, all double glazed, good loft insulation (i dont think we can do any more insulation!). Our only problem is that we might want to put an extension on the side which could see the size of the house increase by a third.

Any thoughts ?

denso13 · 4 Jul 2007

With a combi it's the hot water output which counts as they will all heat plenty of radiators.

Depends on number of showers, baths etc. but if cost isn't a great factor i'd go for the largest one, 33kw.

mark.carline · 4 Jul 2007

denso13 said:
....but if cost isn't a great factor i'd go for the largest one, 33kw.

You mean installation costs ?

Installation costs aren't really a factor (how oftern do you buy a new boiler) but ongoing year on effficiency costs are the biggest factor (especially thinking about the cost of gas in the future!). So I maybe even able to get away with the 24kw ?

denso13 · 4 Jul 2007

mark.carline said:
So I maybe even able to get away with the 24kw ?

Yes it will work ok but the biggest complaint I get is poor hot water flow. That one says 9.8l a minute but in the middle of winter you won't get that if you want nice hot water.

You just need to decide if you want to pay a bit extra for more hot water. The running costs won't be a great deal more either.

mark.carline · 4 Jul 2007

I see - so the pump inside the combi boiler will vary its speed depending on the temperature of the hot water coming out of the hot water supply (which in winter will be lower).

So it makes no difference to the size of the house / number of radiators etc other than the time it would take to change the temperature of the house by 1 degree. ?

I noticed the spec of the ones listed above, as per:

http://www.baxi.co.uk/products/techdetails.php?type=consumer&idx=1128&cat=2

mentions "Maximum DHW Flow Rate (Delta T @ 35? Max)" and the spec of the 24, 28 and 33kw are 9.8, 11.5, 13.5 l/min respectivly - Is this good ??

We only have one shower (a grohe wireless high pressure shower mixer) and one bath and I do want to make sure we have a decent pressure at the shower and also I dont want the bath to take years to fill. The rest of the house I dont realy care about flow rate or water temperature (i'm only washing my hands!).

At the same time as getting this fitted we are getting a new water main and internal pipe work is being renewed (the combi will be on its own 15mm spur from the house 22mm supply) - I dont think I can ask for anything more to get better pressure.

denso13 · 4 Jul 2007

This is from another site which might explain it better than me

b) Water flow rate
The speed at which water passes through the heat exchanger will determine how much heat is absorbed. A good analogy is a finger passing through a candle flame – whether the finger gets burnt or not depends on how quickly it is passed through the flame.

Boiler manufacturers will often quote a hot water flow rate for the appliance in l/min or gal/min. However, it is important when assessing the performance of a combi to remember that this flow rate is based on a specific temperature rise – usually 30C or 35C. If a greater temperature rise is required, the flow rate must be reduced. Conversely, a greater flow rate can be obtained if the temperature rise required is less.

E.g.
A Combi with an output of 23kW gives a flow rate of 9.4 l/min for a temp rise of 35C but;
A flow rate of 8 l/min gives a temp rise of 41C
A flow rate of 10 l/min gives a temp rise of 33C
A flow rate of 15 l/min gives a temp rise of 22C

c) Incoming mains water temperature
We have already seen that the temperature rise depends on the flow rate, but the actual temperature obtained will depend on the initial mains water temperature. Because this varies, the apparent performance of the combi will change according to the season e.g. with the 23kW boiler, at standard water flow rate;
Summer- typical mains temp 15C, hot water temp 50C
Winter – typical mains temp 4C, hot water temp 39C

So the hotter you want the water the slower it will run. A larger boiler output to hot water will give a better flow of hot water. It is not pumped so it depends on the cold water pressure as well

Clear as mud

gas4you · 4 Jul 2007

Go for at least the 28/30Kw. If I ever fitted a 24Kw combi and I wont, it would only be for an industrial unit with one or 2 wash hand basin hot taps.

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Online boiler sizing calculators / sizing my boiler

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