Method for calculating size of boiler in an unvented system.

[Skiny]

Am I correct to assume the following calculation is the correct way to size a system boiler:

Total radiators output +
3KW for the cylinder +
10% extra ?

I've seen various calculators for for radiators, which all come to about the same output, so I expect they're a reliable guide.

I will of course be asking a few different plumbers during quotes, but want to have an understanding of it myself.


Online calculators recommend 8KW for radiators for all rooms, including a loft 'hobby room', that I expect to make in the future.

So, in addition to the 3KW for the cylinder and an extra 10% allowance, a 12KW boiler should be sufficient?

 

[muggles]

The correct way to size a boiler is using a Whole House boiler sizing calculator like this one

Once you've done that, and come up with a number, you use experience to select the best boiler for the job. When you say "unvented system", do you mean that you have an unvented hot water cylinder, or that you're having a sealed heating system?

 

[ianmcd]

Muggles has given you correct advice, however I will add my wee bit and I must stress that this is only my personal opinion and not endorsed by any manufacturer or any other body but newer efficient boilers are only at their most efficient when running at 100% so dont over size your boiler or it will never be 100% efficient as it was designed to be

 

[D_Hailsham]

The correct way to size a boiler is using a Whole House boiler sizing calculator like this one

Baxi Whole House Boiler Size Calculator will give the same result without the hard slog of filling in a sheet of paper.

 

[D_Hailsham]

newer efficient boilers are only at their most efficient when running at 100%

If, by 100%, you mean at maximum output then this will only apply in the few days a year when maximum output is required. The reest of the year the boiler will be running at a lower output so it won't be 100% efficient.

A boiler has to be actually condensing to be at its most efficient.

 

[ianmcd]

OP to be honest with you and you will get so much differing advice on here as lots of folks with lots of experience and lots who have just joined the trade who think that they know everything , the real truth is that modern boilers are really efficient but when they are the third boiler to be fitted to an ancient ****ty system then they cant handle it, modern installers are pushed to day a 1 day swap over and it will never work , bring on all the newbies telling me how they can do two a day

 

[muggles]

The correct way to size a boiler is using a Whole House boiler sizing calculator like this one

Baxi Whole House Boiler Size Calculator will give the same result without the hard slog of filling in a sheet of paper.

Thanks, couldn't find it!

 

[Skiny]

Thanks for all the replies!

...When you say "unvented system", do you mean that you have an unvented hot water cylinder, or that you're having a sealed heating system?

I would like an unvented hot water cylinder, as a complete new system is needed.

OP to be honest with you and you will get so much differing advice on here as lots of folks with lots of experience and lots who have just joined the trade who think that they know everything , the real truth is that modern boilers are really efficient but when they are the third boiler to be fitted to an ancient ****ty system then they cant handle it, modern installers are pushed to day a 1 day swap over and it will never work , bring on all the newbies telling me how they can do two a day

That does make sense. Fortunately I will be replacing the entire system, as the one fitted (I've only just bought the property) is ancient.

I tried both the IDEE and BAXI calculators and they came up as 6.13KW & 6.8KW.
Looking on the Worcester website, the smallest boilers they have, is 9KW. Does a 9KW boiler sound about right?

My property is a mid-terrace cottage, with solid stone walls. The width of the external walls are 3.5m and the length between the neighbors is 6.25m. There are two floors, with one bedroom and bathroom upstairs, though in the future I'd like a loft conversion.

To the rear there is an extension (built in 1994, according to the newspapers stuffed in the walls) with dimensions of 6m long x 2.7m wide. I will work in the extension, but mostly it will be for storage, so heating there will be minimal.

I'm not fussed on combis and would like hot water via an unvented cylinder.

 

[Dan Robinson]

Wooshitter Botch? Sheesh.. .

Anyway.... Decent system boiler, then a 120 or 150 litre cylinder. Job done.

 

[Skiny]

You don't like Worcester? What do you consider as decent?

More importantly, is the calculated boiler output of 9KW, roughly correct?

 

[muggles]

You don't like Worcester? What do you consider as decent?

More importantly, is the calculated boiler output of 9KW, roughly correct?

Very few people on here consider Woofter to be any good. Intergas is generally the boiler of choice, though in your case I'd make a rare exception and suggest something like a Vokera Mynute i20 - 20kW maximum (which will give you really fast reheat times on your hot water cylinder) but will modulate down to just 2.7kW for really efficient room heating. Connected to a decent control, this will give you great efficiency and a five year warranty. Other than that, the Ideal Vogue System 15 seems to be a good boiler and will do 3.0-15.0kW with a 10 year warranty, although it's more expensive to buy and I don't think it uses the high-efficiency control logic of the Mynute, so it'll be more expensive to run too

 

[Skiny]

Ah right. I used their website as a way to see the outputs of various boilers on offer.

Many advise to not oversize the boiler, but others imply it should be.

But really, what I want to know is, are the calculators I was linked to earlier, about right for a 1-2 bedroom, stone wall cottage (with dimensions I posted earlier), when they give recommendations of 6.13KW & 6.8KW?

 

[Dan Robinson]

Impossible to say really, but if you are under 12kw heat loss, then get the closest you can get with the lowest MINIMUM output too.

 

[bernardgreen]

Got mine from

http://www.johnsonandstarley.co.uk/products.asp

 

[Dan Robinson]

Someone has to I suppose