Boiler Sizing

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Can i get some advise on boiler sizing. All the websites have large variance in recommendations and i dont want to sell myself short.

I cant base it on the existing boiler as a large extension has been built and the house is now completely different to its original state.

Its a large 4 bed house with 2 bathrooms, a downstairs WC, kitchen and utility. There are 5 large rads upstairs and 100m2 of UFH downstairs, all to be powered by the new boiler.

Using the online recommendations i have pitched myself at 30KW size. Is this going to be man enough or do i need to go bigger? Can you go too big?
 
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Go smaller, you will not need 30kW.

Go to HeatEngineer.com. Pay £10 and do a one-off paid for calculation using their software. It will take you a couple of goes and a couple of hours. Or, as an alternative, use online calculators from companies who are happy to sell oversized products for properties they've never seen to the uninitiated, guessers or trusting happy go lucky punter.
 
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You'll want a decent boiler that can modulate well and decent boiler controls,

The evohome will be a good option, or Drayton wiser.

As Vulcan said, pay £10 to heat engineers, chances are you'll need smaller than 30kw.

An oversized boiler will be problematic by constantly cycling. Am undersized boiler will keep you cold.

Do it right do it once. You e obviously spent a lot of money on the house don't skimp now it's only £10 and small amount of your time.
 
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As above, sign up to www.heat-engineer.com and do a proper survey. Highly unlikely you need 30kW. My finger-in-the-wind guess based on your description would be 14kW and possibly less.
 
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You will find you’ll need less than you think. I did calcs and for 20 radiator house and it was 26kW - that’s with factoring in hardly any heat insulation (old house), hot water cylinder and few kW loss through pipes; remember you won’t want heat in every room all the time.
When you look at boiler range the cost to size up normally is not that much compared with other costs (sometimes may even be cheaper!) but don’t be tempted, figures won’t lie.
I know with oil boilers you can change the kW to different settings within a range.
 
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Ok I will do the heat engineer survey. Hadn't heard of it before but looks like this will give the best guidance.

Will they ask how many baths / showers are used a day as I want to allow for the proper use of the house rather than just what me and Mrs P use at the moment as that will be under normal use
 
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Ok I will do the heat engineer survey. Hadn't heard of it before but looks like this will give the best guidance.

Will they ask how many baths / showers are used a day as I want to allow for the proper use of the house rather than just what me and Mrs P use at the moment as that will be under normal use

With a system boiler it may not matter, if you get a high gain cylinder,

This one can heat up in 12 minutes with the right size boiler,

https://theintergasshop.co.uk/high-...itre-vented-hot-water-cylinder-900-x-450.html

I'd advise a hot water priority system for maximum efficiency. I'd also recommend a boiler with a high ratio of modulation like the intergas xclusive (though there are many others to choose from)
 
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So this is what it has spat out (if i have done the correct survey). It didnt ask for number of radiators or how the property was heated which has me wondering if i have done the correct exercise?

I am confused as to what part of this data dictates the boiler size.

upload_2021-11-29_10-57-45.png
 
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So this is what it has spat out (if i have done the correct survey). It didnt ask for number of radiators or how the property was heated which has me wondering if i have done the correct exercise?

I am confused as to what part of this data dictates the boiler size.

View attachment 252418
That's the free guesstimate one. You need to sign up for an account and hit the "Buy a survey" button, pay your £12 then do the survey via the app
 
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I paid the £12 and thought that was it. Sorry to be a pain but I just can't work out how to do it. It says that my survey as shown above is in progress? Am I getting this wrong
 
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I have just downloaded the app and seen the survey tool. I guess that is hows its done. It seems very thorough with each room being detailed individually. Is that correct?
 
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I have just downloaded the app and seen the survey tool. I guess that is hows its done. It seems very thorough with each room being detailed individually. Is that correct?

Yup, you are going to do the job properly so you'll have to do the spadework, measure the house room by room and enter it all into the app then upload, then work through the screens. As I said it will take a couple of efforts and a couple of hours but you'll have a figure thaat's worth something and you can adjust the workings later. There are some good help videos on youtube. We suggested this because you seemed to be interested in doing the job properly. This is how to do it.
 
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