Boiler sizing

[dave1]

Hi all, need a bit a help boiler sizing. Existing system has 17 rads, 170L unvented cylinder. Detached house, loft conversion and built around 1995, cavity wall insulation and loft insulated.

I've not had to do boiler sizing before so if you could help point me in the right direction it would be greatly appreciated.

I've had a look online at certain websites to try and do the calculations but it's comes out too low in my opinion.

I will be putting a system boiler in to replace existing.

Cheers.

 

[Simon F]

how long is the piece of string?

room sizes etc.

very crude estimate:-
assuming each rad averages at about 1 kw, add another 3 for the cylinder comes to 20.

what size boiler is being taken out, how efficient?

 

[kris]

Im sure theres even a boiler sizing app for this , app for everything lol

 

[dave1]

how long is the piece of string?

room sizes etc.

very crude estimate:-
assuming each rad averages at about 1 kw, add another 3 for the cylinder comes to 20.

what size boiler is being taken out, how efficient?

All existing rads are staying, just need to replace the system boiler. I've taken all the measurements of all rads. So if I find the approx kw for each plus for the cylinder?

 

[cdodds2004]

if your putting a system boiler in just go for a 24kw it will easily cover that and a bit more if they ever have a extension. I always tend to oversize a little if I can.

 

[kris]

Just as a guess depending on size of rads , 26kw or if real big rads then 30kw with low modulation ,

 

[Chalked]

All existing rads are staying, just need to replace the system boiler. I've taken all the measurements of all rads. So if I find the approx kw for each plus for the cylinder?

You can't trust radiator sizes to calculate heat loss.

You can either use the whole house method.
Or, measure each room and use any of the online calculators. Or, a mears calculator.
Or even easier, take your measurements, including wall and floor types, patio doors etc and take them to your local plumbers merchant. They will do it for you.

 

[wattsie1987]

At a guess I'd go for a 30 kW

 

[newcastle phill]

Depending on system volume you may need to consider a supplementary expansion vessel

 

[Nostrum]

I'd be surprised if it's over 15kw. Do it properly, don't over size.

 

[dave1]

Some big variants in answers, I appreciate all the answers. I'd like to know how to work it out accurately. I'll look into the whole house method, and mears calculator. It's something I'm keen to learn, and learn how to do it properly.

 

[kozak1968]

Ihttp://www.idhee.org.uk/calculator.html I use this website.

 

[Ermintrude]

Use the mcs one

 

[dave1]

Use the mcs one

Have you hot a link erm?

 

[Nostrum]

I don't think MCS do a calculator anymore. You can use any room by room calculator that conforms to BS EN 12831.

Try Stelrad stars to begin with.

 

[village idiot]

Anything over 24kw will be an over kill. 24kw is in my opinion OTT but should more than cover it.

 

[dave1]

I get the whole sizing each room for size of each rad, but the rads that are in place (sized correctly or not will be staying) that's why I was thinking of finding the approx kw rating for each of these then adding them all up plus h/w.

I was pretty sure a 24kw would be fine, but I want to know how to work it out so I can size future jobs correctly.

 

[village idiot]

Ok, try this
[DLMURL]http://www.lomaxtraining.co.uk/media/guides/energy%20efficiency/whole_house_boiler_sizing_method_for_houses_and_flats.pdf[/DLMURL]

 

[dave1]

Ok, try this
[DLMURL]http://www.lomaxtraining.co.uk/media/guides/energy%20efficiency/whole_house_boiler_sizing_method_for_houses_and_flats.pdf[/DLMURL]

OK just used that, with the info I have its come out at about 13kw. Still sounds too low for 17 rads and a large cylinder. ha ha [emoji85]

 

[Nostrum]

Trust me it won't be.

90% of boilers are oversized because nobody does the calculations.

You could have 100 radiators but if the building only requires a certain amount of heat it makes no odds.

Always size the boiler on required heat loss not radiator output.

 

[village idiot]

OK just used that, with the info I have its come out at about 13kw. Still sounds too low for 17 rads and a large cylinder. ha ha [emoji85]

So go for a 15kw boiler. For a worst case scenario, you could go for an 18kw. But as Nostrum has pointed out, it would be OTT

 

[dave1]

OK cool, obviously putting on a massively oversized boiler is going to be in efficient so that's why I'm keen to do this right.