Radiator sizing

[piemmepi]

Hello,

I am going to replace some radiators in my flat. The current radiators are very old and sized pre-double glazing and loft extension, so I am questioning the sizes and numbers, especially as I am going to also replace the floor and I have flexibility with pipe sizing. Going round a few calculators online and I am getting wildly different numbers for the required BTU output.

I have so far used these

Heat adviser	https://heatadviser.co.uk/btu-calculator/
Radiators online	https://radiatorsonline.com/btu-calculator

and I get the following BTU figures

Room	Heat adviser	Radiators online
Lounge	8102	5440
Study	2301	1735
Spare room	4049	2218
Hallway	964	2084
Kitchen	4609	5679

I figured radiators online would be more reliable as it asks for more inputs (e.g. what's above/below the room), are there any other good ones out there?

 

[Exedon]

We use Stelrad stars there are 2 versions one aimed at diy the better one you have to register for though still free.
You will probably need a understanding of how Hest loss calculations are done to get best out of it.

 

[piemmepi]

Wow that tool is amazing. I do have appreciation of heat losses, just what I needed. Thank you very much!

 

[piemmepi]

Quick question, the home page suggests:

Heat loss is presented in BTU (British Thermal Unit) and Watt. A figure of 20% is often added to all room heat loss calculations to fully comply with the requirements under BS 5449 concerning intermittent use of the system.

Does this mean the 20% is already built in or that I should build it in?

 

[Exedon]

All aliwances buit in you can opt faster warm up on first screen.