Advice on replacing radiators

[Perfidy]

I can calculate the BTU but I'm befuddled by all the advice out there about radiators. Where they should be placed, whether the modern cast iron ones are any good or all show and no go...

New house has very old radiators, some have 10 mm pipes some have 15 mm. They mostly work but the they are v big and very 1970s ugly. It's a big old house - with draughty windows (secondary glazing is on the list).

Are you thinking of doing them yourself then? A plumber would advise but our experience of replacement old radiators with modern was that the new ones needed to be a fair size smaller than those they replaced.

Can't answer about 10mm pipe. Ours was a combination of 15 and 22 MM and he used plastic pipe to make new connections which was dead easy.

A friend has a big cast iron one. It is no way as effective as what it replaced. It is a large vertical one

Search on here (and elsewhere) for Acova radiator recommendations - you'll see they are highly rated. Used the column ones throughout our extension and were so impressed we're gradually replacing all the other ones with them.

@Africa2gothats perfect thank you.

As MarieG10 has said, a plumber would be best suited to calculate and do the job for you properly. If you are thinking of doing the job yourself, you will likely need to redo all the pipework to fit to the new radiators. If the radiators are all very old like you say, they will likely be in imperial as well.
The first thing you will need to do before you go any further with radiators, is work out what style you want for the radiators in your house. Do you want old, school style, column radiators? These will push up the price and size that you will need as the heat output is lessened due to materials used in manufacture. I would personally recommend looking at Stelrad for ideas on radiators. They are the industry leader and preferred by most plumbing and heating engineers. There website has a large range of different radiators and if you use their heat loss calculator, it will give you the ideal match of radiators depending on your preference. You don't necessarily have to go with their radiators once you have calculated everything, as you can find just as good but cheaper radiators elsewhere, but it is a good starting point. Kartell are also a good manufacturer, as well as centre brand (Wolseley). Acova aren't too bad on the designer radiator side either, but it can always be a bit of pot luck with designer radiators, due to the way they are manufactured.
Ideally, you would want to place your radiators under windows as this is where most of your heat loss is and the radiators positioned there will help mitigate that. Modern cast iron radiators should be a lot better than previous rads, as they are now designed to be a lot lighter and give out more heat.
The most important thing you will need to remember if you are doing the work or having someone else do the work for you is that the system is cleaned once all the radiators have been replaced and inhibitor is added to your system. This will prolong the life of your radiators and boiler.
There are many other things to take into account on pipe sizing and pipe runs too, but I would probably run out of space. Feel free to ask, if you do have more questions.

I've replaced all mine, mostly with kudox (from screwfix) they are no frills white but after 5 years still.look new. I swapped to type 22 as the BTU was greater for a smaller size. I would change the tails to metric either behind the rad or buried in the wall, there are adapters between sizes. Definitely change the tail stocks and TRVs. I put type A drain cocks in every drop loop but to be honest that was probably overkill.

That’s really helpful thanks. We would be using a plumber but wanted to get an idea First.