Trying to size up radiators for our new house. Simple, there’s lots of handy online tools that I can just plug the information in to and it tells me what BTU I need….. except they are all wildly different. One example I have right now is of one room at less than 5000 BTU in one tool and over 11,000 in another! I started with the Best Heating one as it seemed the nicest to use and took into account more factors than many others, but it’s consistently well below anything else.
Last thing I want is to spend thousands on radiators that are too small, or to spend twice as much on radiators much bigger than we need. So which one should I use that is accurate?

Go bigger than required so you can use a slightly lower water temperature.

I used the best heating calculator for a couple of rooms when renovating our house. It’s ok but in hindsight wish I’d gone with slightly bigger radiators! With standard radiators, cost wise it isn’t that much more for slightly larger ones.

As it happens,I’m doing a course now to tell me what size radiators and boiler size is required.
It’s not simple. Heat loss is a big thing. You need to know the U value, or a typical U value for the rooms walls, along with window size, what insulation is above and below, what exterior walls are in the room and some other factors. I could give you a very accurate forecast, whereas the online calculators give an average size. Theres also the heat requirement in the room, does it want to be cooler, or tropical in there?
Go bigger, fit thermostatic valves. Make sure your boiler has the capacity to give the heat required.

Screwfix has a radiator calculator on their website

Screwfix has a radiator calculator on their website

Yes but this is my issue, many websites have a calculator, they give wildly different values.

If it were me, I think I’d pay the couple of hundred quid to have heat loss calculations done specific to your home.

I’d also be speccing them for the inevitable move to heat pump, like them or not, they seem to be just around the corner.

Never used one, but when replacing radiators I’ve always gone for double panel ones, type 22 from memory. I’d rather go too big and have a TRV throttling them than too small and a cold house. Plus it helps that our house is Victorian with no cavity insulation, so I know heat loss is a bitch for me.

If it were me, I think I’d pay the couple of hundred quid to have heat loss calculations done specific to your home.

I’d also be speccing them for the inevitable move to heat pump, like them or not, they seem to be just around the corner.

Yeah good shout on both, though not sure I can stomach speccing up for heat pump right now, rule of thumb seems to be 2x normal size, which will lead to some insane pricing given my other half is committed to something a bit more design-y (read more expensive…..)

New combi probably going in anyway so think heat pump is far enough away that I can forget about it for now.

Its kind of academic if your house leaks heat like a Sieve.

I have done everything I can (currently) to insulate our kitchen. We had a new radiator put in last year that is apparently twice the size it needs to be, yet it still now only just heats the room enough

Is underfloor an option thats how I would do it

Its kind of academic if your house leaks heat like a Sieve.

Well, yes….. Victorian house with all the sieve like qualities you might expect. Plenty improvements we can make in the short to medium term but it’s always going to be pretty sieve like I think.

Do you heat loss calc to get the room size in kW.

Then go to the manufacturer radiator spec sheets and look at the small print. You’ll see that the stated output are for 70°c flow temp. There should be an asterisk with a conversion formula for your desired temp. Convert to 50° for condensing boiler or 35° for heat pumps.

Years ago we couldn’t work out why our (new) house was cold – turned out that rad sizes etc were based on a standard UK calculation which was way out for our top of a Yorkshire hill facing North location…

You can always turn them down.

Same position recently. Different calcs gave different values

I went for the biggest possible rad 1800×560. And then also specd a wet plinth heater “just in case”. I’m hoping I don’t need it, but as said you can always turn them down / off if you have over specd

Do you heat loss calc to get the room size in kW.

Then go to the manufacturer radiator spec sheets and look at the small print. You’ll see that the stated output are for 70°c flow temp. There should be an asterisk with a conversion formula for your desired temp. Convert to 50° for condensing boiler or 35° for heat pumps.

All done, I’m 100% confidence everything is calculated correctly but the output each calculator gives still varies wildly.

if you go over with a view of turning it down if it feels to hot – are you liable to be running the boiler above its capacity?

The heat loss calcs are a guide as any house built to UK building standards will not be checked for air leakage, thermal bridging etc, so take them with a pinch of salt.

As others have said, get as large as possible. This is why UFH is popular, as you have a huge surface area running at low temperature.

The lower the temp needed to heat a room the cheaper the fuel running cost.

Making sure that the pipework is adequate is equally as important. Push fit is the spawn of the devil!

Sorry, I’d missed your point.

I use: https://www.heat-engineer.com/ for my HL calculations.

Wickes do stelrad homeline range of rads are a good starting point for price. You can then compare to screwfix and the the like.

not sure I can stomach speccing up for heat pump right now

Maybe go for radiators with a big spacing between the pipes, ie, long and low, so that if you want to put bigger ones in later you don’t have to move the pipes?