What Does BTU Mean?

BTU stands for British Thermal Unit. It’s a measure of heat, and on a radiator, the BTU figure tells you how much heat it gives out each hour, so you can check whether it’s big enough for the room.

If you’re shopping for a new radiator, you’ll have spotted a BTU number in the product details. One BTU is the heat it takes to warm a pound of water (about 450ml) by one degree Fahrenheit. A burning match gives off roughly one BTU, which is why radiator ratings run into the thousands.

Is BTU the same as BTU per hour?

Near enough. Strictly speaking, a BTU is an amount of heat and BTU/h is how fast it’s delivered, but when a radiator is listed as 5,000 BTU, it means 5,000 BTU of heat for every hour it’s running. Some retailers write BTU/h and some just write BTU. They mean the same thing.

How do you convert BTU to watts?

Divide the BTU figure by 3.412. A radiator rated at 5,000 BTU outputs about 1,465 watts, or roughly 1.5kW. To go the other way, multiply watts by 3.412.

You’ll want this more often than you’d think. Electric radiators and electric towel rails are rated in watts, boilers are rated in kW, and a lot of water-filled radiators still lead with BTU, so comparing any two of them means converting one.

What is ΔT50 on a radiator?

ΔT is the gap between the average temperature of the water in the radiator and the air in the room. Radiators are rated at ΔT50 as standard, and that’s what our BTU calculator works to, so match your target against the ΔT50 figure. Some manufacturers also publish a ΔT60 figure. It’s higher because it assumes hotter water, so don’t set a ΔT60 number against a ΔT50 target, or you’ll end up with a radiator that’s too small.

What affects how many BTU a room needs?

Room size is only part of it. What counts is how quickly the room loses heat, and that depends on how big the windows are and what type of glazing they have, how many walls are external and what those walls are made of, and what’s above and below the room. A big living room with single glazing and three outside walls will need far more than a small bedroom with double glazing and just one.

The temperature you’re aiming for matters too, which is why our calculator asks for the type of room: living room, bedroom, hallway or kitchen, or bathroom. They’re usually heated to different temperatures, and our guide to the ideal home temperature is a good place to start.

How do you work out the BTU you need?

Grab a tape measure and a notepad. Measure the length, width and height of the room and the area of the windows, then note what type of windows they are, how many walls are external and what they’re made of, and what’s above and below the room, such as a heated room, a roof or a floor over soil. Put all of that into a BTU calculator, like our own free one at HeatandPlumb.com, and it’ll give you a target figure in both watts and BTU.

Then look for a radiator that matches that figure or sits a little above it. Going slightly over is easy to live with, because a thermostatic radiator valve lets you turn the heat down. Going under means a room that never quite warms up.

Is a higher BTU always better?

A higher BTU does mean more heat, but bigger isn’t automatically better. An oversized radiator costs more to buy, takes up wall space you might not have and can overheat the room. An undersized one will struggle to get the room warm however long the heating’s on.

Do heated towel rails use BTU too?

The water-fed ones do, and the same sums apply. Bathrooms tend to be heated a bit warmer than the rest of the house, so it’s worth working the figure out properly rather than guessing. Electric towel rails are rated in watts, so use the conversion above if you’re weighing one against the other. If you’re choosing, have a look at our range of heated towel rails.

Still stuck between two sizes? Give the team a ring and they’ll talk it through with you.