Choosing a radiator that looks right is the easy part. Choosing one that actually keeps the room comfortable depends on heat output, and the right output depends on the room. This guide explains how to work out the heat output you need, how to read the numbers on a product page, and what to check before you order.
Heat output in plain terms
A radiator's heat output is the amount of heat it gives off, shown in watts (W) or British thermal units per hour (BTU/h). Both measure the same thing, so you can convert between them:
- 1 watt is about 3.412 BTU/h
- 1,000 W (1 kW) is about 3,412 BTU/h
The output you need is the amount of heat your room loses. If a room loses 1,500 W on a cold day, a radiator that gives 1,500 W (about 5,100 BTU/h) keeps it at the temperature you want. A radiator with a lower output will struggle, and one with a slightly higher output gives you some margin.
Step 1: Decide the temperature you want
Rooms are usually heated to different temperatures. These are commonly used design temperatures in UK homes:
| Room | Typical target |
|---|---|
| Living room and dining room | 21 °C |
| Bedroom | 18 °C |
| Bathroom | 22 °C |
| Kitchen | 18 °C |
| Hall and landing | 18 °C |
Treat these as a starting point. If you prefer a warmer or cooler home, adjust accordingly.
Step 2: Understand what makes a room lose heat
Two rooms of the same size can need very different radiators. The main factors are:
- Insulation: loft, cavity or solid-wall insulation reduces the heat a room loses.
- Glazing: large windows and single glazing lose more heat than small, double or triple glazed windows.
- External walls: a corner room with two outside walls loses more heat than a room in the middle of the house.
- Ceiling height and room volume: more air to warm means more heat needed.
- What is above and below: a room over an unheated space, or directly under the roof, loses more.
- Draughts and ventilation: open fireplaces and leaky doors add to the load.
Step 3: Estimate the heat you need
There are two practical routes:
- A proper heat loss calculation. A heating engineer or designer works out the heat loss of each room from its dimensions and construction. This is the most accurate method and is worth doing for a new system, an extension or a whole-house renovation.
- An online BTU or watt calculator. These multiply the room volume by a factor that reflects insulation, glazing and room use. They are quick and useful for a single replacement radiator, but they are an estimate. If your house is unusual, or you are unsure, ask a heating professional.
If you are replacing an existing radiator, the output of the old one is a useful guide, provided the room was comfortable with it. If the room was always cold, aim a little higher. If it was always too warm, you can go a little lower.
Step 4: Read the output on the product page correctly
Radiator outputs are normally published at a standard test condition called delta T 50 (ΔT50). It assumes the water in the radiator averages 50 °C above the room temperature. The result is a comparable number, so you can compare one radiator with another fairly.
Your own system may not run that hot. Modern condensing boilers often run at lower flow temperatures, and heat pumps run lower still. A radiator on a lower flow temperature gives out less heat than its ΔT50 figure. If your system runs cooler, choose a radiator with a higher ΔT50 output, or ask the manufacturer or your installer for the output at your system's temperature.
Step 5: Check the size fits the wall
Output and size are linked. A taller, narrower radiator and a low, wide one can give the same output. Before you choose a shape, measure:
- the width and height of the wall space, allowing for skirting, furniture, curtains and windowsills
- the distance between the pipe connections (the pipe centres) if you are replacing an existing radiator
- the depth available if the radiator sits close to a door or a corridor
Our vertical radiators suit narrow walls, while horizontal radiators often work well under a window. If you have tall walls and little floor space, take a look at tall radiators and slim radiators.
Common questions
Is it a problem if my radiator is a little too big?
A radiator with a slightly higher output than the room needs is usually fine, especially if you fit a thermostatic radiator valve, which reduces the flow when the room reaches the temperature you set. A radiator that is far too large wastes space and money.
Can two smaller radiators replace one large one?
Yes, as long as their combined output meets the heat the room needs and your pipework can support them. This is useful for long walls or rooms that need heat in more than one place.
Does the finish change the output?
Product outputs are tested for the radiator as supplied, so use the published figure for your chosen model and size. Do not paint a radiator in a different type of paint, or cover it, as this can change how it behaves.
Next steps
Once you know the output you need, see our radiator buying guide for the rest of the checklist, or compare materials in our radiator material guide. If you would like a second opinion on a room, you can call us on +44 7729 799515 (Monday to Friday, 09:00–17:00) or email info@poshrads.co.uk.