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Choosing the right heat output for your radiator

How do I calculate what heat output I need from my radiator for a specific room?

We offer a simple heat calculator on The Radiator Centre website to provide you with a guide range of heat required for a specific room. However, you should always ask for a second opinion from your plumber or heating engineer as they can make a specific assessment of your property’s needs and note any elements that might potentially impact the heat that you need.

If you believe your property has specific issues due to its age, location, heating system, etc. there are more complex methods available for calculating the precise heat requirement required and in these situations we advise you seek out the advice of a heating consultant or architects.

Heat Calculations

The formula below is for all basic heat calculations. Instead of manually using the below though, you can instead just use our heat calculator to do the below for you!

Step 1. - Volume

Calculate the volume of the room in cubic metres (length x width x height)

Step 2. – Room Type

Take the room volume and multiply it be the relevant figure below.

  • Lounges and Dining Rooms - multiply by 50.
  • Bedrooms - multiply by 40.
  • Hallways, Landings and Kitchens - multiply by 30.
  • Bathrooms - multiply by 90.
  • Conservatories – see the section below.

Step 3. Any Adjustments

Take into consideration any relevant factors that may later the amount of heat required.

  • For rooms facing north - + 15%.
  • For French Windows + 20%.
  • For double glazing + 10%.
  • For very exposed sites, or very cold weather + 10%
  • For new builds (which have much better insulation) - 20%

Total = heat output in watts needed from one or more radiators to heat the room.

Every radiator in every size on www.theradiatorcentre.com has its output shown in watts and BTUs (British Thermal Units).

Why do you show outputs as Delta 60 (Δt 60ºC) and Delta 50 (Δt 50ºC)?

A radiator’s output is expressed in watts. The wattage is based on the system’s likely operating temperature or the correlation between the flow temperature in the system, the return temperature and an average room temperature and is either shown as Delta 60 (Δt 60ºC) or Delta 50 (Δt 50ºC), i.e. 90/70/20 (delta T60) or 80/60/20 (delta T50).

Delta 60 (Δt 60ºC) is the British Rating System whilst the newer European Rating is Delta 50 (Δt 50ºC ). In our experience, most UK plumbers and heating engineers still calculate heat requirements using the original British Delta 60 rating. To comply with this and to avoid confusion, our heat outputs are shown in both.

  • To convert Delta 50 to Delta 60 multiply Delta 50 heat output by 1.264
  • To convert Delta 60 to Delta 50 divide Delta 60 heat output by 1.264

The heat outputs of your chosen radiators are unlikely to be exactly the same as your heat requirement. So, always go for radiator size with a higher rather than lower heat output and fit thermostatic radiator valves to control the room temperature.

How do I convert from BTUs to Watts?

Divide by 3.412.

How do I convert from Watts to BTUs?

Multiply by 3.412.

How do I calculate the heat needed in a conservatory?

There are many factors affecting heat loss in conservatories –age, quality of glass, full height walls, etc. The installer or your plumber is probably best qualified to give you a proper estimate of the amount of heat required. However, we are used to working the approximate heat outputs required for conservatories, based on the information provided by you – e.g. dimensions, materials etc. Please call and speak with one of our team who will work it out for you.