The formula for the thermal unit

Different countries use different units for heat output. If you convert them, you strangely get different kW values. It is all to do with how the thermal unit is defined.


Again and again, you get customers from the USA asking about a blower from a European catalog. Later on, they are surprised because the blower is suddenly designed for a higher heat output (kilowatts) than is stated in the catalog. Why is that?

Jörg Scheuerlein, Engineer Heating Mechanics at ebm-papst in Landshut. (Photo | ebm-papst)

Sometimes, units are like zombies and they live on after they have been killed off. The majority of people, for example, still think of horsepower when they buy a car or of calories when it comes to a diet — rather than using the modern units of watts or joules respectively. The same applies to the thermal unit. In the majority of European countries, specifying the thermal unit is based on an old definition of how much heat energy you can obtain from one cubic meter of natural gas. In Europe, the reference gas G20 has 34.02 MJ/m³. This is the European thermal unit Hi. Heating systems are now considerably more efficient. The outdated definition of the thermal unit remained, however.

In the USA, on the other hand, the heat energy that can be obtained from the reference gas G20 is justifiably defined with a higher value, namely 37.76 MJ/m³. This is the US thermal unit Hs. Now, the USA is well known for having issues with the metric system. This is why they use their own unit for heat output — which you cannot calculate without the thermal unit. The unit the Americans use is the BTU/h (British Thermal Unit per hour, which despite its name is only used in the USA, and not in the United Kingdom). This is why you get a higher kW value in the USA compared to in Europe when converting the heat output from BTU/h into kilowatts — because the definition of the thermal unit is higher in the USA. By the way, in the Netherlands, they use the more realistic American thermal unit Hs, but specify the output in kilowatts.

China uses yet another system

In China there is a particular heat output zombie: the steam ton/hour. This is based on the old coal stoves in the cellar which directly produced steam for the radiators. The quantity of steam per hour resulted in a certain heat output in people’s homes. Even today, the heat output in steam ton/hour is still required for large plants in China. There is, however, no method for precisely converting it into kW. Using practical comparisons, ebm-papst has come to a useful equivalent of 635 kW (Hi) ≙ 1 t steam/h.

So do not be surprised if you encounter some confusion when it comes to the units and values used to denote heat output — it is all down to how it is defined. 

Required fields: Comment, Name & Mail (Mail will not be published). Please also take note of our Privacy protection.