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 The outdated defi­n­i­tion of the thermal unit remained, however.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. 

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