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Measuring room temperature - the right way!

Knowing the room temperature is the basis for saving on heating costs. But how do you measure it correctly? I have some tips - also for Home Assistant.

Energy-Saving Temperature
Measuring room temperature - the right way!

He who measures a lot, measures a lot of rubbish. At least that’s what a saying from research and technology tells us. The same applies to something that seems so simple at first glance - measuring room temperature. If you want to save on heating energy, the measurements you use for regulation and control must of course also be correct.

Especially among smart home enthusiasts, the question of the accuracy of a temperature or humidity sensor is often at the forefront. It should be accurate to 0.1 °C and 5% humidity. Yet with analog thermometers, you can perhaps still distinguish 0.5 degrees, which is also completely sufficient.

Much more important is the position of the thermometer in the room and the correct mounting location. Ideally, this would be at a height of 150 cm, in the middle of the room, without direct heat sources or sunlight nearby and without drafts. This is rather inconvenient for practical use - who wants to put a pole in the middle of their living room?

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In any case, there is no such thing as “the” room temperature. You can practically measure a slightly different value at every height and every position in the room.

Therefore, there are some rules for how thermometers and hygrometers should be placed or mounted:

  • At about 150 cm height
  • Avoid exterior walls
  • Not near or directly opposite heat or cold sources such as radiators, fireplaces or windows.
  • Energy-intensive devices such as refrigerators, PCs, televisions, amplifiers, etc. in the immediate vicinity can falsify the measurement.
  • Avoid exposure to sunlight
  • Sensors in flush-mounted boxes and switches practically always need to be calibrated, as they could otherwise measure the waste heat from the electronics

This already ensures that you don’t capture effects that have nothing to do with the room temperature.

However, a pleasant indoor climate includes not only temperature, but also humidity and air flow. The latter cannot be measured with common means. Humidity, however, can. For example, we perceive a lower temperature with higher humidity as warmer than a higher temperature with low humidity. Only both values together give a picture of the perceived temperature. The Thermal Comfort Integration for Home Assistant can calculate meaningful values using temperature and humidity:

The accuracy of the sensors, on the other hand, plays a rather subordinate role: 2-3% deviation in temperature is just as acceptable as 10% measurement inaccuracy in humidity.

If you want it to be precise, place several thermometers/sensors in the room and take the average. Here, of course, all thermometers must also work with similar accuracy. To do this, place them directly next to each other for some time until the measurements have settled and compare the values.

In smart home applications such as Home Assistant, you can set the deviation for individual sensors as an offset value and automatically have averages calculated from multiple measuring devices.

If, for example, you find a deviation of 0.8°C between two sensors, the deviation can be included as an offset in a new sensor entity:

yaml 
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  - platform: template
    sensors:
      temperatur_wohnzimmer:
        friendly_name: "Living room temperature"
        value_template: "{{ states('sensor.mein_wohnzimmersensor') | float + 0.8}}"
        device_class: temperature
        unit_of_measurement: '°C'

sensor.mein_wohnzimmersensor is the actual value of the sensor. A new entity temperatur_wohnzimmer is created from this, which returns the measured value corrected upwards by 0.8 °C.

Multiple sensors calibrated and aligned in this way can be very easily merged with Home Assistant using the Min/Max integration . This not only outputs the average of all sensors, but also minimum and maximum values.

yaml 
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sensor:
  - platform: min_max
    name: Temp living room average
    entity_ids:
      - sensor.temperatur_wohnzimmer
      - sensor.temperatur_wohnzimmer_esstisch
      - sensor.temperatur_wohnzimmer_couchecke

Accurate sensors are important, but correct application is even more important.

Hi! I'm Markus – the Fricklr. ⚡ Electrical engineer, 🎵 musician (bass, guitar, keys) and professionally 'something with the internet' 🌐 for over 30 years – nowadays with AI too 🤖 – a 55-year-old generalist from Bavaria, Germany.

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