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Temperature and Humidity - Humidex and Heat Index

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  • Temperature and Humidity - Humidex and Heat Index

    A strong wind in the winter makes exposed skin much colder, and for that, many people are aware of the wind chill factor.

    Less known is something similar for hot climates, where Humidex (Canada) or Heat Index (U.S.A.), are similar ways to indicate how humidity affects temperature.

    Looking at the Humidex scale, considering the dew point and temperature, we can see that we can be comfortable even up to 30 degrees C (86 F), as long as the dew point is 5 or less. In this condition, the air is very dry, though, only 20% relative humidity. So it's a bit extreme. But this table doesn't even consider ventilation, or rather, air movement. Using a fan or other ways to shift the air will make even higher temperatures comfortable.

    Using humidity as an additional parameter to temperature, and maybe even adding in air movement, is a great way to save a lot of energy. There are ways to remove moisture from the air which requires less energy than actual cooling an entire building. If using solar thermal power, where heat is used to dry the air, it becomes even better: Apart from a few fans, pumps and a control system, we can keep comfortable and dry, using little or no external energy source at all.

    Look at this table. In Hong Kong, the temperature is above 30 in the daytime, most of the summer, and the dew point is most often 25 or higher. Shifting right in the table, from 30 degrees C, we can get to a comfortable range. Cooling the air just a few degrees, and the energy use is minimal:

    Click image for larger version

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    Hong Kong is expending 1/4 of all end energy on cooling and hot water. Thermal solar power can cut all that energy, when combining it with desiccant dehumidification and absorption chillers.

    But we need to change our mindset that it's only the temperature that controls our comfort. Look in the table again: It is NOT the case.

    Two reasons we are led to believe that temperature is the only important parameter:

    1) Most HVAC system controllers only have temperature as a parameter the user can see and select

    2) Both A/C and heating systems most often inherently dry the air.

    By being more smart about how we regulate indoor comfort, and with what appliances, we can save huge amounts of carbon emissions.

  • #2
    Working on the concept above, the basic values are temperature and dew point.

    From those two, we can calculate RH (relative humidity) and the HX (Humidex) index number.

    As long as the HX stays within 20 - 29, and RH stays within 30% - 75%, we can vary the temperature and keep the same comfort. By comfort, also looking at not getting the air too dry, as that will cause dried out mucous membranes and causing static electricity. Hence, 30% to 75% is a good range.

    Looking at Humidex first, an area is already framed in to mark RH:

    Click image for larger version  Name:	Humidex comfort Zone with RH 1of2.png Views:	23 Size:	60.4 KB ID:	149

    Continuing from HX to RH, let's look at the range 30% - 75% Relative Humidity:

    Click image for larger version  Name:	Humidex comfort Zone with RH 2of2.png Views:	20 Size:	86.9 KB ID:	150

    The same area with both charts is framed in. It shows that there are instances where either of RH or HX are OK viewed in isolation, but if we are concerned about both of these, we should look at this somewhat more narrow envelope.

    Interesting points to note:

    a) This envelope still allows both the temperature and dew point to vary quite a bit.

    b) A dew point in the range of 9-12 gives the largest flexibility with regard to varying the temperature

    c) A temperature of around 22-23 degrees gives the largest flexibility with regard to varying the dew point

    d) This is important as some times it is easier/cheaper to vary either the temperature OR the dew point. Using this envelope means we can keep a comfortable and healthy indoor climate and yet in many cases save energy.

    Heating, cooling and drying does not have to be switched on as long as you can stay within that envelope.
    Rather than chasing 20-22 C, we have a lot more flex.

    e) Regarding mold, fungus, mildew and so on, the dew point will still have to be looked at in isolation. This will depend on several factors, like how the building is constructed, how the weather is outside and so on. It is important to avoid that sections of a building is of a lower temperature than the dew point of the air, as this is where condensation sets in. Condensation in a building can lead to problems with both the building and its inhabitants. Water is life, not only for humans.

    A dry building is a happy building - one that doesn't invite unwanted pests and growths.
    Last edited by Core; 03-15-2021, 02:54 AM.


    • #3
      Simplified Comfort measure:

      Using temperature alone to measure our comfort (and building health), is not in itself enough.

      If you have access to measurements of the dew point, here is a simplified comfort measure:

      ( 2 x Temperature + DewPoint) / 3

      In other words, the average of temperature and dewpoint, but where the temperature is weighed 2/3.

      Keep this value below 23 C (73 F), and you won't be uncomfortable.

      You will still have to look at the relative humidity, though. I suggest keeping that below 75% as well. Hence, a temperature and dew point that are both 23 won't work either.

      We can call that measure TTD, indicating it is twice the temperature and one times dew point.

      Keep the TTD below 23, and the RH% below 75%.

      Unless you live in a very cold climate, you normally don't need to monitor the lower end of the humidity scale. Just keep the temperature at least 20 C (68F).