How to assess the thermal comfort and how to address the most efficient factors

This post is also available in: Spanish

One of the works in which I am currently involved, for the Research Centre for the Development and Innovation in Enterprises, of the Technical University of Catalonia (CERpIE-UPC), consist of testing applications for diverse uses. Afterwards, we draft a review article that is published in their site “Prevención Integral”.

Last week, it was the turn of an app named “FANGER by Aqualogy”. The application is available for free in Android 2.2 or higher versions in Google Play, and iOS 7.0 or higher versions in Apple iTunes.

Despite that the app intends to assess the workplaces thermal comfort, it is also useful to measure the thermal comfort of interior spaces, including your home, and to know which factors are worthwhile to take action.

In this post, I explain how to do it in an approximate and simplified way, but that will allow you to know which are the most effective factors upon which to act in order to improve the thermal comfort at home, applying energy efficiency criteria.

data_netatmo_ontinar_additional_module

As example, I will use the temperature and humidity data from the NetAtmo station installed in Ontinar which, while drafting this post (17th july, 12,30 hrs) are the one of the image at the right. Up to now, this was the hottest day of the year.

You can get your own temperature and moisture data using any thermometer and a hygrometer.

The date is important, as you will see later.

The variables to assess the thermal comfort are six

Four variables depend on the environment:

  • Fanger_data_inputMean radiant temperature which, since it is not usual to have the measuring instruments to get a precise reading (black globe thermometer), we will consider the 24,2ºC given by the station;
  • Air temperature which, without any climate control appliance working, we will consider the same 24,2ºC in the room;
  • humidity = 64%;
  • air speed = 0,0 m/s, without any climate control, as with the air temperature;

Two variables depend on the individual:

  • metabolic rate, in W/m2, measures te heat per square meter of skin that the human body needs to dissipate to the environment in order to reach the thermal balance according to the effort made. Such activity is measured in MET and depends on each individual´s metabolism. In the example, we´ll consider a MET=2,5 (145,5 W/M2) [1 MET=58,2 W/m2] for a homework relatively intense and continuos.
As a reference, these are some MET figures according to different physical efforts:
Sleeping, 0.8 met = 46,56 W/m2;
Seated, relaxed, 1 met = 58,2 W/m2;
Walking, 3.4 met = 197,88 W/m2;
Running, 9.5 met = 552,9 W/m2.
  • clothing thermal insulation, that the own application explains and is measured in “clo”. Wearing a “summer clothing”, clo = 0,5.

Fanger_results

In these conditions, the result brought by the application are the following thermal comfort rates:

  • PMV (predicted mean value) = 1,15. This is a very high value and means that we were experiencing heat. An acceptable PMV is between +0,5 and -0,5;
  • PPD (predicted Percentage of Dissatisfied) = 32,94%. That is, in such conditions, almost one third of people felt uncomfortable at home.

The conclusión drawn from these data (PMV=1,15) is that in the moment of the measurement (july 17th at 12,30hrs) a relatively intense and continuous homework brings a significant thermal discomfort for heat.

Variables upon which take action to improve the thermal comfort

In addition to the results, the application shows figures simulating the comfort rates according to different values of the variables, highlighting in green the cases in which the variables put the PMV rate within the acceptable range between -0,5 and +0,5:

Fanger_suggestions

As you can see, the influencing variables in the PMV comfort rate follows this order:

  1. air temperature;
  2. metabolic rate;
  3. thermal isolation of the clothing;
  4. air speed;
  5. humidity.

The objective consist of establish an acceptable comfort level (between -0,5 and + 0,5 PMV) applying energy efficiency criteria, what means that we must take action on the comfort variables by means of tasks organization and planning, changes in the mindset and the day-to-day customs:

Fanger_variacion_temp

  • The hard work, in the early morning. A temperature of 22ºC (just 2,2ºC less than the measured temperature as the reference, 24,2ºC), means that the PMV goes from 1,15 to 0,67, reaching almost the acceptable thermal comfort level between -0,5<PMV<+0,5. The day of the measuring, there was 22ºC until 11 a.m. This is a technical demonstration of the proverb “the early bird catches the worm”.

 

Fanger_variacion_clo

  • A simple piece of clothing makes a difference. If, in addition to make the hardest tasks in the very morning when the temperature is lower, you wear “light summer clotting” (clo=0,3), then the PMV=0,23, which is within the thermal comfort range.

 

Fanger_variacion_met

  • Once the thermal comfort on the hardest work is solved, the rush hours are available to make the lighter tasks. A metabolic rate of 1,2 MET for a regular stay at home, neither resting nor making intense and continuous homework, causes an activity level of 69,6 W/m2, which means that PMV=-0,05, very close to the optimal thermal comfort. Furthermore, consuming cold drinks and light food, helps to balance the metabolic rate even more.

Demonstrated: without any action on the temperature, humidity and air speed by means of climate control devices, it is possible to reach a thermal comfort inside the house, even in the hottest days of the year, in an energy efficient way and, therefore, more cost-effectivelly. All this, apart from the health benefits that a habit improvements may bring.

If someone does´t believe in the traditional proverbs, you can use this app to explain it.

Next winter, we will repeat the exercise of assessing the thermal comfort and addressing the variables when the houses are colder.

Bibliography: HERNÁNDEZ CALLEJA, A. NTP 779: Bienestar térmico: criterios de diseño para ambientes térmicos confortables. http://www.insht.es/InshtWeb/Contenidos/Documentacion/FichasTecnicas/NTP/Ficheros/752a783/nTP-779.pdf. Instituto Nacional de Seguridad e Higiene en el Trabajo. Consulted 17 jul 2014.

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