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My latest post in the PI&ORP site (Polytechnic University of Catalonia) is titled “Confort térmico en invierno: cálculo y variables más eficientes sobre las que actuar” (“Thermal confort in winter: calculation and the most efficient factors to act on”) and is all about a practical exercise consisting of a thermal confort assessment in a simulated enclosure and the determination of the most efficient variables to act on during the coldest season of the year.
The post at PI&ORP may be considered as the second part of a post I published last summer, making the same exercise applied to the hottest season of the year. To know the details of the thermal confort calculation and the determination of the most efficient factors in winter and summer, I refer you to those posts.
Now, what I want to do is to watch the outcomes from the respective calculations and the influence of certain variables in the thermal confort.
- First, it must be taken into account that, depending on the enclosure features and the location climatic conditions, the outcomes may vary. In the case of the two analysis -winter and summer-, I used the data from the weather station installed in my family house in Ontinar (Zaragoza).
- In both cases, the most influential factors in the thermal confort rate PMV (predicted mean vote) are the same:
- air temperature (ºC);
- metabolic activity (MET) ;
- clothing thermal insulation (CLO) .
- Despite of the climatic peculiarities in each place, these three factors will typically be the most influential ones in any analysis, whatever the location of the studied enclosure.
- The actions on the air temperature following energy efficiency criteria, are different in summer and in winter:
- in summer, it is possible to reach a lower air temperature in a natural way in the very morning. So, the thermal confort rate PMV may be improved by taking advantage of the natural air temperature, in addition to actions on the metabolic activity (harder work early in the morning) and the clothing thermal insulation;
- in winter, it is more difficult to take advantage (raise) of the air temperature in a natural way. To improve the thermal confort, actions on the metabolic activity and clothing thermal insulation are more influential.
- It is easier to improve the thermal confort when the metabolic activity increases, than when it must be decreased. Noting the data of the analyzed cases, the reduction of the metabolic activity in 10W/m2 segments mean a PMV improvement in 0,14 points (left table, summer); the metabolic activity raise in the same 10 W/m2 segments, improve the PMV index faster, a minimum of 0,20 points per each 10W/m2 segment (right table, winter)
- It is easier to improve the thermal confort rate when the clothing thermal insulation is increased, than when it must be decreased. With the data of the analysed cases, increasing the clothing thermal insulation brings a better thermal confort rate: 0,44 points at least in each segment; whilst diminishing the clothing thermal insulation pursued in summer, improves the PMV rate in 0,38 points per segment at best (right table, summer):
Finally, a thought that comes me from the influence of the metabolic activity and the clothing thermal insulation on the thermal confront: in many cases, these will be the most influential factors when it comes to get an acceptable thermal confort, even ahead of the air temperature. However, the common practice in most of the enclosures -public or private spaces, workplaces or home- is to go to the air HVAC systems as the first (or the only) option to improve the thermal confort.
It is right to take advantage of the development in HVAC systems to improve our confort, but… won´t we be going too far?
Referencias:  WIKIPEDIA. 22 feb 2014. Índice metabólico. http://es.wikipedia.org/wiki/Índice_metabólico. Consultado 27 nov 2014.  WIKIPEDIA. 8 nov 2013. Clo. http://es.wikipedia.org/wiki/Clo. Consultado 27 nov 2014. Bibliografía: ARAGON VALLEY. 22 julio 2014. How to assess the thermal comfort and how to address the most efficient factors. http://www.aragonvalley.com/en/assess-thermal-comfort-address-most-efficient-factors/. Consulted 28 nov 2014. MAIRAL, David. PREVENCIÓN INTEGRAL. 26 nov 2014. Confort térmico en invierno: cálculo y variables más eficientes sobre las que actuar. http://www.prevencionintegral.com/comunidad/blog/aragon-valley/2014/11/24/confort-termico-en-invierno-calculo-variables-mas-eficientes-sobre-que-actuar. Consulted 28 nov 2014. 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 28 nov 2014.