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Thermal Comfort in Buildings: How to Better Control and Predict

Thermal Comfort in Buildings

Controlling and predicting thermal comfort in buildings is essential in HVAC systems design. How many times have you had small problems related to the operation or the location of heating or air conditioning? Particularly in office buildings, we often have to ask for a change in the desk position due to the vent being too close to our seat. It’s quite unpleasant as it directly affects our productivity.

Many times, the sources of heat / ventilation operation schedule don’t fit the diurnal evolution of outside temperatures or the windows exposure to direct sunlight. In the cold mornings, the air is set by default to a higher temperature, which unfortunately cannot be changed because the heating system does not allow it. These are common situations, even in modern office buildings.

Why do we pay so much attention to thermal comfort in buildings?

Buildings Thermal Comfort SimScaleIt’s simple: having optimal conditions makes us think and work better. And we eliminate a potential health hazard. All these situations related to individual and group comfort should be considered very seriously. It is relatively easy to understand and to plan it even from the design stage for individual rooms or single office spaces. Things become more complicated if we need to design an office building, where each room and floor have quite different thermal comfort parameters: different number of persons, different size, and placing of windows, diversity of the electronics equipment or the vicinity of special areas such as server rooms, lift room, central heating, staircases and other service premises.

The design and construction of a building that fits in all aspects the thermal comfort standard requirements is an art. But if we need this ideal building to provide all expected benefits related to the specific inside activities, we need more than the building design art. We need the essential contribution of the HVAC systems simulation algorithms able to offer full thermal comfort in buildings and the optimization of energy balances according to predictive models.

What is thermal comfort?

According to international standard EN ISO 7730 thermal comfort is:(…) that condition of mind which expresses satisfaction with the thermal environment”. In simple words, is the comfortable condition when anyone is not feeling either too hot or too cold. [1]

Buildings Thermal Comfort SimScaleISO 7730:2005 presents methods for predicting the general thermal sensation and degree of discomfort (thermal dissatisfaction) of people exposed to moderate thermal environments. It enables the analytical determination and interpretation of thermal comfort using the calculation of PMV (predicted mean vote) and PPD (predicted percentage of dissatisfied) and local thermal comfort, giving the environmental conditions considered acceptable for general thermal comfort as well as those representing local discomfort [2]. This standard was reviewed and confirmed in 2015.

The human thermal environment cannot be expressed in degrees and can’t be defined by an average range of temperatures. It is a very personal experience function of many criteria and is different from person to person in the same environmental space. The Health and Safety Executive estimate the reasonable comfort can be established when minimum 80% of indoor occupants are feeling comfortable with the thermal environment. [3]

What influences thermal comfort?

Thermal comfort is a cumulative effect resulted from a series of environmental and personal factors. As environmental factors we can find [1]:

  • Air temperature – The air contact temperature measured by the dry bulb temperature (DBT);
  • Air velocity (AV) – The air contact velocity measured in m/s;
  • Radiant temperature (RT) – The temperature of a person’s surroundings; generally expressed as mean radiant temperature (MRT) – weighted average of the temperature of the surfaces surrounding a person and any strong mono-directional radiation, like the sun radiation;
  • Relative humidity (RH) – The ratio between the current amount of water vapour in the air and the maximum amount of water vapour that the air can hold at that air temperature, expressed as a percentage.

Personal factors are also important but not depending on the environment:

  • Clothing – Clothes insulate a person from exchanging heat with the surrounding air and surfaces;
  • Metabolic heat – The heat we produce through physical activity. Usually, a person which stays is feeling cooler than other who is moving.

There are also other contributing factors that could be considered like availability of drinks and food, acclimatisation device, or the health status.

How to control thermal comfort in buildings?

Buildings Thermal Comfort SimScaleWe can take some actions to control the environment thermal comfort:

  • Easiest is to self-control the active systems (HVAC) and passive influence by monitoring and control the thermal environment;
  • For modern building this control could be made in all indoor spaces using some automatic and pre-set sensor system and a central management console;
  • Studying the thermal comfort factors with simulation models to establish better solutions for vans positions, the power of cooling/ heating system, humidity, and velocity;
  • Using thermodynamic and CFD simulation analysis for ergonomic positioning of the working places.

The personal factors could not be controlled by engineering methods. For buildings’ working places employers could establish a series of internal rules and recommendations. During the very cold or very hot seasons, the function of the geographic position specific rules can be applied to flexible working hours, adjusting tasks, adapted dressing code or personal equipment allowing (PPE – Personal Protective Equipment).

How can we predict thermal comfort?

We can use a large palette of actions and techniques to estimate the environment factors, including the predictions for effective, resultant, or equivalent temperature, and Wet Bulb Globe Temperature (WBGT).

According to suggestions of BS EN ISO 7730 and BS EN ISO 10551 standards, thermal comfort can be expressed in terms of Predicted Mean Vote (PMV) and Percentage People Dissatisfied (PPD).

These thermal comfort indexes were developed by Professor Ole Fanger, using the research from the Kansas State University and the Technical University of Denmark. The research purpose was to study how people are comfortable in different conditions and served to develop the comfort prediction equations. The equations are considering air temperature, mean radiant temperature, air movement, humidity, clothing, and activity level[4]. PMV is an index that predicts the mean vote of a group of people voting on how comfortable they are in an environment. PPD is a function of PMV.

Many times we are facing non-uniform conditions and therefore have to assume multiple assessments, considering complex environments, and Computational Fluid Dynamics (CFD) analysis may be necessary too.

How does a simulation software help us better control and predict thermal comfort in buildings?

 

Buildings Thermal Comfort SimScale

Comparison of velocity streamlines for variation in placements of inlets/outlets. The inlet is on top (and outlet on bottom) is shown on left while the opposite on right.

SimScale provides effective cloud-based simulation analysis for the design of HVAC systems. If you are an architect, a civil engineer or an HVAC systems designer, you can use CAE solutions to simulate workspaces optimal thermal conditions. 3D simulations can offer better solutions to evaluate thermal comfort. You can also optimize inlet and outlet vanes size and positions to minimize energy costs. Moreover, you can use SimScale for preliminary virtual testing of innovative smart buildings systems, such as underfloor heating or on the roof or passive ventilation systems.

The optimization of HVAC systems is based on multiple thermodynamic process analysis. You can simulate the airflow distribution and dynamics in any building space, starting from basic aspects like infusion of fresh and removal of stale air, heating infusion produced by the electronic devices, peoples’ working position, walls insulation, office cubicles or windows/ doors exposure to external factors.

You can find more details about the optimal simulation of thermal comfort factors and other practical aspects related to better control and predictions in office locations reading this specialised article: “How to Improve Thermal Comfort in an Office Environment”.

Moreover, in SimScale’s Public Projects library you can find many templates that you could use as starting model to better understand thermal comfort and to optimize HVAC systems designs for different rooms, office spaces, and special buildings scenarios.

White Paper: How to Ensure Thermal Comfort in Buildings with CFD


References:

[1] “Thermal comfort in buildings”, Designing Buildings Wiki, 2016

[2] ISO 7730:2005 – Ergonomics of the thermal environment – Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria, 2005

[3] “Health and Safety Executive HSE”, Designing Buildings Wiki, 2016

[4] – Fabbri, K. – “Indoor Thermal Comfort Perception”, A Brief History of Thermal Comfort: From Effective Temperature to Adaptive Thermal Comfort, Springer International Publishing Switzerland, 2015.

[5] – Harish, A. – “How to Improve Thermal Comfort in an Office Environment”, SimScale Blog, July 2016.

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