Maintaining comfortable thermal conditions in a building is very important. Even minor deviation from the comfort zone may lead to reduced productivity and a higher risk of illness. Workers who are already stressed are far less tolerant of temperatures and levels of humidity outside of the comfort zone.
Comfort Zone
According to the West Midlands Public Health Observatory in the UK, 21°C (70°F) is the recommended temperature for a typical domestic living room/lounge. 18°C (64°F) is the recommended temperature for bedrooms. The ability of a heating system to maintain this temperature depends upon effciency and controls-sensitivity. A minimum "heating-on" setting should be 19ºC and the maximum "heating-off" setting should be 23ºC. These are approximate temperatures. More sensitive heating systems may operate using a proportionate control philosophy, allowing the heating system to react in accordance with the heating demand. See controls philosophy for more information.
Humidity & Ventilation
Now it is all good and well to heat an internal environment, but as soon as a door or window is opened your nice warm air flows straight out, and this reduces the ambient temperature. "Ambient temperature" is simply the average/general room temperature.
When relative humidity is kept at about 50%, office workers have fewer respiratory problems. This is particularly true in colder months. As a result, they generally feel better. Higher levels of relative humidity make the office feel stuffy. The word "stuffy" refers to the high level of moisture in the air, which at extremely high percentages makes the air actully feel damp. More importantly, prolonged high levels of humidity contribute to the development of bacterial and fungal growth and aside from being unsightly this can be hazardous to health - some strains of fungus can even be fatal. See mould & prevention in the bathrooms section for more information.
Humidity deals with the amount of moisture that a given amount of air can actually hold, and is understood in the industry using two terms. One is absolute humidity and the other is relative humidity.
Absolute humidity is the percentage of water vapour actually present in the air i.e. of all the air in the room, this value represents how much is actually water vapour.
Relative humidity is the absolute humidity divided by the amount of water that could be present in the air. Relative humidity indicates the degree of comfort or discomfort one feels from the humidity, because it indicates the amount of perspiration that can evaporate from the skin. See sweating for a brief explanation of why.
Levels of relative humidity below 50% cause discomfort by drying out the mucous membranes. This contributes to skin rashes and electrostatic shocks. Any static you may "build up", for example by walking on synthetic carpets, is not readily dissipated into moisture in the air, but is stored in your body until you become grounded. Common means by which people experience this grounding and hence shocking are by touching table legs or chair legs etc.
Quick Calculation
Various methods have been devised to assess human comfort by presenting temperatures in different ways. A quick method of calculating potential internal temperatures based on minimal known information is as follows:
First of all we need to establish the mean radiant temperature. This is the temperature of a room due to the surfaces within it radiating heat. It does not take the presence of air into account, so for now imagine the room has been emptied of air. A suitable internal room temperature is 23oC, so I will assume this to be the mean radiant temperature. This assumption should be made having taken into account the location of the building geographically, because hotter countries generally tolerate higher internal air temperatures.
Now measure the temperature of the air inside the room. For simplicity say I measured the air temperature and it was 20ºC. This is the dry bulb temperature and it should be taken near the middle of the room at approximately mid-height. A mecury-in-glass thermometer will suffice.
Now use the following formulae to calculate the potential internal room temperature. This is a good guide to what the temperature within the room will be when there are no unusual draughts:
Temp (inside) = 2/3 x Mean Temp Of Room + 1/3 x Air Temperature Of Room
The resulting value is a combination of air temperature and radiant temperature. It is an approximation of the room comfort level, presented as a temperature. Radiant heat is felt and will travel in a vacuum. The air temperature may be a result of many factors, hence we measure or assume it for quickness and simplicity.
Comfortable Temperatures
Using the method above, 20oC to 22oC is fine for situations where people are generally sitting down, but if physical activity is taking place then a lower temperature is necessary.
Another factor that should be considered is the age of room occupants. Small babies and elderly people require a higher room temperature, possibly up to 24oC in order to feel comfortable.
Air Movement
High rates of air movement in a room can cause discomfort, especially if the air is too cold i.e. in winter. Cooler air tends to travel at floor level and can cause discomfort around the ankles. Low levels of air movement can also cause discomfort, because moisture in the air will not be removed quickly enough, leading to stuffiness.
Air velocities between 0.10 m/s and 0.45 m/s are generally acceptable, but this depends on conditions such as dry bulb temperature, humidity and clothing and the nature of work taking place within the space.
To allow for dry bulb temperature the graph below gives acceptable values for air velocity to maintain a level of comfort.
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Section:
Thermal
Comfort
In More Detail
Created:
24 Oct 07
Updated:
06 Nov 07
For general use in buildings where the air temperature is acceptable, 0.15m/s is an acceptable air velocity to use.
Air velocities less than 0.10m/s may cause a feeling of discomfort. At higher values of over 0.45m/s draughts may become a problem. Air velocities of less than 0.01m/s generally result in stagnant conditions.
