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Make it comfortable for real People
by Hakan Falk at Energy Saving Now. (energysavingnow.com)
The majority of installations in the world are neither designed or dimensioned to minimize the energy comsumption nor to maximize the comfort. The most common methods to calculate and dimension the systems are grossly misleading and waste energy. The method of regulate the systems with only the air temperature, representing only 24% of the human body system, adds to a large waste of energy. The only way that you can regulate a system with a parameter that only represent 24% of the environment, is if the system is correctly designed and dimensioned and therefore is in balance with the other factors.
The result is that enormous savings can be done only by correcting mistakes. We are talking about savings in the range of 10% to 50% and ROI (Return Of Investments) of typically less than 1 to 3 years. It is unbelievable that Sweden still is the only country in the world that mention surface temperatures in it's Building Code. The predominant use of "closed constructions", also provide for a humidity control. The result of this can easily been seen in the EU commission suggest common building code for thermal insulation. Sweden is the only country in Europe that would use more energy with the EU suggestion, it is of course no expressed demands for that.
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A person will feel comfortable when he/she does not feel too cold or too warm and can maintain the body temperature with ease. The human system for maintaining and regulating is very good and make use of a number of environmental factors for it's comfort, it is sensitive for the following,
- Radiation exchange with hot and cold surfaces.
- Air temperature.
- Humidity.
- Air movements.
- Symmetry between the factors.
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The person is also a part of his environment and produces energy corresponding to the above factors. In total a person produces approx. 80 watt of energy without working and more if exercise is performed. The person must get rid of the heat generated and are using the body's systems to do so. If the person cannot do this the body temperature will go up and with more than 0.5 centigrade movement of body temperature, it is uncomfortable, If the body looses more than the produced energy, the feeling is that of freezing. The energy is divided as follows,
- Radiation 50% (surfaces temperature)
- Convection 24% (air temperature)
- Transpiration 22% (humidity)
- Misc. 4%
Radiation to and from surfaces
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A person will feel radiation from and to surfaces and it will contribute to his perception of temperature and feeling of comfort. It is easy to identify radiation from warm surfaces, but radiation to cold surfaces is often wrongly identified as air movement (draft). If it is both a warm and a cold surface radiating in the same direction the person will feel the sum of them and they compensate each other. This is the reason for always place radiators on outside walls and under windows in the rooms. Placing radiators on partition walls and especially opposite outside walls, is a design error.
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Air temperature
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Because air temperature is easy to measure it is often wrongly used as a general measurement of comfort. Under good conditions 18-20 centigrade air temperature will be perceived as comfortable by most people. It is very often a large difference between the measured temperature and the temperature perceived by people. The perceived temperature is a combination of the earlier mentioned comfort factors. Air temperature can often compensate variations in radiation, air movements or humidity, i.e. during a sunny, cold and dry spring day, the temperature in the sun can be perceived as more than 22 centigrade even if the air temperature is 10-15 centigrade.
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Humidity
The relative humidity is also an important factor for the feeling of comfort. The air can at a certain temperature only contain a certain maximum amount of water, this is described as 100% relative humidity. Warm air can contain more water than cold air and if warm humid air is cooled down it will condense the surplus of water i.e., fog. If surfaces are colder than air temperature it will cool down the air close to the surface and if the air is humid it will result in condensation on the surface i.e. mirrors in bathrooms during hot showers. The following diagram shows the relationships between temperature, relative humidity and amount of water in the air.
People are dependent on evaporating water from the skin (transpire) to keep an ideal body temperature. If the air is very humid, it cannot take up more water, the evaporation cannot take place and people will feel uncomfortable. This can vary with the metabolism for different people i.e., slim people that do not have to watch the weight, have a high burning rate and will be less effected by warm humid conditions, where people with low burning rate will feel very uncomfortable. Normally people will feel comfortable between 30% to 70% relative humidity.
Air movements
Air movement (wind and draft) can have a major effect on the perceived temperature. One sample is the wind chill effect that now is popular to mention in weather reports. Everybody also knows the effect of ventilators during hot summer days. This must be considered when doing the construction design. A good construction is little effected by outside winds and does therefore include wind breaking wall construction, doors and windows. Air movements can also be caused by cold surfaces, since the cold air close to the surface will move downwards by thermal forces. This kind of air movements will cause draft and negatively influence the comfort, but it's influence is heavily overstated by the common methods of design. This is the reason why ideal placement of radiators is under the windows, where it will both compensate for draft and the radiation to a cold surface.
Symmetry
We have means of calculating a comfort factor, which considers the directions of which the body radiates the heat and the air temperatures. If the symmetry is unbalanced the feeling is uncomfortable i.e. an open fire in a cold room which gives a lot of heat to the front, but the back is very cold. The comfort factor (symmetry) is a measurement of the influence of the difference of temperature of the surfaces and is described as operative temperature.
Dimensioning factors for energy saving and waste.
Everybody talks about the air temperature as the most important factor for
our comfort. It is simply not true!!!
Dimension systems for real People
It is few "experts" that understands this, but it is now used in several important areas, were the body needs to keep its temperature. The modern rescue blankets or cloths for antartic climats are samples of this. The Swedish Building Codes is an other sample.
A person will feel comfortable when he/she does not feel too cold or too warm and can maintain the body temperature with ease. The human system for maintaining and regulating is very good and make use of a number of environmental factors for it's comfort, it is sensitive for the following,
- Radiation exchange with hot and cold surfaces.
- Air temperature.
- Humidity.
- Air movements.
- Symmetry between the factors.
The person is also a part of his environment and produces energy corresponding to the above factors. In total a person produces approx. 80 watt of energy without working and more if exercise is performed. An athlet can momentarely produce up to 1 kW. The person must get rid of the heat generated and are using the body's systems to do so. If the person cannot do this the body temperature will go up and with more than 0.5 centigrade movement of body temperature, it is uncomfortable, If the body looses more than the produced energy, the feeling of freezing occurs. The energy is divided as follows,
- Radiation 50% (surfaces temperature)
- Convection 24% (air temperature)
- Transpiration 22% (humidity)
- Misc. 4%
The majority of installations in the world are neither designed or dimensioned to minimize the energy comsumption. The most common methods to calculate and dimension the systems are grossly misleading and waste energy. The method of regulate the systems with only the air temperature, adds to the waste of energy. The only way that you can regulate a system with a parameter that only represent 24% of the environment, is if the system is correctly designed and dimensioned and therefore is in balance with the other factors.
The result is that enormous savings can be done only by correcting mistakes. We are talking about savings in the range of 10% to 50% and ROI (Return Of Investments) of typically less than 1 to 3 years. It is unbelievable that Sweden still is the only country in the world that mention surface temperatures in it's Building Codes.
Calculation methods
This is a controversial subject.Mainly because the most used calculation methods and dimensioning rules, should have been replaced many years ago. Some can be compensated with the knowledge that we are communicating on this site and the really bad overdimensioning can be avoided. The reality is that over 80% of what is done today is still overdimensioned and based on regulation of air temperature and enormous waste of energy resources. Together with inadequate control policies, it is also the major reason for peak demand problems.
With overdimensioned systems, it can be very difficult to save energy and maintain comfort. With time, we will build up a good understanding and advices for different situations.
Accumulation of energy in constructions
It is necessary to take in account the accumulation effect in the construction. This is very important for buildings with heavy constructions, since the effect of changes in weather will be accounted for in a proper way. It also shows the obvious need for considering the type of construction, when designing and regulating the heating or air conditioning system i.e. a light construction benefits from system with sensors outside, inside and on the water, a heavy construction benefits from a more simple system or a system who can learn the behavior of the building.
Building Codes
Building Codes show large variations in its need of energy. We have studies of office buildings in US that demands 800 kWh/year and square meter and corresponding office buildings done in Sweden with the help of simulations and a realistic Standard that demands 70 kWh/year and square meter.
In the new Swedish Building Codes, considerations is taken to surface temperatures and more natural demands. This give considerable savings, compared with Building Codes built on more or less constant air temperatures.
To achieve significant energy savings, the Building Codes needs to be subject to overhauls. If the respective societies with its technical and political resources, decides to revise its Building Codes, very large savings can be done.
Hakan Falk
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