Why is the Supply Air Temperature 55°F?

Most experienced HVAC engineers, when questioned about what supply air temperature they typically use for comfort air conditioning applications, will answer 55°F almost without exception.  As consistent as this answer will be repeated, one would think that there must be some basic law of physics that makes it so universal.  In fact, the answer is not simple at all.  It is based on a series of principles that span several interrelated topics from physics and chemistry to the basic temperature regulation systems of the human body.  This discussion will provide a very brief overview of the parameters that conspire to make that 55°F supply air answer appropriate for comfort air conditioning.

The first element of the answer has to do with what constitutes human comfort (after all, we do call it comfort air conditioning).  But before we can address human comfort, we need to talk about the sea of atmospheric air that we all live in.  Atmospheric air is composed of a mixture of several gases, principally oxygen and nitrogen, but also containing a very small amount of water vapor.  Water vapor is always present in atmospheric air, and though its relative weight in the atmosphere averages less than 1% in temperate climates, it is nevertheless one of the most important factors in human comfort.  Its impact on human activities is in fact altogether disproportionate to its relative weight.

The human body, at the most fundamental level, is a heat machine with a complex set of mechanisms to control core body temperature.  The food we take in is used in chemical processes to provide the energy for our body to function.  In turn, this heat must be rejected to our environment (usually atmospheric air) to maintain what we refer to as “comfort” – not enough heat rejected, we feel warm; too much heat rejected, we feel cold.  The heat rejection that takes place from our bodies occurs by three principal means:

·       Convection – heat transfer based on the temperature of the air around us.  The higher the air temperature, the less heat transfer that occurs tending to make us feel hot; the colder the air temperature, the more heat transfer that occurs making us feel cold.

·      Radiation – heat transfer based on the difference in temperature between our skin temperature and the temperature of surfaces around us; we lose heat sitting next to a cold window surface in the winter even though the air temperature around us may be warm.

·       Evaporation – heat transfer based on the relative humidity or the amount of water vapor in the air.  Water leaving the surface of the human body (sweat) evaporates and cools the body.  The higher the relative humidity of the air, the lower the evaporation that takes place making us feel warmer; the lower the relative humidity, the more the evaporation that takes place making us feel colder.

In the years before about 1950 when our buildings were typically heated only, making people comfortable relied principally on convection and radiation – maintaining the air temperature was the only parameter that was needed for control in the winter.  If there were large windows, we installed warm radiators below them to (1) warm the air and (2) offset the radiation heat loss from the window.  Since the air in the winter was very dry (low relative humidity), evaporation heat transfer occurred at a relatively constant rate as needed and convection based on air temperature could be controlled to make us comfortable.  In the summertime, everyone inside buildings knew they were going to be uncomfortable since the air temperature was generally high (reduced convection heat transfer), the building surfaces were warm (less or no radiation heat transfer), and the air relative humidity was generally high (reduced evaporation heat transfer).

When air conditioning became common after the early 1950’s, the picture changed dramatically.  Suddenly the relative humidity became an equally important parameter in comfort.  In humid climates like St. Louis, summer outdoor relative humidity is high and the evaporation component of body cooling is appreciably reduced.  Exhaustive tests were performed in controlled settings to determine what combination of space temperature and relative humidity in indoor environments made people comfortable.  The result was that 75°F, 50% relative humidity became the target that seemed to please the maximum number of test subjects.  Thus 75°F, 50% relative humidity became the indoor air standard for summer air conditioning.  To maintain the indoor air temperature at 75°F in the summer, colder air needed to be introduced into rooms to provide cooling.  But the physics of the cooling process could be satisfied by introducing supply air into the room at any temperature below 75°F, as long as the quantity of such air could be controlled (much more 70°F air would be required than 50°F air for the same amount of room cooling).  But the evaporative side of the heat transfer equation turned out to be a bit more complicated.

In the summertime, the relative humidity in air conditioned spaces increases principally due to the evaporation of water vapor from the skin surfaces of the people in the spaces.  The process of controlling indoor humidity occurs in the same basic process as that of temperature control described above – drier air (below the 75°F, 50% relative humidity) is introduced into the spaces to limit the relative humidity rise to 50%.  Again, the physics of the drying process could be satisfied by introducing supply air into the room at any dryness below 75°F, 50% relative humidity, as long as the quantity of such air could be controlled (much more 75°F, 49% relative humidity air would be required than 75°F, 30% relative humidity air for the same amount of room air drying).  Though our technology provides many options to heat, cool, and add water vapor to air, only one simple option is available to remove water vapor from air.  And that option is only available because of a unique property of the air/water vapor atmosphere we live in – as air is cooled, it can physically hold less water vapor.

If we pass air over a cooling coil that is at a colder temperature than the air entering it, cooling of the air occurs.  If the air is humid as on a rainy spring morning, the air is cooled but because the cooler air can hold less moisture, water vapor begins to condense out of the airstream and onto the coil as liquid water.  If the coil is very efficient, the air leaving the coil is at nearly 100% relative humidity (a condition referred to as saturation).  The air leaving the coil now becomes both cooler and drier.  It turns out that if you pass 75°F, 50% relative humidity air across the coil, at a coil temperature of a little below 56°F, water begins to condense on the coil and air cooling and drying begins to take place.  At a coil leaving air temperature of about 55°F, the drying effect of this air leaving the coil is sufficient to reduce the water vapor increase from the building occupants for typical people densities in our modern buildings.  And because of the property of air/water vapor described above, the 55°F air is at nearly 100% relative humidity and a single specification of 55°F supply air temperature satisfies both the cooling and the drying of the air necessary for comfort air conditioning.  Thus, all of these parameters together make the simple answer- deliver supply air at 55°F.

As year-round air conditioning systems were then developed over the ensuing years, most of these systems were based on delivering 55°F supply air to satisfy the air cooling and drying required during summertime conditions.

8760 Engineering would like to thank Jerry Williams for this blog post, and stay tuned for several more insightful entries from our very own senior engineer!

To end this blog post we present and energy conservation measure that almost always exist and is never found unless an engineer employs data logging.  This is the case of the broken heating valve, which will heat the air up in excess of 100°F before it is cooled back down to 55°F.

ECM:  Replace Cracked / Failed Heating Valve

kWh Saved per CFM: 86

Article Author:  Jerry Williams
jwilliams@8760engineering.com

Blog Post:  Ryan Corrigan 
rcorrigan@8760engineering.com