Air Conditioning - The Black Art of Air Quality Control

"Contradictory nonsense from wikipedia"

It amazes us at kwiqly that Air Conditioning and Heating is controlled so inefficiently, but on closer inspection it is completely understandable - it must be due to a combination of factors as follows :

Firstly there is no clear and universal fresh-air policy - take the following contradictory nonsense from wikipedia and try to interpret it. While humans are broadly similar all over the planet it appears  :

  • UK schoolchildren can manage 1,500 ppm, 
  • National Institute for Occupational Safety and Health says 1,000 ppm are a marker that suggests inadequate ventilation
  • ASHRAE the experts in America suggest it should be better by  600 ppm than outdoors.
  • Europeans can handle 3500 ppm.
  • US Occupational Safety and Health Administration says 5,000 ppm is OK for prolonged periods

From ASHRAE we must assume that every city and mountain peak have equally fresh air ! ...
and ASHRAE is the American Society of Heating, Refrigerating and Air-Conditioning Engineers - who ought to know ???  So what is the lowly engineer to do ?

Extract from ...

Carbon dioxide

Carbon dioxide (CO2) is a surrogate for indoor pollutants emitted by humans and correlates with human metabolic activity. Carbon dioxide at levels that are unusually high indoors may cause occupants to grow drowsy, get headaches, or function at lower activity levels. Humans are the main indoor source of carbon dioxide. Indoor levels are an indicator of the adequacy of outdoor air ventilation relative to indoor occupant density and metabolic activity. To eliminate most Indoor Air Quality complaints, total indoor carbon dioxide should be reduced a difference of less than 600 ppm above outdoor levels. NIOSH considers that indoor air concentrations of carbon dioxide that exceed 1,000 ppm are a marker suggesting inadequate ventilation. ASHRAE recommends that carbon dioxide levels not exceed 700 ppm above outdoor ambient levels.[14] The UK standards for schools say that carbon dioxide in all teaching and learning spaces, when measured at seated head height and averaged over the whole day should not exceed 1,500 ppm. The whole day refers to normal school hours (i.e. 9.00am to 3.30pm) and includes unoccupied periods such as lunch breaks. European standards limit carbon dioxide to 3500 ppm. OSHA limits carbon dioxide concentration in the workplace to 5,000 ppm for prolonged periods, and 35,000 ppm for 15 minutes.
So Wiki talks utter nonsense (and represents one assumes the combined intelligence of a lot of contributors) -I have no idea what the right answer is - and I might be considered an expert. (but let's not forget Moulds, NOx, Legionallae, CO, So and all sort of other factors that come into play before we have even considered heat and humidity !!!

A pragmatic approach
So lets move on and just assume that there exists some minimum policy for fresh air ventilation, even though defining it is surely a black art.
Then we have to consider targets for humidity and air temperature.  These are also culturally diverse, but generally it is easy to define an adequate envelope of suitable conditions.
Having done this the controls engineer must simply decide how best to achieve this, which is dependent on a huge array of possible plant configurations (yes almost every building is different), but we can break out a theme or two:
Continual vs non-continual occupancy
Which functions are available ? - 
Passive or Forced ventilation, Humidification, Dehumification, Recirculation, Heat recovery (run around coils, crossover plates, enthalpy or heat wheels etc), Heating, post- dehumidification reheat, Cooling and so on...
Now the controls engineer sitting in his clean office (who may understand bits and bytes, but has probably never seen a valve actuator) overlooks the fact that when you switch off cooling and switch on heating, the two co-exist for a while (a radiator coil does not get cold the moment you isolate the heat source). So the control engineer maintains the system he has designed never heats and cools at the same time.  He is right to maintain that he has told it not to, but then there are those inconvenient rules of physics that stand in the way of the mighty controls engineer.
And guess who loses due to this systematic misunderstanding  - We do !
PS - Are you aware that you can get free heating and cooling degree-day data for anywhere in the world from us ?