Engineering & Technology · Building Systems, Services & Environmental Performance
Comfort, ventilation, air quality and indoor conditions
Two rooms have the same thermostat reading, yet one feels cold beside a window and the other feels stuffy. Air temperature is only one part of indoor conditions. Thermal comfort and the concentration of a particular airborne substance require different observations and models.
Use heat transfer, civil systems, electrical quantities and the preceding instrumentation/control chapters. Elementary energy and material balances support the worked examples; all numerical building inputs are supplied teaching assumptions.
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Distinguish comfort and air-quality questions and interpret a simple ventilation mass balance within its assumptions.
Comfort depends on people and their surroundings
Air temperature, surrounding radiant temperatures, air movement and humidity influence heat exchange with a person. Clothing and activity also matter, and individuals can experience the same room differently. A thermostat at one location does not describe every occupied zone or every surface temperature. Specify where, when and for whom a comfort observation applies.
Indoor air quality concerns a range of substances and conditions, rather than one universal clean-air number. Source control, outdoor-air ventilation and pollutant-appropriate cleaning serve different functions. Recirculating air is not automatically outdoor-air ventilation. A filter’s action depends on the substance, collection mechanism and flow; removing particles does not establish removal of all gases.
A fictitious tracer makes the assumptions visible
Consider a 150 m³ room with equal outdoor inflow and exhaust of 300 m³/h. Supply outdoor tracer concentration 10 mg/m³ and constant internal generation 6000 mg/h, beginning at time zero. Assume instantaneous uniform mixing, constant volume and no other sources, reaction, deposition or filtration. The tracer is hypothetical and has no assigned exposure limit.
The material balance is V dC/dt=G+Q(Cout−C). Equilibrium is Cout+G/Q=30 mg/m³, and the time constant is V/Q=0.5 h. Starting at outdoor concentration, C(t)=10+20(1−exp(−2 t)) mg/m³ when t is in hours. At half an hour it is about 22.64 mg/m³, not yet at equilibrium.
A useful model does not describe every breathing location
A short circuit between supply and exhaust can leave another area poorly mixed. Sources close to a person can create a local concentration different from the room average. Outdoor air can carry its own pollutants and moisture, and ventilation changes can affect thermal loads. A measured value therefore needs location, timing and relevant source information.
Temperature or a convenient tracer can help investigate conditions, but neither establishes the concentration of every other substance. Establish which quantity a sensor measures and which inference connects it to the concern. Retain maintenance, occupancy and system-mode information so an apparently stable reading does not hide a changed source or an ineffective airflow path.
Separate dilution arithmetic from a design decision
In the supplied steady model, doubling outdoor exchange to 600 m³/h would reduce the excess G/Q from 20 to 10 mg/m³ and the time constant from 0.5 to 0.25 h. Equilibrium becomes 20 mg/m³ because outdoor concentration remains 10. Halving generation at the original flow gives the same equilibrium excess but retains the original 0.5 h time constant.
Neither comparison selects a ventilation rate for an actual building. The invented substance, assumed mixing, source history and absent removal mechanisms define only a teaching account. Actual evaluation must address the relevant contaminants, occupied locations, environmental conditions and intended criteria. Comfort observations remain necessary even if one tracer balance agrees with measurements.
Try the reasoning
Why is indoor equilibrium not zero when generation stops?
Show the answer and reasoning
Under this model it approaches the outdoor concentration, which is 10 mg/m³, rather than zero.
Does twice the recirculation necessarily mean twice the outdoor exchange?
Show the answer and reasoning
No. Recirculation can move indoor air without replacing it with outdoor air; the flow paths must be identified.
Do equal thermostat readings prove equal comfort?
Show the answer and reasoning
No. Radiant conditions, air movement, humidity, clothing, activity and individual experience can differ.
What to carry forward
Specify the indoor quantity and occupied conditions, then test the airflow and source assumptions behind the interpretation.
References
Teaching resources and research supporting this lesson’s explanations and examples.