QUALITY V COMFORT

The Building Code (Section G4) suggests two methods of obtaining acceptable indoor air quality.

1) Opening windows equal to at least 5% of the floor area is acceptable.

2) Mechanical ventilation, NZ Standard 4303 "Ventilation for Acceptable Indoor Air Quality" specifies ventilation for classrooms at the rate of 8 litres per second per occupant.

To comply with NZS4303 and the Building Code a mechanical ventilation system needs --- to exhaust air at 8 litres per second per occupant, and to supply air into the classroom at the rate of 8 litres per second per occupant.

There is a significant difference between Indoor Air Quality and Indoor Air Comfort.

In winter, ventilation at the rate of 8 litres per second per occupant may make the classroom environment too cold for comfort, and in summer, ventilation at the rate of 8 litres per second per occupant may ensure the indoor air quality is acceptable, but this rate may be insufficient to provide comfort conditions.

Our experience is that there is a wide variation in the needs of classrooms to achieve both indoor air quality and indoor air comfort.

On the left is a selection of installations where we have successfully achieved these results.

The most common needs for improvements to classroom ventilation are those rooms where computers are installed. These rooms are often over hot, and have "stuffy" atmospheres.

When an indoor atmosphere is referred to as "stuffy", the cause is most commonly excessive humidity, and carbon dioxide. Both are the by-products of human breathing and occupation, and in a room of relatively small cubic capacity in relation to the numbers of occupants, the entire atmosphere of the room becomes affected by the number of occupants.

Body odours add to the problem. A relatively new unit of measure "the Olf" enables odour to be quantified.

The solution is ventilation. The rate of ventilation to achieve healthy indoor air quality is 8 litres of fresh air per occupant. (This rate is in a normal classroom -- in science, or other special purpose rooms where chemical experiments etc may take place higher rates may be necessary).

However indoor air quality does not mean that indoor comfort is achieved.

For example; In a classroom with windows exposed to North and West, where the roof of the classroom is painted a dark colour (absorbing heat radiated from the sun), and where there are 30 pupils and a teacher, each with a computer, plus lighting, the amount of heat that is being added to the room on a sunny Summer's day, may be in the order of 15kW to 25 kW.

Consider that a large capacity Office Window Air Conditioner or home "heat pump" as in a lounge room may be able to remove 4kW of heat, in cooling mode.

A really hot classroom may need 5 to 8 window air conditioners just to remove the heat from the room, and cool the classroom a few degrees.

Add to this, the fact that, even if the classroom air is cooled (or heated in winter), ventilation is required. Cooling or heating the same air does not make it fresh and safe to breathe. 8 litres per second per occupant is approx 250 litres per second for an average classroom.

Comparatively, two large domestic clothes dryers, or two range hood fans are approximately this capacity. Because the classroom has to be "ventilated" there must be exhaust and supply fans equal to this rate -- not just an exhaust, or just a supply fan.

Most Air Conditioners, Heat Pumps, and Heaters simply recirculate the same indoor air over and over.

Cooling indoor air in summer does not mean that the quality of the indoor air is healthy.

NZS4303 specifies that acceptable Indoor Air Quality is achieved if Carbon Dioxide is less than 1000 parts per million (ppm).

We have data logged over 100 classrooms the length and breadth of NZ, and in winter all are similar:

1000 ppm CO₂ is reached after approx. 30 minutes occupation, and after 1½ hours 3000 ppm CO₂ is almost consistent in every classroom on a typical winter’s day.

Before entering into any heating or ventilation contract, schools should demand that the supplier certifies that the indoor air quality is guaranteed to be 1000ppm CO₂, or less, with the system operating as designed. If this cannot be guaranteed, then the system does not meet NZ Standards.

If opening windows is necessary to achieve acceptable Indoor Air Quality, then request running costs and energy waste figures.  What point is there building an energy efficient classroom block, if the windows have to be opened in winter, to gain ventilation at the required rate of 250 litres of fresh outdoor air per second???