Thinking Inside the Box - Volume 4
Ventilation: Fresh Air or Hot Air
By Moses D. F. Ling, PE, RA
At this time, all municipalities in Pennsylvania have either opted in or opted out of the administration responsibilities of the Uniform Building Code. The code being enforced is the International Building Code and a number of associated codes. The intention is to have building designs be code compliant regardless of their location within Pennsylvania . Some locations such as Centre Region will see very little impact while more rural areas will be facing some of the code provisions for the first time.
One of the sections that is sure to impact the building industry, as implementation becomes more widespread, is Chapter 4 of the International Mechanical Code – Ventilation. The code requires that outside air be either naturally or mechanically introduced to occupied spaces.
Natural ventilation can be satisfied by providing operable windows, door and louvers with area equal to or exceeding 4% of the floor area. Mechanical ventilation rates are specified in Table 403.3. This appears rather straight forward. However, the impact of this quantity of outdoor air, if not properly treated, can negatively impact the conditions of the occupied space.
The psychrometric properties of the outside air vary as the weather conditions change throughout each day. It can be very desirable at one point and unacceptable later the same day. The psychrometric conditions can be divided into several general categories. The impact of three of these categories can cause significant problems.
- Cold and Dry – In the winter, outside air contains very little moisture. When low moisture contentcold air is brought into the building and heated, the resultant relative humidity drops drastically. Low humidity indoor conditions below 20% are common.
- Hot and Humid – In the summer, hot and muggy conditions demand that the cooling system be capable of reducing the temperature and wringing out the humidity. It is often assumed that the air-conditioning system can achieve the task. In reality, dehumidification is a by product of cooling. Often, the system would maintain the desired temperature but the humidity is out of controlled. If occupant comfort is the only objective, some variation in indoor humidity may be acceptable. People are not very good thermometers. High humidity, however, will foster the growth of mold and other micro-organisms which can cause air quality problems.
- Cool and Humid – If dehumidification is achieved by the cooling effect of the air-conditioning system, then how is dehumidification accomplished when the temperature of the air does not demand cooling? It is NOT, with most conventional commercial HVAC systems. Systems with better performance and controls can be designed to handle these conditions. One common technique is cooling with reheat dehumidification.
So Moses, what does this have to do with architecture?
- This brings the discussion back to several points in the initial presentation of Thinking Inside the Box. Is the project set up for failure? is adequate attention paid to these issues? Is the project design and construction budget allocated properly? To view the past Inside the Box discussions, log on to www.lingpartnership.com
- Uncontrolled humidity can reap havoc on architectural woodwork. Dry conditions will cause the joints to come apart. Humid conditions will cause swelling. If the swelling is excessive, buckling can occur. Don’t look at the carpenter. Look at how design decisions were made.
While outside air issues affect every building, its impact is most pronounced where the occupancy load is high.