Occupant Controlled Windows with HVAC
People like to think they have control of their environment. For air-conditioned buildings without operable windows, there is a desire “to have a thermostat in my office.” In fact, many maintenance staff have discovered that the presence of a thermostat can be very satisfying even when it is not connected! Hence the use of the phrase ‘dummy thermostat.’
This desire for control is often successfully exercised in the demand for occupant controlled windows, operable windows. Unfortunately, people are not good at assessing when to have the window open or when to close it. This is where good communication can have a very beneficial effect. People are generally cooperative if they understand why they should be cooperative. You would be surprised at how many buildings have occupants running win- dow air-conditioners while the windows are open. The owners make no effort to explain the waste and lack of dehumidification that occurs when the air-conditioner is cooling while the window is open on a hot, humid day. The result is fewer satisfied occupants and the owner has a higher electricity, or energy, bill. If you are faced with a situation like this, try to let the occupants know the benefits that will affect them if they use the system more efficiently.
Actual ventilation depends on orientation, building height, wind direction and wind speed. In narrow buildings with windows on both sides, a cross flow can be very effective. One problem is that on the incoming side occupants may experience an unacceptable draft if they are close to the windows.
In winter, in colder climates, the warm, less dense air in buildings tends to rise. As a result, there is a constant inflow of air through openings that are low in the building and a outflow high in the building. An occupant who opens a ground floor window in a three story apartment building receives an incoming icy blast. The window is quickly shut and remains closed. On the other hand, the person on the third floor can open their window wide and the warm air from the building will flow outward. They can leave their win- dow open, letting the warm air, and energy, of the building continuously vent outside. In this situation, the windows are unusable low in the building and a great waste of energy for negligible ventilation high in the building. The problem of providing enough ventilation without a huge energy waste is addressed in Canada and parts of northern Europe by requiring mechani- cal ventilation in residences. This has, in turn, made a variety of heat recov- ery units quite popular, and in many places mandatory, although their cost is often not recovered from the energy savings when fan power is included in the calculations.
In mild climates, operable windows can be used to both ventilate the build- ing and provide overnight pre-cooling with judicious building design and use.
The ventilation benefits of windows, and the challenges of their operation are being addressed in some new buildings by having the windows controlled automatically. The control system may have sensors for wind direction and speed, solar intensity as well as interior and exterior temperature sensors to aid in the decision making process.