cvanoverbeke I agree with your approach. The majority of owners/developers are not in tune with ASHRAE recommendations and thus are looking for standard residential split systems or packaged rooftops for corridors.
For residential building’s corridor HVAC systems, the design intent is to provide constant positive pressure, temperature and humidity control utilizing 100% conditioned outside air at a rate of 0.5 cfm/sq.dt. or 3 AC/Hr (whichever is greater) following ASHRAE guidelines. The supply air is constantly tempered (cooled or heated) to maintain acceptable indoor temperature (68°F - 72°) and dehumidified (40% - 60% RH summer) to maintain recommended humidity levels based on ASHRAE Standard 55-2004 – Thermal Environmental Conditions for Human Occupancy. In addition, required outdoor air ventilation rate as prescribed per ASHRAE Standard 62.1-2004 – Ventilation for Acceptable Indoor Air Quality is maintained.
ASHRAE Applications Handbook – 1999 Edition recommends the corridors are maintained under positive pressure to prevent infiltration of warm, moisture laden air through building cracks, gaps, shafts, etc. Assuming a residential split system DX unit is used for pressurization and temperature control (humidity is uncontrolled), the unit would operate as follows. The unit’s thermostat would be set to the “On” position, meaning the fan would run 24 hours a day/7 days a week and outdoor air would be constantly introduced thru the unit’s return air plenum and then to the space. The compressors would cycle on based solely on indoor space temperature requirements. Thus the supply air would only be dehumidified when there is a call for cooling via the local thermostat. ASHRAE states:
“….introducing untreated outside air directly into the return air plenum of the HVAC unit at part load or low load creates a severe high humidity problem, which is one of the causes of mold and mildew. The situation is further aggravated when the HVAC unit operates in on-off cycle during part or low load conditions”
The cooling load profile for the corridors is such that the HVAC equipment must only handle part or low cooling load and peak cooling loads rarely happen. Thus the unit will cycle on and off only to satisfy the space temperature set point. Under the more common part load conditions, satisfying space temperature set point can occur quite rapidly. In “short cycling”, the unit is not in cooling mode for sufficient time period to address the latent load of the moisture in the air – i.e. humidity. ASHRAE further states:
“The quantity of outdoor air introduced into the rooms or corridors is usually slightly higher in excess of the exhaust quantities to pressurize the building. To avoid adding any load to the individual systems, the outdoor air should be treated to conform to indoor air temperature and humidity conditions. In humid climates, special attention must be given to controlling the humidity from outside air. Otherwise, the outdoor air may reach corridor temperature while still containing a significant amount of moisture.”
Changing the operation of the unit (thermostat in “Auto” position) for the fan to only cycle on when there is a call for cooling to prevent untreated outside air from being introduced to the space creates additional issues. Outside air for building pressurization is now only introduced when the unit is in cooling cycle, thus not maintaining ASHRAE recommended building positive pressurization. And again, the humidity levels remain uncontrolled.
Utilizing residential split system DX HVAC units will accomplish space temperature control but is not capable of maintaining constant building positive pressurization, nor control humidity to ASHRAE recommended levels. Residential split system DX HVAC units are not suitable for this application.
Andy W.
