Choosing a System

In determining the best HVAC system for a given installation, the HVAC system designer is well advised to consider as many relevant factors of performance and cost as possible. For hydronic and air-based HVAC systems, safety isn’t a paramount consideration because these systems don’t generally pose a potential hazard to building occupants. This is not true of VRF systems because they use refrigerant throughout the occupied and unoccupied spaces in a structure.

The major concern about VRF systems is the safety factor stemming from a potential refrigerant leak. As stated earlier, refrigerant is a toxic fluid that can be lethal in sufficient concentrations and, as such, is classified as a hazard to human health in the relevant codes, namely, ANSI/ASHRAE Standards 15 and 34 and the International Mechanical Code (IMC).

These codes define how refrigerant systems are applied, including classifying refrigerants in terms of their hazard to human health and the refrigerant concentration limits required to avoid immediate and long-term negative effects on human health. These codes place restrictions on how much refrigerant can be discharged into a living area or workspace. Refrigerant gas is heavier than air, and because of this, it displaces oxygen in a room—if it takes enough oxygen out of a space, a person exposed to it can suffocate. Because refrigerant can’t be readily detected by human senses—it cannot be seen or smelled—codes also require refrigerant alarms and ventilation systems in spaces where the concentration of the fluid is high enough to cause a lethal accident.

ASHRAE Standard 34 defines both a refrigerant concentration limit (RCL) and an oxygen-deprivation limit (ODL). The RCL is “intended to reduce the risks of acute toxicity in normally occupied, enclosed spaces” and is typically expressed in units of pound per 1000 ft3 or concentration in parts per million (ppm). The RCL deals primarily with physiologic effects on humans to the heart, lungs, and central nervous system. The ODL defines “the concentration of a refrigerant or other gas that results in insufficient oxygen for normal breathing.” The ODL deals with asphyxiation.

These two parameters can be used to determine the maximum amount of refrigerant that can be released into a space in the event of a catastrophic leak of refrigerant to prevent injury and death. Because of the nature of hydronic and air-based systems, these restrictions don’t apply because the systems pose no similar threat.

Safety concerns aside, in general, it is advisable to choose a heating/cooling system on the basis of its total life-cycle cost rather than solely on the initial installation cost. Making a choice based on first costs can prove to be very expensive over the life of a building despite apparent savings in the beginning.

Energy and maintenance costs vary widely for each type of HVAC system. Recently, variable-speed equipment, which increases efficiency at part loads, further complicated making system cost comparisons. There can be so many variables involved that making a wise determination unaided becomes nearly impossible. However, today there are computer programs that designers can use to greatly simplify the very complex calculations involved.