Two-pipe HVAC systems are so named because they use the same pair of pipes to bring hot and cold water into a building for heating and cooling purposes. This is different from a four-pipe system, which utilizes two pairs of lines -- one pair for hot and one pair for cold water. The idea behind a two-pipe HVAC system is that it can serve as a cost-effective means of adding air conditioning to buildings.
According to the University of California at Davis Energy Efficiency Center, two-pipe HVAC systems "fell out of favor" because they were high-maintenance, compared to four-pipe systems. However, newer designs for two-pipe systems have again made them a viable option for efficiently and cost-effectively heating and cooling commercial and residential structures. New two-pipe HVAC systems can be installed directly to a home or business during construction, or an existing system can be retrofitted.
A primary feature of a two-pipe HVAC system is its heat trap. The heat trap is where the boiler and chiller outlets meet and allows for the system to send hot or cold water to the boiler or chiller without using any additional parts. Cold water from the system's chiller cannot pass through the heat trap, which protects the boiler. The simplicity of a two-pipe system's design means that it uses less pumping power to move water through the system, which results in lower energy costs.
Energy Savers, a U.S. Department Of Energy website, notes that homeowners who currently have a steam heating system may wish to convert it to a two-pipe HVAC system to save space. This is best done if the existing steam system is a two-pipe setup. According to the University of California at Davis Energy Efficiency Center, installing a two-pipe HVAC system will not result in compromised indoor air quality or affect the ability to control humidity in a structure.