Seasonal/Heating Transition Published Nov. 23, 2009 By Mary Lumsdon 17th Civil Engineer Squadron GOODFELLOW AFB, Texas -- With the winter season nearing and temperatures begin to drop, you may notice the drop of indoor temperatures as well. This is because the older heating, ventilating and air-conditioning (HVAC) systems work on a two-pipe principle. Two-pipe HVAC systems cannot be switched back and forth from cooling to heating with daily fluctuations in temperatures. They are designed to run in either heating or cooling mode for extended periods of time. Two-pipe HVAC systems heat a space in the winter season by pumping hot water through a coil which transfers heat to an air stream. The same system will be used to cool a space in the summer season by pumping cold water through the same coil which absorbs heat from the air stream. These systems cannot heat and cool at the same time. During the switch over to the heating mode, valves must be manually opened and closed to reroute the water from a chiller to boiler. The HVAC system piping, boiler, and chiller are susceptible to thermal shock caused by abrupt water temperature changes. A two-pipe HVAC system must be allowed to rest for a day or two until its water temperature becomes moderate and seasonal changeovers can be made to prevent damage to the system. Determining the best time for seasonal changeover on these HVAC systems is a judgment call. The 17th Civil Engineer Squadron personnel have already turned the heat on in the two-pipe systems due to the lower night time seasonal temperatures. Since the weather in West Texas is unpredictable this time of year, we can expect some outdoor temperatures that aren't consistent with the two-pipe system mode. There will be times when these two-pipe system facilities are uncomfortable. Please be patient during this seasonal/transition until the weather is consistently cooler when we need heat every day. If you have questions about the system in your facility please call 654-5285.