Testing For and Determining the Prevalence of Crossover in a Multifamily Central Domestic Hot Water Distribution System
Year: 2016

Crossover allows the unintended flow of hot water into the cold water system, and vice versa, and may have a significant impact on energy losses in the system, particularly in central DHW systems in multifamily and commercial buildings. In a CEC funded study of over 100 multifamily buildings throughout California, over 50% exhibited crossover failures in their central DHW distribution. This occurs most often when a cartridge in a shower or faucet's mixing valve wears out and is no longer able to effectively control the mix of hot and cold water at a fixture or point of use. Many times these issues are asymptomatic and the effects not readily felt by the user. The implication being that the problems will not receive proper attention or repair, and thus will persist over a long period of time. According to past CEC funded research into multifamily hot water energy use, approximately 1/3 of all energy use, on average, is lost in the distribution due to existing issues and conditions. Current efforts are underway to determine what impact that crossover failures have on these distribution losses and what the potential for energy savings are on a statewide level, given the pervasiveness of the problem.  Crossover occurs in a DHW distribution system and is the result of a combination of a faulty fixture valve, or an uncontrolled open circuit at the point where cold water and hot water pipes join to mix, and a pressure imbalance between the hot and cold water subsystems. Crossover allows the unintended flow of hot water into the cold water system, and vice versa as shown in the diagram below (without crossover: left, with crossover: right) 1 1 (c)2016 ACEEE Summer Study on Energy Efficiency in Buildings 1 1 It may have a significant impact on energy losses in the system, particularly in central DHW systems in multifamily and commercial buildings. Likewise, crossover may create issues that compromise hot water service that cause tenant complaints and discomfort, such as: Long hot water wait times, sudden and rapid temperature fluctuations (most common symptom) lukewarm water or no hot water. Changes to the system can impact the degree in which the symptoms are felt, such as adding recirculation pump controls or temperature reset strategies, and this increases the likelihood that controls measures for energy efficiency are bypassed and/or temperature set points are increased in an attempt to counteract the effects. In many cases, however, the symptoms are masked which cause the issues to go unnoticed and untreated, and energy losses persist over long periods of time, unabated. The frequency of these issues is not sufficiently documented but may be the best explanation as to why recirculation pump controls, though part of CA Title 24 since 2005, still have a very low market acceptance. According to one report by engineering group HMG, most buildings do not have any type of controls (HMG 2005) This may be due, in part, to a lack of awareness of available energy saving hot water technologies, but it also may be due to controls being taken out or deactivated because of unrelated crossover issues that were misdiagnosed. This would have great consequence, as PIER funded research has shown that recirculation pump controls on their own can reduce total loop heat losses by over 50% and total hot water gas use by 15% (Bonneville 2010) Anecdotal data suggests that crossover may occur more frequently than previously thought. For example, in one article on PIER funded research in this area, the principal investigators noted that two of the three buildings monitored had shown signs of crossover (Howlett and Stone 2007) There is also anecdotal data on the energy impact of crossover in buildings, however there are varying degrees of crossover severity, and according to a report by ACEEE, crossover flows may account for 5 10% of total hot water energy use in multifamily buildings (Sachs et al. 2011) Another study that tracked the energy use of a multifamily community with 104 units where all the cartridges in every shower valve were replaced showed savings of $23,328 or 62% (Pfaff 2009) This high level of variation highlights the fact that there is too little data to make any conclusions and that more research and data collection must be performed. For this reason, Enovative Group has embarked on CEC funded research to help determine the frequency and energy impact that crossover has on buildings in California. Project Goals and Tasks Determine the frequency and scale of crossover problems in multifamily buildings throughout California Determine the impact that crossover has on the energy performance of a typical CDHW system Determine potential energy savings for California Recommend changes to building codes and standards to help prevent crossover from occurring in buildings Testing buildings for crossover The first challenge in determining an estimate for the prevalence of crossover is that there is little documentation on how to detect crossover. A search on the internet yielded some 1 2 (c)2016 ACEEE Summer Study on Energy Efficiency in Buildings 1(c)2016 ACEEE Summer Study on Energy Efficiency in Buildings instructions and methods for detecting crossover, but did not have data nor a detailed description of how and why the detection method worked. Oftentimes these would be buried deep, many pages from the first page of an internet search. The research team identified a list of potential methods for detecting crossover via a paper study, internet research, and by way of interviews with various experts in the field of plumbing and hot water systems. Crossover Detection Methods The following descriptions are of the detection methods gathered from the interviews. Pressure gauge method. The pressure gauge method requires a standard pressure gauge that is commercially available and relatively inexpensive. The pressure gauge can be attached to the domestic hot water system anywhere on the hot water subsystem (i.e. on a hot water pipe) however it is recommended to use a hose bib on a return line, which is commonly available. To start, close the valve to the cold water supply that feeds into the water heater, while keeping the cold water supply to the cold water subsystem open. Then release hot water from the hot water subsystem via any fixture, thus relieving pressure on the hot water subsystem. After some or all of the pressure is released, the fixture should be closed so that water is no longer coming out. If crossover is present, then the pressure gauge will show the pressure in the hot water subsystem slowly rebuilding to normal. If crossover is not present, the pressure gauge will continue to register at lower levels or even come to rest at zero pressure, but should not increase. The temperature and pressure profiles monitored during the test procedure is displayed in the plot below against time: 20 0 20 40 60 80 100 120 140 1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81 85 89 93 97 10 1 Te m pe ra tu re (F ah re nh ei t) P re ss ur e (P SI Time (Seconds) Pressure Gauge Method Scenario 2 Test 3 P_SV3_C P_SV3_H T_WS_H T_WS_C T_SV3_H P_WS_C T_SV3_C P_WS_H 3 5 1 3 (c)2016 ACEEE Summer Study on Energy Efficiency in Buildings Annotations from the chart above (Without Crossover Scenario) 1. SV3 Hot section opened increasing temperature and dropping pressure. 2. SV3 Hot section turned off, regaining pressure 3. SV3 hot section isolated, dropping pressure but avoiding crossover 4. P_WS_H turned off preventing it from gaining full pressure and dropping P_SV3 fully. 5. Hose bib opened, relieving all pressure from hot sections of the system. 6. Pressure stays at zero indicating absence of crossover when the SV3_H was isolated. The above figure shows the pressure gauge method being tested on the test platform with the Scenario 2 treatment, a situation without crossover in the system. Similarly the following chart displays the temperature and pressure profiles for the pressure gauge method for a scenario with crossover present: Annotations from the chart above (With Crossover Scenario) 1. Shower Valve 3 turned on causing drop in pressure and spike in temperature. 2. Shower valve 3 closed off causing pressure stabilization and temperature loss. 3. Cold line water supply inlet to heater turned off. 4. Hose bib opened causing pressure drop and instant crossover at about 1.4 GPM 5. Hot water supply temperature starts to drop from cold water crossing over 6. Hose bib closed allowing pressure to slowly build up. Verified with P gauge. The above chart shows the data collected from the Pressure Gauge method being tested in scenario 1, a condition when crossover is present in the test platform system. The graph results confirm that there is crossover in the system. First, the needle on the pressure gauge attached to 0 20 40 60 80 100 120 140 1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96 10 1 10 6 11 1 11 6 12 1 12 6 13 1 13 6 14 1 Te m pe ra tu re (F ah re nh ei t) P re ss ur e (P SI Time (Seconds) Pressure Gauge Method  3 P_SV3_C P_SV3_H T_WS_H T_WS_C T_SV3_H P_WS_C T_SV3_C P_WS_H 2