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Resolving the CirculationDilemma in Multifamily Buildings by Derek Zobrist M uch like a beating heart, a circulation Water Heater Losses Circulation Loop Losses (Distribution Losses) 35 End-Use Energy (i.e. Hot Water) % pump circulates heat- ed water throughout a 34 % building for residents’ use and 31 % back to its source. Some single- family homes have circulation Figure 1. In hot water systems about 31% is lost at the water heater—either from combustion and heat transfer pumps, but for multifamily inefficiencies or through the standby heat losses of the storage tank; and 34% is lost in the distribution system. housing, a circulation pump is an essential part of the water-heating system, and in some states it cold-water supply. When someone opens a hot-water tap, water is required by the building code. Homeowners don’t like the idea of exits the system, and the flow sensor senses that cold water is letting a pump run 24/7 consuming electricity and wasting natural coming into the system to replace it. But this does not necessarily gas, when they need hot water only a few times a day, so they often mean that the system will turn the circulation pump on—espe- unplug the circulation pump installed in their hot-water distribu- cially in a multifamily building, where there are hundreds of hot tion system. As for multifamily buildings, without some kind of water draws each day. If hot water is already available in the hot- circulation pump, the tenants will waste a substantial amount of water supply pipes near the user, the circulation pump will remain water and time. These are two sides of the circulation dilemma. off. Turning it on will not do anything except waste energy. In a An efficient hot-water delivery system for a multifamily (or multifamily building, if anyone else in the building is using hot single-family) building balances timely delivery and energy ef- water, then hot water is probably available. Normal water pres- ficiency. Such a system operates a pump only when one of the sure, with the pump off, will deliver it to the new user’s hot-water residents needs it. The rest of the time it pumps less or not at all. tap. So how does a demand-controlled pump determine whether This is how a demand-controlled circulation pump works. (For hot water is available? The system has a second sensor that mea- information about demand-controlled pumps in single family sures water temperature in the pipes. It is installed at the coolest homes, see “Demand-Controlled Pumps—Sticker Shock Versus point in the circulation loop, on the return line coming back into Value,” p. 12.) the boiler room from the building. If at this point the water is at a predetermined temperature (hot), then the rest of the pipes will The Logic of Demand-Controlled Pumps be as hot or hotter. A hot water distribution system that uses flow An electronically controlled system turns the circulation pump and temperature sensors, properly located, and an adjustable elec- on only when it is needed to deliver hot water quickly; it works tronic control, has the information that it needs to run the pump on demand. The pump receives on-off signals from the system’s only when it is necessary. electronic controls, which monitor hot-water demand continu- ously. The signal to turn the pump on can be a user pushing a Research on Multifamily Hot-Water Distribution button or triggering a motion sensor in the bathroom. Or the Theoretically, a demand-controlled pump sounds as if it would signal may come, indirectly, from a flow sensor. provide the best balance between water and energy efficiency in The flow sensor signals to the hot water distribution system multifamily hot-water distribution. But utilities, governments, that someone is using hot water. The sensor is installed on the and property owners can’t rely on theory to achieve their energy22 Home Energy | S e pt e m be r / O c t o be r 2 0 1 2
multifamilyreduction goals. In the case of hot-water distribution, several engineering organiza-tions have tackled the question of energy reduction within the last five years. Theyhave done this by measuring the energy used in central water-heating systems andfiguring out how to make them more efficient. The first important study of multifamily hot-water distribution was conductedin 2006 by the Heschong Mahone Group (HMG) and funded by the Public InterestEnergy Research program, a California Energy Commission program that is ultimate-ly funded by the California ratepayers (1). HMG studied three multifamily buildingsin the San Francisco Bay area (one each in Emeryville, Saint Helena, and Oakland)in the spring and early summer. The study modeled energy flow of a central water-heating system to determine what measures could be implemented to save energy.They studied systems that ran the pumps continuously as well as systems with pumps enovative Kontrol syste msthat were controlled by timers, temperature controls, or demand controls. They foundthat demand controls outperformed them all. The researchers found that hot-waterdistribution systems were more variable and complex than they had supposed, andthat additional research was needed. This resulted in the creation of a second, morecomprehensive, study (2). This study was conducted over many years and included 50 The photo above and the two 3-D models below all showbuildings with central hot water distribution systems. The results of this second study typical installations of demand-controlled hot-water circulation systems.were published in the fall of 2011. All the buildings were profiled and then modeled for energy flow of the water-heating system. A subset of the buildings, roughly 30 of them, was hooked up withfield-monitoring equipment, and the actual field data were then compared to the energymodels. The field data and the energy models matched up well, which led the researchersto draw three important conclusions. The most interesting conclusion was this: Of all the energy that is put into a multi-family central water-heating system, only 35% is used in the form of hot water comingout of a tap. The remaining 65% of the energy is lost (see Figure 1). The researchers were enovative Kontrol syste msastonished to find out how inefficient water-heating systems are, especially since waterheating is the second-largest use of residential energy in the United States. These inef-ficient hot-water delivery systems create a multibillion-dollar savings opportunity. The second major conclusion of the HMG long-term study was that demand-controlled pumping can achieve the highest efficiencies and the quickest paybackswhen it comes to energy measures for central water-heating systems. One graphicfrom a presentation on the study showed that demand controls could reduce the heatloss in the distribution system by over 50% (see Figure 2). This translates to an aver-age 15% reduction in energy use for the water-heating system. The study estimatedsavings at 1,014 therms and 1,228 kWh for low-rise multifamily housing and 2,035therms and 1,255 kWh for high-rise multifamily housing (see Table 1). The third conclusion was that the circulation loop can be zoned to achieve higherefficiencies, and that when the zoned loop is paired with a demand-controlled pump,this saves the most energy for multifamily hot-water distribution. The extreme heatlosses in the distribution are due to the large surface area of the pipes. These pipesare radiating away the heat being pumped through them during the excessive pumprun time. By zoning the circulation loop, the surface area of the pipes can be reduced,and the pump run time can be lessened, because each zone has its own demand-controlled circulation pump. The caveat is that designing a zoned circulation loop enovative Kontrol syste msis feasible only for new construction projects. Another important study was fundedthrough a Southern California Gas Company program that has been providing a re-bate to multifamily building owners for demand-controlled pumps within the com-pany service territory (3). Program follow-up studies were conducted to measure andverify the savings. The program studied 35 multifamily buildings that had received w w w. h o m e e ner g y.o r g 23
Table 1. Savings from Demand-Controled Hot-Water Systems Low-Rise High-Rise the pros of a demand-Control Technology Electricity Demand Natural Gas Electricity Demand Natural Gas pump retrofit project Savings Savings Savings Savings Savings Savings are that it is quick, easy, (kWh) (kW) (Therms) (kWh) (kW) (Therms) and cost-effective. TheTemperature Modulation 0 0 405 0 0 535 con is that there areContinuous Monitoring 0 0 461 0 0 771 generally only one orTemperature Modulation + 0 0 785 0 0 1,199 two pumps to upgradeContinuous Monitoring per building, so theseDemand Control 1,228 0.140 1,014 2,035 .233 1,255 are mostly low-cost projects; thus it is more rebated demand pumps starting in 2008. The results showed an profitable for the professional to promote these projects to poten- average of 1,526 therms and 1,236 kWh of savings per year per tial customers with enough buildings to make the job worthwhile. demand pump among the study group, which broke down to In general, the best applications are buildings that use a lot of do- 34.7 therms of savings per year per apartment unit. This study mestic hot water. represents the largest set of The first step is to make sure that there is a circulation pump. Recirculation Loop Losses Reduction field data on the efficacy of There will be a pump only if the building has a central water-60% demand-controlled pumps heating system, but not all central water-heating systems have a in multifamily hot-water pump. The next step is to see if there are local utility programs50% Demand Control distribution systems. that can help subsidize the cost of the retrofit. Most utilities do40% Timer not have a prescriptive rebate—meaning a flat rebated dollar Temperature Get Involved amount—but that doesn’t mean you can’t get some incentives Modulation30% The multifamily hot- from a local incentive program. Often the utilities offer a cus- water system studies have tomized approach, meaning that you would write a short sum-20% yielded exciting results. mary of the estimated savings with your calculations, and the10% The research will continue utility will evaluate it. If your calculations are backed up with and will be expanded be- references that support your savings estimate, the utility will 0% yond multifamily buildings. approve your project and let you know how much of the cost itFigure 2. The results are normalized by gallon There is an opportunity is willing to subsidize. The reports that were mentioned in thisof water. here for energy profession- article are good references to back up the calculations. When>> learn more als to help build up the data you are doing an audit or inspection, make sure to get pictures set on hot-water distribu- of the existing system and document anything that you can use Studies cited in this article: tion systems in multifamily in a report to justify the savings. 1. Water Heaters and Hot Water Distribution Systems Pier Final Project buildings and larger com- A licensed plumbing contractor should install an on-demand Appendices, CEC-500-2005-007-APA. mercial applications, and circulation system in a multifamily building, but this is a simple Prepared for the California Energy to take part in cutting-edge procedure that should take one to two hours. Most of the time is Commission Public Interest Energy research into hot-water dis- spent cutting in and soldering a copper tee that houses the flow Research program by Lawrence Berkeley National Laboratory, May 2008. tribution. The findings of sensor. The photos on p. 23 show typical installations. 2. Codes and Standards Enhancement this research will eventually To measure the results, get a motor run time detector. I per- Initiative (Case): Multifamily Central be reflected in utility rebate sonally like the one manufactured by Dent Instruments that DHW and Solar Water Heating, 2013 and incentive programs, as costs $99. You can track the run time of the pump. In general, if California Building Energy Efficiency well as in building codes. a pump is running approximately one hour in a day, everything Standards. This report was prepared by the California Statewide Utility Codes Based on the research that should be working as intended, and you will be helping to re- and Standards Program and funded by has been conducted, there solve the circulation dilemma. the California utility customers under seems to be great potential the auspices of the California Public —Derek Zobrist Utilities Commission, October 2011. for improvement in the area 3. PY 2009 Monitoring Report: of multifamily hot-water Demand Control for Multifamily Central distribution. In the multi- Derek Zobrist is the founder and CEO of Enovative Kontrol Domestic Hot Water. Prepared by family application, demand Systems, a firm dedicated to assisting building owners reduce en- the Benningfield Group for Southern controls are just beginning California Gas Company, October 30, ergy consumption through simple, innovative technologies and 2009. to emerge. From an energy strategic design. His area of expertise focuses on domestic hot professional’s viewpoint, water distribution in commercial and multi-tenant buildings.24 Home Energy | S e pt e m be r / O c t o be r 2 0 1 2