Resolving the Circulation Dilemma in Multifamily Buildings
Much like a beating heart, a circulation pump circulates heated water throughout a building for residents’ use and back to its source. Some single-family homes have circulation pumps, but for multifamily housing, a circulation pump is an essential part of the water-heating system, and in some states it is required by the building code. Homeowners don’t like the idea of letting a pump run 24/7 consuming electricity and wasting natural gas, when they need hot water only a few times a day, so they often unplug the circulation pump installed in their hot-water distribution system. As for multifamily buildings, without some kind of circulation pump, the tenants will waste a substantial amount of water and time. These are two sides of the circulation dilemma.
An efficient hot-water delivery system for a multifamily (or single-family) building balances timely delivery and energy efficiency. Such a system operates a pump only when one of the residents needs it. The rest of the time it pumps less or not at all. This is how a demand-controlled circulation pump works. (For information about demand-controlled pumps in single family homes, see “Demand-Controlled Pumps—Sticker Shock Versus Value,” p. 12.)
The Logic of Demand-Controlled Pumps
An electronically controlled system turns the circulation pump on only when it is needed to deliver hot water quickly; it works on demand. The pump receives on-off signals from the system’s electronic controls, which monitor hot-water demand continuously. The signal to turn the pump on can be a user pushing a button or triggering a motion sensor in the bathroom. Or the signal may come, indirectly, from a flow sensor.
The flow sensor signals to the hot water distribution system that someone is using hot water. The sensor is installed on the cold-water supply. When someone opens a hot-water tap, water exits the system, and the flow sensor senses that cold water is coming into the system to replace it. But this does not necessarily mean that the system will turn the circulation pump on—especially in a multifamily building, where there are hundreds of hot water draws each day. If hot water is already available in the hot-water supply pipes near the user, the circulation pump will remain off. Turning it on will not do anything except waste energy. In a multifamily building, if anyone else in the building is using hot water, then hot water is probably available. Normal water pressure, with the pump off, will deliver it to the new user’s hot-water tap. So how does a demand-controlled pump determine whether hot water is available? The system has a second sensor that measures water temperature in the pipes. It is installed at the coolest point in the circulation loop, on the return line coming back into 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 be as hot or hotter. A hot water distribution system that uses flow and temperature sensors, properly located, and an adjustable electronic control, has the information that it needs to run the pump only when it is necessary.
Research on Multifamily Hot-Water Distribution
Theoretically, a demand-controlled pump sounds as if it would provide the best balance between water and energy efficiency in multifamily hot-water distribution. But utilities, governments, and property owners can’t rely on theory to achieve their energy reduction goals. In the case of hot-water distribution, several engineering organizations have tackled the question of energy reduction within the last five years. They have done this by measuring the energy used in central water-heating systems and figuring out how to make them more efficient.
The first important study of multifamily hot-water distribution was conducted in 2006 by the Heschong Mahone Group (HMG) and funded by the Public Interest Energy Research program, a California Energy Commission program that is ultimately funded by the California ratepayers (1). HMG studied three multifamily buildings in 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 that were controlled by timers, temperature controls, or demand controls. They found that demand controls outperformed them all. The researchers found that hot-water distribution systems were more variable and complex than they had supposed, and that additional research was needed. This resulted in the creation of a second, more comprehensive, study (2). This study was conducted over many years and included 50 buildings with central hot water distribution systems. The results of this second study 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 with field-monitoring equipment, and the actual field data were then compared to the energy models. The field data and the energy models matched up well, which led the researchers to draw three important conclusions.
The most interesting conclusion was this: Of all the energy that is put into a multifamily central water-heating system, only 35% is used in the form of hot water coming out of a tap. The remaining 65% of the energy is lost (see Figure 1). The researchers were astonished to find out how inefficient water-heating systems are, especially since water heating is the second-largest use of residential energy in the United States. These inefficient 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 paybacks when it comes to energy measures for central water-heating systems. One graphic from a presentation on the study showed that demand controls could reduce the heat loss in the distribution system by over 50% (see Figure 2). This translates to an average 15% reduction in energy use for the water-heating system. The study estimated savings at 1,014 therms and 1,228 kWh for low-rise multifamily housing and 2,035 therms 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 higher efficiencies, 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 heat losses in the distribution are due to the large surface area of the pipes. These pipes are radiating away the heat being pumped through them during the excessive pump run 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 is feasible only for new construction projects. Another important study was funded through a Southern California Gas Company program that has been providing a rebate to multifamily building owners for demand-controlled pumps within the company service territory (3). Program follow-up studies were conducted to measure and verify the savings. The program studied 35 multifamily buildings that had received rebated demand pumps starting in 2008. The results showed an average of 1,526 therms and 1,236 kWh of savings per year per demand pump among the study group, which broke down to 34.7 therms of savings per year per apartment unit. This study represents the largest set of field data on the efficacy of demand-controlled pumps in multifamily hot-water distribution systems.
The multifamily hot-water system studies have yielded exciting results. The research will continue and will be expanded beyond multifamily buildings. There is an opportunity here for energy professionals to help build up the data set on hot-water distribution systems in multifamily buildings and larger commercial applications, and to take part in cutting-edge research into hot-water distribution. The findings of this research will eventually be reflected in utility rebate and incentive programs, as well as in building codes. Based on the research that has been conducted, there seems to be great potential for improvement in the area of multifamily hot-water distribution. In the multifamily application, demand controls are just beginning to emerge. From an energy professional’s viewpoint, the pros of a demand-pump retrofit project are that it is quick, easy, and cost-effective. The con is that there are generally only one or two pumps to upgrade per building, so these are mostly low-cost projects; thus it is more profitable for the professional to promote these projects to potential customers with enough buildings to make the job worthwhile. In general, the best applications are buildings that use a lot of domestic hot water.
Studies cited in this article:
1. Water Heaters and Hot Water Distribution Systems Pier Final Project Appendices, CEC-500-2005-007-APA. Prepared for the California Energy Commission Public Interest Energy Research program by Lawrence Berkeley National Laboratory, May 2008.
2. Codes and Standards Enhancement Initiative (Case): Multifamily Central DHW and Solar Water Heating, 2013 California Building Energy Efficiency Standards. This report was prepared by the California Statewide Utility Codes and Standards Program and funded by the California utility customers under the auspices of the California Public Utilities Commission, October 2011.
3. PY 2009 Monitoring Report: Demand Control for Multifamily Central Domestic Hot Water. Prepared by the Benningfield Group for Southern California Gas Company, October 30, 2009.
The first step is to make sure that there is a circulation pump. There will be a pump only if the building has a central water-heating system, but not all central water-heating systems have a pump. The next step is to see if there are local utility programs that can help subsidize the cost of the retrofit. Most utilities do not have a prescriptive rebate—meaning a flat rebated dollar amount—but that doesn’t mean you can’t get some incentives from a local incentive program. Often the utilities offer a customized approach, meaning that you would write a short summary of the estimated savings with your calculations, and the utility will evaluate it. If your calculations are backed up with references that support your savings estimate, the utility will approve your project and let you know how much of the cost it is willing to subsidize. The reports that were mentioned in this article are good references to back up the calculations. When you are doing an audit or inspection, make sure to get pictures of the existing system and document anything that you can use in a report to justify the savings.
A licensed plumbing contractor should install an on-demand circulation system in a multifamily building, but this is a simple procedure that should take one to two hours. Most of the time is spent cutting in and soldering a copper tee that houses the flow sensor. The photos on p. 23 show typical installations.
To measure the results, get a motor run time detector. I personally like the one manufactured by Dent Instruments that costs $99. You can track the run time of the pump. In general, if a pump is running approximately one hour in a day, everything should be working as intended, and you will be helping to resolve the circulation dilemma.
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