Try These On for Size: 
New Guidelines for Multifamily Water Heating

Fredric S. Goldner, C.E.M., is principal of Energy Management and Research Associates in Brooklyn, New York. He is the author of the new 1995 ASHRAE guidelines discussed in this article.

The 60-unit building in the photo above houses middle-income families. As part of a study conducted in New York City, researchers monitored the building's hot water consumption, which fell into the medium usage category under the 1995 ASHRAE guidelines.

Energy professionals have long been frustrated by the lack of reliable data for sizing domestic hot water (DHW) equipment in multifamily buildings. To be on the safe side, many designers oversize the equipment, resulting in systems with higher equipment costs, operating costs, and energy use. Now ASHRAE has incorporated data from recent studies into new guidelines for properly sizing DHW systems.

Using previous (pre-1995) ASHRAE guidelines resulted in serious undersizing (see Figure 1). In practice, however, DHW systems and combined heating/DHW boilers are often oversized by 30%-200%, according to the New York City Department of Housing Preservation and Development, Energy Conservation Division. Discussions with designers in other parts of the country revealed similar oversizing.

What happens is that the individual responsible for installing a boiler will often size it with a what was there before, looks like ..., or other rule-of-thumb method. Even when they do try to calculate the loads, designers use enormous safety factors because they know the DHW demands estimated with the old methods tend to undersize (see Evolution of an Oversizing Rule). The safety factors cause considerable oversizing even when the space heating portion is calculated properly, which is rarely the case. I've seen factors that double the size of the boiler relative to the space heating load (a rule of thumb that is particularly inaccurate for the New York climate).

The 1995 guidelines also take a new approach. Rather than a single value for volume of water used per apartment, they offer a range of values for different types of users. The residents or likely residents of a building are separated by their demographic characteristics into three usage categories: low, medium, or high (LMH). Additionally, the usage factors are provided per capita rather than per apartment. This reflects the fact that people, not apartments or square footage, use water.

To help in the design process, the new ASHRAE tables give more detailed levels of consumption for the peak 5 minutes and the peak 15 minutes (the old tables had only 60-minute peak values). These values more closely represent the instantaneous demand peak that a building will experience.

Table 1 lists a variety of occupant classifications, one or a combination of which should describe the occupants of any particular multifamily building. For example, a luxury condominium in an area inhabited predominantly by young couples will tend to fall into the all occupants work category of low anticipated water consumption. By contrast, a low-income housing project will generally fall somewhere between the low income and no occupants work categories of high-volume water consumption. An abundance of hot-water-consuming appliances, such as clothes washers or dishwashers, will tend to increase hot-water consumption. If the condominium building example above intended, or allowed, the future installation of a clothes washer in each unit, the demographic category should be augmented from low to medium. It is up to the system's designer to determine this category.

Once this LMH factor has been determined, values for hot-water consumption can be selected from Table 2. Values are indicated per capita in peak or maximum flows of 5 minutes, 15 minutes, one hour, two hours, three hours and one day, as well as average daily flow. From these values, anticipated demand can be determined for the estimated maximum building population.

Author Fredric Goldner discusses meter equipment with building superintendent John Perkins. The meter he is pointing to monitors hot water recirculation, and above it is a domestic hot water consumption meter.

The number of occupants per apartment should be estimated based on local standards or regulations. For example, in a given city, studios may accommodate two persons; one-bedroom apartments, three persons; two-bedroom apartments, three to five persons; and so on.

In buildings where corrective maintenance cannot be done, a safety factor of 20%-30% may be employed to compensate for poorly maintained fixtures and distribution piping. However, this should be done only in extreme cases.

The figures presented in Table 2 are for centrally fired systems; individual apartment water heater systems are likely to have lower levels of consumption because the resident usually pays for fuel directly, which encourages conservation. There isn't a set of values for individual systems in ASHRAE, but a suggested rule of thumb for sizing these would be to use a low-end estimate for a single-family home load.

ASHRAE based its 1995 guidelines (published in the 1995 HVAC Applications Handbook) on new research conducted in New York City (see Collecting Usage Data in New York City) as well as data from studies in seven other areas of the United States and Canada. Both research and practical experience in different areas of North America indicate that there are variances in DHW use among geographical locations. There is, however, no distinctive pattern that can be identified with the available data.

The joint ASHRAE/ASPE (American Society of Plumbing Engineers) Domestic Hot Water Design Manual, to be published this fall, will go into greater depth than the ASHRAE standards, including the patterns of consumption and demand derived from the New York study. Becoming familiar with these patterns can help designers choose the best equipment and help auditors troubleshoot related system problems.

Figure 2. Seasonal variations in weekend consumption, gallons per person (composite of data from New York City apartment buildings).

There is generally a slightly higher daily consumption on weekends than on weekdays. This holds true in all seasons. The average weekend daily consumption is 7.5% greater than the average weekday daily consumption.

Weekday and weekend hot-water consumption patterns have distinct differences (see Figure 3). Weekdays have little overnight usage; a morning peak; lower afternoon demand; and an evening or nighttime peak. Weekends have just one major peak, which begins later in the morning and continues until around 1 pm to 2 pm. The usage then tapers off fairly evenly through the rest of the day. The weekend peak is greater than any of the weekday peaks.

The highest peaking level occurs during winter weekends. Thus, the best tactic for an engineer who has the time and money to custom-design a retrofit system is to monitor current consumption for two or three winter weekends to determine a building's actual peak usage, rather than estimating it with Table 2. A system designed to meet these draws should satisfy all other year-round requirements.

Two morning peaks occur on the weekdays, the first between 6 am and 8 am and the second between 9:30 am and noon. Individual buildings tend to exhibit one of these two peaks. Generally, the buildings with large numbers of working tenants and middle-income populations experience the early morning peak, while buildings with many children exhibit the later morning peak (especially during the summer period).

This knowledge of flow patterns can come in particularly handy when troubleshooting hot water complaints. For instance, a large fluctuation in water temperature at a time when the usage was extremely low recently helped me to identify a problem with a faulty hot water coil. If the fluctuation was observed only during a high usage period, the cause-perhaps an undersized coil or a problem with the mixing valve-would have been harder to determine.
 
 

Although recirculation pumps should be sized to meet each individual building's requirements, common practice is one size fits all. Thus we found the same pump size at all sites. (A methodology for proper pump size selection can be found on page 45.5 of the 1995 ASHRAE HVAC Applications Handbook.)

Our monitoring showed that water consumption has an inverse relationship with recirculation flow. In the overnight period, when there is little or no consumption, the pump reaches its maximum capacity rate. Designers should consider this and the flow curves in Figure 3 when choosing between recirculation control strategies (see The Best Boiler and Water Heating Retrofits, HE Sept/Oct '95, p. 27, and Controlling Recirculation Loop Heat Losses, HE Jan/Feb '93, p. 9). A new study investigating three very low-cost approaches to reduce recirculation system losses while maintaining resident comfort and satisfaction should be completed in early 1997.

Figure 4. Parts of three-hour peak and 60-minute peak consumption.

The 5-, 15-, 60-, 120-, and 180-minute peak demand times coincide with each other. These volumes should therefore be addressed as different (time length) measurements within the same peak DHW draw, so the system can be designed to satisfy this load. An instantaneous system designed to meet the peak 5-minute draw will have no problem meeting the rest of the load. Generation and storage systems should be designed both to provide hot water for the average load and to meet the short, sharp peaks.

If the current practice of defensive oversizing is applied to the new guidelines, this will only exaggerate the capital and energy inefficiencies experienced in the past. It is therefore important for the designer to recognize the inherent safety nets in the new approach. The most significant of these is that the method uses the building's maximum potential occupancy, which may never actually occur. Also, using the new guidelines, an engineer designs a system to satisfy the higher-volume but short-duration peaks (not delineated in the old guidelines), which occur only a few times during the year. Even if the system were not able to satisfy that load, the problems would probably be minor-for instance, the occupants might experience slightly lower temperature hot water at their taps a few times per year.

The main question concerning acceptance and use of the new guidelines is whether the designers and energy professionals are comfortable with their reliability and professional backing. ASHRAE's Technical Committee 6.6 (Service Hot Water) was the main force in the call for a new sizing tool based on the vast quantity of real-time data that has been collected. The new joint ASHRAE/ASPE Domestic Hot Water Design Manual, scheduled for publication this fall, should also provide substantial support for those who wish to size systems properly. It includes a how-to sizing guide for 17 different building types-from residential buildings to commercial, industrial, and recreational facilities.

Goldner, F.S. DHW System Sizing Criteria for Multifamily Buildings. ASHRAE Transactions 100, No.1 (January 1994): 147-65.

Goldner, F.S. Energy Use and DHW Consumption Research Project, Report No. 94-19. Final Report: Phase 1. Prepared by Energy Management and Research Associates for New York State Energy Research and Development Authority, November 1994.

Goldner, F.S., and D.C. Price. Domestic Hot Water Loads, System Sizing and Selection for Multifamily Buildings. In 1994 ACEEE Summer Study on Energy Efficiency in Buildings Proceedings, 2.105-2.116. Berkeley, CA: American Council for an Energy-Efficient Economy,1994.
 
 

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