Size Calculator Multi Family on Demand Water Heater With Storage
This commodity was originally published in the July/Baronial 1996 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older annal content.
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Home Energy Mag Online July/August 1996
Try These On for Size:
New Guidelines for Multifamily Water Heating
by Fredric S. Goldner
Fredric Due south. Goldner, C.E.M., is principal of Energy Management and Research Assembly in Brooklyn, New York. He is the writer of the new 1995 ASHRAE guidelines discussed in this commodity.
ASHRAE has published new sizing guidelines for hot water systems, based on recent studies of water-heating employ in apartment buildings. If adopted in exercise, the new sizing method should prevent the costly oversizing that is now common.
The 60-unit of measurement edifice in the photo in a higher place houses middle-income families. Equally part of a study conducted in New York City, researchers monitored the building'southward hot h2o consumption, which roughshod into the medium usage category under the 1995 ASHRAE guidelines.
Energy professionals take long been frustrated past 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 information from recent studies into new guidelines for properly sizing DHW systems.
Using previous (pre-1995) ASHRAE guidelines resulted in serious undersizing (encounter Figure 1). In practise, however, DHW systems and combined heating/DHW boilers are often oversized by 30%-200%, co-ordinate to the New York Urban center Section of Housing Preservation and Development, Energy Conservation Sectionalisation. Discussions with designers in other parts of the state revealed similar oversizing.
What happens is that the private responsible for installing a boiler will oft 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 considering they know the DHW demands estimated with the former 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 example. I've seen factors that double the size of the boiler relative to the space heating load (a rule of thumb that is specially inaccurate for the New York climate).
A review of manufacturers' literature uncovered at least half a dozen different methods for sizing both combined estrus/DHW units and stand-alone systems.
Many of these methods were initially based on the pre-1995 ASHRAE approach. I once sat downwards with the VP of marketing and i of the pattern engineers for a prominent manufacturer and asked them how the information sheets in their itemize decide system size. They replied the ASHRAE Handbook method. After running some calculations, nosotros found that in fact their results were somewhere between two to three times greater than the results obtained using the Handbook method.
What probably happened was that the engineer who had written the sizing sheets (many years ago) started with the Handbook values as a base. Just from his feel, he recognized that the numbers were not sufficient to meet a edifice'south demand, and so he added a safety gene based on that experience. Subsequently, equally the catalog has been revised, each engineer given the responsibleness to update the sheets has said to himself or herself, Well, I'm not going to exist responsible for at that place not existence plenty hot h2o in a building and has added another safety gene on top of the previous one. And and then the chief engineer in charge of the revision says, I'm not going to be responsible for there not being enough hot water . . . and adds yet another safe factor. Thus over fourth dimension these values have in some cases go grossly inflated.
To aggravate this already bad situation, the contractor on the job may look at the data sheets and say, Well I'g non . . . and add another level of so-called condom cistron. The task then gets sized out and a call is made to the warehouse, whose staff, feeling similar all the other parties, applies the adjacent-size-up approach earlier sending the heater/boiler out to the job site.
The 1995 guidelines also take a new arroyo. Rather than a single value for volume of water used per apartment, they offering a range of values for different types of users. The residents or probable residents of a building are separated by their demographic characteristics into three usage categories: depression, medium, or high (LMH). Additionally, the usage factors are provided per capita rather than per apartment. This reflects the fact that people, non apartments or square footage, use water.
To assistance in the blueprint process, the new ASHRAE tables give more than detailed levels of consumption for the peak 5 minutes and the peak 15 minutes (the old tables had just 60-infinitesimal height values). These values more closely stand for the instantaneous demand summit that a building will experience.
Using the New Method The showtime footstep in computing DHW demand is to determine the demographic profile of the project and building occupants. Different types of building occupants swallow hot water in adequately predictable patterns. Users tin exist lumped into one of the 3 typical LMH categories of water consumers.Table 1 lists a diversity of occupant classifications, one or a combination of which should describe the occupants of any particular multifamily edifice. For example, a luxury condominium in an area inhabited predominantly by immature couples will tend to fall into the all occupants work category of low anticipated water consumption. By contrast, a depression-income housing projection will generally fall somewhere between the low income and no occupants work categories of loftier-volume water consumption. An abundance of hot-water-consuming appliances, such as clothes washers or dishwashers, will tend to increase hot-h2o consumption. If the condominium building example higher up intended, or allowed, the hereafter installation of a clothes washer in each unit, the demographic category should be augmented from low to medium. Information technology is up to the system'south designer to determine this category.
Once this LMH factor has been determined, values for hot-water consumption tin exist selected from Tabular array 2. Values are indicated per capita in peak or maximum flows of v minutes, 15 minutes, one hour, two hours, iii hours and one day, also every bit average daily flow. From these values, anticipated demand tin can be determined for the estimated maximum edifice population.
| Table 1. Demographic Characteristics Correlation to DHW Consumption | |
| Demographic characteristics | Usage cistron |
| No occupants piece of work Public assistance and low income (mix) Family unit and unmarried-parent households (mix) High percentage of children Low income | High |
| Families Public assistance Singles Single-parent households | Medium |
| Couples College population density Middle income Seniors Ane person works, i stays abode All occupants work | Depression |
| Table 2. National DHW Sizing Guidelines (Low-Medium-High) | |||||
| Hot Water Demands and Use for Multifamily Buildings | |||||
| Maximum hour | Superlative 15 minutes | Maximum twenty-four hours | Average day | ||
| Low | 2.8 gal (ten.5 l)/person | i gal (iv l)/person | xx gal (76 l)/person | 14 gal (53 l)/person | |
| Med | 4.8 gal (eighteen 50)/person | 1.vii gal (6.4 l)/person | 49 gal (185 fifty)/person | thirty gal (114 l)/person | |
| Loftier | eight.5 gal (32.5 l)/person | 3 gal (xi.5 fifty)/person | 90 gal (340 l)/person | 54 gal (205 l)/person | |
| Peak 5 minutes | Peak xxx minutes | Maximum ii hours | Maximum 3 hours | ||
| Low | 0.iv gal (1.5 l)/person | 1.7 gal (half dozen.v l)/person | 4.5 gal (17 50)/person | 6.ane gal (23 l)/person | |
| Med | 0.7 gal (2.6 l)/person | 2.9 gal (11 l)/person | viii gal (31 50)/person | 11 gal (41 l)/person | |
| Loftier | 1.2 gal (4.5 50)/person | five.1 gal (19.5 fifty)/person | 14.v gal (55 50)/person | xix gal (72 l)/person | |
| Note: These volumes are for DHW delivered to the tap at 120oF. | |||||
| Sources: Data from Chapter 45: Service Hot Water, In 1995 ASHRAE Handbook: HVAC Applications, Atlanta: ASHRAE, 1995, and Goldner, F.S., and D.C. Price. Domestic Hot Water Loads, Organisation Sizing and Selection for Multifamily Buildings. In 1994 ACEEE Summer Study on Energy Efficiency in Buildings Proceedings, ii.105-2.116. Berkeley: American Council for an Energy-Efficient Economy, 1994. | |||||
Author Fredric Goldner discusses meter equipment with edifice superintendent John Perkins. The meter he is pointing to monitors hot water recirculation, and above information technology is a domestic hot water consumption meter.
The number of occupants per flat should be estimated based on local standards or regulations. For example, in a given city, studios may adapt two persons; 1-sleeping accommodation apartments, 3 persons; two-bedroom apartments, iii to five persons; and so on.
In buildings where corrective maintenance cannot exist done, a safety factor of 20%-30% may be employed to recoup for poorly maintained fixtures and distribution piping. However, this should be done but in extreme cases.
The figures presented in Table ii are for centrally fired systems; individual apartment h2o 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 exist to use a low-end judge for a single-family dwelling load.
ASHRAE based its 1995 guidelines (published in the 1995 HVAC Applications Handbook) on new inquiry conducted in New York Urban center (see Collecting Usage Data in New York City) besides as data from studies in vii other areas of the United States and Canada. Both research and practical feel in different areas of North America indicate that there are variances in DHW employ among geographical locations. In that location is, all the same, no distinctive pattern that can be identified with the available data.
The articulation ASHRAE/ASPE (American Society of Plumbing Engineers) Domestic Hot Water Pattern Manual, to be published this fall, volition 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 cull the best equipment and help auditors troubleshoot related arrangement problems.
Collecting Usage Data in New York City
The data were collected past computerized heating controllers, which monitored burner on-off times and the post-obit temperatures: apartment air, outdoor air, boiler water (aquastat), and DHW. Eight buildings had boosted monitoring equipment installed to record stack temperature, boiler makeup h2o flow, DHW flow in 15-minute increments, oil flow, and DHW temperature earlier and after the mixing valve and on the return line.
In 1993, we equipped a subset of three of the sites to record DHW flow in five-minute increments and to record recirculation flows. This was done to go a more precise picture of short-term/instantaneous need peaks and to collect the missing information necessary to create an authentic simulation of real-time operations. We collected data in these three buildings for 100 days.
EMRA too collected building operation and tenant information from superintendents and property managers via questionnaires and interviews, and building and apartment occupancy records. We conducted energy audits to determine the type and condition of equipment and buildings.
Within the New York research, we tried to include a variety of edifice sizes, income levels, ethnic backgrounds, and locales. The study buildings are characteristic of the older and predominant stock of the over 120,000 New York Metropolis multifamily buildings. The buildings range in size from 17 to 103 apartments in either five or half dozen above-ground stories. These buildings were built earlier 1902 or between 1902 and 1928. All accept combination steam-space-heating and DHW-generating steel tube boilers, which use primarily #4 or #6 oil in air-atomizing burners. DHW is generated by a tankless curlicue merely nether the surface of the boiler h2o.
Energy utilisation Analysis
For farther details, a copy of Study No. 94-xix, Free energy Apply and Domestic Hot H2o Consumption: Phase 1, is available from NYSERDA. Tel:(518)465-6251, Ext. 250.
Effigy 2. Seasonal variations in weekend consumption, gallons per person (blended of data from New York Metropolis apartment buildings).
In that location 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.five% greater than the boilerplate weekday daily consumption.
Weekday and weekend hot-water consumption patterns have distinct differences (run into Effigy three). Weekdays have lilliputian overnight usage; a morning time peak; lower afternoon demand; and an evening or nighttime peak. Weekends have only one major peak, which begins afterward in the morning and continues until around one pm to 2 pm. The usage so tapers off fairly evenly through the balance of the day. The weekend height is greater than any of the weekday peaks.
The highest peaking level occurs during winter weekends. Thus, the all-time 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 bodily peak usage, rather than estimating information technology with Tabular array 2. A system designed to come across these draws should satisfy all other year-round requirements.
2 morning peaks occur on the weekdays, the outset between 6 am and 8 am and the 2d between 9:30 am and noon. Individual buildings tend to exhibit one of these two peaks. More often than not, the buildings with large numbers of working tenants and middle-income populations experience the early forenoon peak, while buildings with many children exhibit the later morning height (specially during the summer period).
This knowledge of flow patterns can come up in peculiarly handy when troubleshooting hot h2o complaints. For instance, a large fluctuation in water temperature at a time when the usage was extremely low recently helped me to identify a trouble with a faulty hot h2o coil. If the fluctuation was observed simply during a loftier usage menses, the cause-perchance an undersized gyre or a problem with the mixing valve-would have been harder to determine.
Although recirculation pumps should exist sized to meet each private building'due south requirements, common practise is i size fits all. Thus nosotros establish the same pump size at all sites. (A methodology for proper pump size pick can exist institute on page 45.5 of the 1995 ASHRAE HVAC Applications Handbook.)
Our monitoring showed that h2o consumption has an changed relationship with recirculation flow. In the overnight period, when there is lilliputian 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 (meet The Best Boiler and Water Heating Retrofits, HE Sept/Oct '95, p. 27, and Controlling Recirculation Loop Heat Losses, HE Jan/Feb '93, p. ix). A new written report investigating iii very depression-cost approaches to reduce recirculation system losses while maintaining resident comfort and satisfaction should exist completed in early 1997.
Peak Demands and Boilerplate Consumption In the New York Metropolis buildings, the average hourly consumption is but 42% of the consumption in the peak hour. Instead of sizing a system to be able to provide the peak demand, information technology's possible to generate and store hot water during the periods of average and below-average demand to meet the superlative. This could be accomplished by installing a system with a heater designed to generate the average hourly load, running essentially continuously, and providing plenty storage tank chapters to shop unneeded hot water during the night and furnish it during periods of peak demand (such as morning shower time).
Effigy 4. Parts of three-hr tiptop and lx-minute peak consumption.
The five-, 15-, 60-, 120-, and 180-infinitesimal peak demand times coincide with each other. These volumes should therefore exist addressed equally different (fourth dimension length) measurements within the same peak DHW draw, so the organisation can be designed to satisfy this load. An instantaneous system designed to run across the superlative 5-minute depict volition accept no problem coming together the rest of the load. Generation and storage systems should be designed both to provide hot water for the average load and to see the short, abrupt peaks.
Let's take a 58-unit flat edifice whose occupants are a mix of families, middle income couples, and some singles. Most adults work exterior the dwelling. There is a public laundry in the basement with a few washers, and the leases prohibit both wearing apparel washers and dishwashers in the apartments (although conversations with the edifice superintendent have confirmed that a number of people have such appliances.)
Step 1. Compute the maximum potential occupancy, based on local standards and expectations, and conversations with the edifice possessor or manager.
Step 2. Determine the Low, Medium, or Loftier (LMH) usage cistron of the building'southward occupants from Table ane, based on cognition of the building, conversations with the building possessor or managing director, and observations. Consider the effect of either currently installed or potential future additions of appliances that might motility a edifice up to a higher usage category.
Based on the information higher up, the Medium usage factor was selected.
Instantaneous Systems
Step 3a. Compute the system load using the 5-minute height demand values in Table 2.
1/Boiler Organization load Conversion Temp ascension combustion efficiency DHW load 1,663 gal/hr x 8.33 lb/gal x 90oF ten 1/0.eight (80% CE) = 1,558,439 Btu/h Instantaneous DHW-But Heater. The 1,558,439 Btu/h should be the size of the DHW heater. (Note that a higher combustion efficiency should actually be used for sizing an instantaneous heater; use 85% or the efficiency specified in the equipment documentation.)
Combination Heat/DHW Banality. When sizing a tankless coil in a combination oestrus/DHW system, the i,663 gallons per 60 minutes is the coil size to be ordered. The one,558,439 Btu/h is the additional load capacity for DHW to exist added to the space-heating load to size the boiler. (In an existing steam heating distribution system, the space-heating load should exist computed by the EDR-equivalent direct radiations-methodology.)
Generation and Storage System
Step 3b. Compute the system load using the peak thirty-minute and maximum three-hr hot h2o values in Table 2.
Pace 4b. Side by side, convert the load into equipment ratings.
Estimating Consumption in Existing Buildings
Step 3c. Summate organization load using the average twenty-four hours values in Table two.
If the current practice of defensive oversizing is applied to the new guidelines, this will merely exaggerate the capital and energy inefficiencies experienced in the past. It is therefore important for the designer to recognize the inherent condom nets in the new approach. The nearly pregnant 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 just short-duration peaks (not delineated in the old guidelines), which occur only a few times during the yr. Even if the system were non able to satisfy that load, the problems would probably exist small-scale-for example, the occupants might experience slightly lower temperature hot water at their taps a few times per year.
The principal 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'southward Technical Commission half-dozen.6 (Service Hot Water) was the main force in the call for a new sizing tool based on the vast quantity of existent-time data that has been collected. The new articulation ASHRAE/ASPE Domestic Hot Water Pattern Transmission, 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.
Further Reading Affiliate 45: Service Hot Water, In 1995 ASHRAE Handbook: HVAC Applications, Atlanta: ASHRAE, 1995.Goldner, F.South. DHW System Sizing Criteria for Multifamily Buildings. ASHRAE Transactions 100, No.i (January 1994): 147-65.
Goldner, F.Due south. Energy use and DHW Consumption Research Projection, Report No. 94-xix. Final Report: Phase one. Prepared past Energy Management and Enquiry Assembly for New York State Free energy Inquiry and Development Authorisation, November 1994.
Goldner, F.Southward., and D.C. Price. Domestic Hot Water Loads, System Sizing and Choice for Multifamily Buildings. In 1994 ACEEE Summer Study on Energy Efficiency in Buildings Proceedings, 2.105-2.116. Berkeley, CA: American Council for an Free energy-Efficient Economic system,1994.
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