Comparative Energy Performance of Domestic Hot Water Systems in Multifamily Buildings

The overall efficiencies of four types of central domestic hot water (DHW) systems used in multifamily buildings were determined, along with the savings associated with converting to more efficient systems. Commercial tank heaters with integral flue dampers (IFD) were compared to tank heaters without IFDs in three buildings, and a high efficiency, low-mass, front end boiler (FEB) was compared to a space heating boiler also heating DHW in a single building. The tests were conducted using a side-by-side, alternating mode test procedure.

In the buildings with commercial tank heaters, the overall system efficiencies and savings varied depending on the method of analysis used. System efficiencies for the heaters without IFD ranged from 53 to 64%, while the efficiencies for models with IFD varied from 56 to 67%. The best estimates of savings were 23.8, 13.2 and 3.9 MBtu/yr ($107, $60 and $18/yr). The incremental cost of a heater with IFD is $540 to $640. Paybacks on the added cost ranged from 6 to 30 years, which is long compared to those desired by multifamily owners.

The yearly DHW system efficiencies for the space heat boiler site were 68% for the FEB and 62% for the existing space heat boiler. This results- in a savings of only $228 per year, suggesting that the use of a large space heat boiler for DHW may sometimes be relatively efficient, if, as in this case, it has low off-cycle losses.

Data on daily, weekly and seasonal patterns of hot water use were also collected. The patterns observed were similar to those reported elsewhere, and the peak flows were generally in line with ASHRAE sizing guidelines. Leakage was measured in one building and determined to be 23% of total hot water use, clearly a significant factor in DHW energy consumption.

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Comparative Energy Performance of Domestic Hot Water Systems in Multifamily Buildings