Measured Savings from Domestic Hot Water Control Strategies in Multifamily Buildings with Recirculat

Multifamily buildings larger than 40 units commonly have a pump on the domestic hot water system that constantly circulates heated water through a piping loop. Such a system ensures continuous delivery of hot water, but high piping losses decrease overall efficiency. To investigate the strategy of reducing system temperatures during periods of low demand, time-based and demand-based temperature controls were installed in three apartment buildings. The purpose was to assess the savings potential, operation, and tenant acceptability of these controls when compared to a constant temperature aquastat. Tests were conducted using an alternating mode design monitored by a computerized data acquisition system.

Even in relatively small buildings of thirty-five to fifty units, this control strategy is worthwhile. Energy savings averaged 10.3% of domestic hot water heating consumption for the time-based control and 16.2% for the demand-based control, with mean simple paybacks 2.2 and 1.9 years, respectively. Savings tended to be seasonally biased, with the largest savings for both controls during summer operation and lower savings during the shoulder and winter months. On average about 14% of the total savings observed for the time control are attributable to reduced demand and 86% to reduced distribution and off-cycle tank losses. By comparison, 32% of the savings observed for the demand control are attributable to reduced demand and 68% to reduced losses. Both controls improved DHW system efficiencies by about the same amount, from 35% to about 38%.

Initial malfunctions with all of the time controls and one of the demand controls lead us to recommend that contractors include a monitoring service for a brief period after installation to assure proper control operation. While neither control generated specific tenant complaints, the time-based control did require periodic reprogramming and in one case aggravated the on-site caretaker considerably. In comparison, the demand control proved easier to operate and appears less likely to be overridden by the building operato\

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Measured Savings from Domestic Hot Water Control Strategies in Multifamily Buildings with Recirculation Loops