FINAL REPORT: Evaluation of New DHW System Controls in Hospitality and Commercial Buildings
Executive Summary
The energy usage of hot water systems in non-residential facilities is often overlooked. Hot water systems, particularly large central systems, are typically among the larger systems in a commercial building. In Minnesota’s climate, however, they are a much lower priority than space heating systems. This presents an opportunity for system improvement and energy savings.
Hot water usage patterns in hospitality, commercial, and multifamily applications tend to reflect periods of heavy use (weekday mornings and early evenings), as well as periods of low or no use (between midnight and 5 a.m.). To ensure that hot water is immediately available at all times, building owners typically install central recirculation loops that operate continuously, even during periods of low or no use. This results in continuous circulation pump electricity consumption, as well as wasted energy to constantly reheat water as it circulates through the building.
To limit these pump and reheat costs, building managers often use aquastat and time clock recirculation loop controls. However, because the controls work on schedules preset for low use, hot water draws are significantly delayed during periods when the pump is off. When occupants complain about the delays, building managers often bypass or remove the time clock-based controls. In the hospitality sector, where guests’ hot water usage patterns are particularly unpredictable, managers are left with little choice beyond continuous recirculation to avoid risking customer dissatisfaction.
Unlike crude traditional controls, the new demand systems use temperature and demand inputs, so the controller activates recirculation when both (a) the recirculation loop return water has dropped below a prescribed temperature and (b) a DHW demand is sensed as water flow into the system. Demand controls respond more quickly to deliver hot water during low use periods, resulting in greater building manager acceptance and persistent savings.