When designing a building’s heating hot water system, determining the peak heating hot water supply temperature is a critical decision. These systems provide heat to air handling systems, perimeter radiation, radiant panels, fan coil units, reheat VAV boxes and other hydronic heating elements. The hot water supply temperature (HWST) must be high enough to satisfy the heating loads in the building, but also be low enough to achieve high system efficiency. Once determined, a HWST reset is then often incorporated into the design. The main purpose of a HWST is to reduce the hot water temperature during low-load conditions, resulting in reduced heating energy consumption. Due to the linear nature of space heating loads, a linear reset based on outside air temperature is typically used. As the outside air temperature rises, the HWST setpoint will decrease. As the outside air temperature decreases, the reverse will occur. See the figure below for example.
|Outside Air Temperature (°F)
||Hot Water Supply Temperature (°F)
Now there are a few important considerations when implementing a HWST reset. To start, let’s look at how the reset is operating on a warmer day where the outside air temperature is high and the HWST is low. With a lower HWST also comes a lower return water temperature. Modern condensing boilers achieve the majority of their high-efficiency performance factors when operating at lower water temperatures. For boilers such as these, the low-end reset temperature limit is typically not a concern. However, older cast iron boilers can begin to experience corrosion and damage if entering water temperatures fall below 140°F, as a result of acidic condensation forming from the lower flue gas temperatures. On colder day when the HWST is operating at the maximum setpoint, the setpoint should match the design entering water temperatures of the downstream heating equipment. Taking into account the above-mentioned information, the minimum and maximum HWST setpoints can be determined.
Once the HWST range of the reset is determined, the coinciding outside air temperature (OAT) setpoints must be established. The lower OAT setpoint should match the design outside air condition of the heating system. This is the OAT that the heating system was designed at to satisfy the building’s heating load. The higher OAT setpoint should typically be at or close to the building’s “balance point”, which may require adjusting based on the operation and use of the building. The building’s “balance point” is the typical outside air temperature at which the building does not need heating or cooling. Additional savings can be achieved by disabling the heating system when the outside air temperature reaches the building’s balance point, however some operations such as dehumidification and demand control ventilation may require reheat during the cooling seasons.
When reviewing the hot water system, both the OAT and HWST parameters should be evaluated to optimize overall system performance. For example, if the lower OAT range limit is higher than the design heating condition (e.g., OAT lower limit of 10°F vs. design condition of 0°F) or the upper OAT range limit is higher than the balance point, the system is generating excess heat during partial load conditions. Whenever this occurs, excessive energy consumption is also occurring.
Over time, a HWST reset is often adjusted and fine-tuned to account for changes to the equipment served by it, changes to the heating load, building usage, occupancy, building drift and system degradation. However, it should always be good practice to assess the heating load of the facility and ensure the reset is properly scaled based on the building performance. In a previous project, retro-commissioning of a hot water supply temperature reset resulted in $1,500 in annual savings by simply adjusting the setpoints to match the operation of the facility.
This strategy can be combined with many other strategies that we have already discussed or will review in upcoming articles in this retro-commissioning series. Take a look at our other ECM tips here. If you have any questions about this article or think you might be able to apply this strategy in your facility and would like to learn more, please contact us here.