HVAC Operations Chilled Water and Hot Water Reset Buildingsone

HVAC Operations Chilled Water and Hot Water Reset BuildingsOne | April 29, 2019

During lower demand cooling and heating conditions, the building’s central HVAC system - chiller and boiler plant - can operate at moderated reset temperatures, reducing energy and related operating costs. Chilled water reset (HVAC cooling operation) and hot water reset (HVAC heating operation) are typically associated with temperate outdoor weather conditions. However, the practice of resetting chilled water temperature is also a key operating strategy implemented with Demand Management Response (DMR) utility programs.

In its most basic form, during cooling operation, the Building Automation System (BAS) will reset (increase) central HVAC chiller water output temperature based on moderated (cooler) outdoor air temperatures. The energy required to produce 48°F chilled water is much less than the energy to produce 43°F chilled water. Conversely, during heating operation, the building’s BAS or boiler controls will reset (decrease) hot water output temperature based on moderated (warmer) outdoor air temperatures.

Depending on the complexity of a building’s HVAC BAS system, chilled water and hot water reset can be effectively accomplished through various operating strategies or, what is referred to as, Sequence of Operation. For example, a basic algorithm can be established in adjusting chilled water temperature based on the average position of primary Air Handling Unit (AHU) chilled water valves. More specifically, a building with 4 AHU chilled water valves, having a current average 15% open position, would indicate a moderation of outdoor air temperatures, and reduced cooling requirements. With average chill water valve position data, providing input to the BAS system, the BAS will increase the temperature of the chilled water until the average AHU chilled water valve position is, let’s say, 50% open.

In another example, chilled water or hot water temperature could be adjusted (reset) based upon secondary-loop chilled water or heating valves, situated on individual sides of the building’s perimeter cooling-heating zone. This finite, zone-control would provide for cooling and heating of conditioned space on a more enhanced occupant-comfort basis, while optimizing energy.

HVAC Operations Chilled Water and Hot Water Reset

Water-Cooled Chillers | Chilled Water Temperature Reset

During the cooling season, increase chilled water supply temperature while still maintaining good humidity ranges. Chilled water temperature is reset automatically based on outdoor-air temperature, or the current cooling load - average AHU or secondary system cooling-coil valve commands. Chillers operate more efficiently by raising-resetting chilled water loop/circuit temperatures during periods of lower demand. For example, by applying an outdoor air reset schedule, the operating range of a building’s chilled water temperature could be optimized by programming a chilled water operating range of 48ºF - 43ºF against an outdoor air temperature (OAT) range of 60ºF - 90ºF.

Basic Outdoor Air / Chilled Water Temperature Reset Schedule

Outdoor Air Temperature (OAT) Chilled Water Supply Temperature

60ºF 48ºF

65ºF 47ºF

70ºF 46ºF

75ºF 45ºF

80ºF 44ºF

85ºF+ 43ºF

90ºF 43ºF

‘A basic algorithm can also be established in adjusting chilled water temperature based on the average position of primary Air Handling Unit (AHU) chilled water valves’.

HVAC Operations Chilled Water and Hot Water Reset

Heating Control | Hot Water Reset

During the heating season, decrease (reset) boiler or steam plant hot water temperatures based on outdoor-air temperature or current heating load in the building. Boilers operate more efficiently and, for buildings connected to district steam, energy consumption decreases, by lowering hot water loop-circuit temperatures during periods of lower demand. For example, by applying an outdoor air reset schedule, the operating range of a building’s hot water loop could be optimized by programming a hot water operating range of 140ºF-180ºF against an outdoor air temperature (OAT) range of 60ºF-20ºF.

Heating Control | Hot Water Reset Schedule

Outdoor Air Temperature (OAT) Hot Water Supply Temperature

60ºF 140ºF

55ºF 145ºF

50ºF 150ºF

45ºF 155ºF

40ºF 160ºF

35ºF 165ºF

30ºF 170ºF

25ºF 175ºF

20ºF 180ºF

HVAC Operations Chilled Water and Hot Water Reset

Newer, high efficiency condensing boilers are designed to operate with return water temperatures lower than 140ºF. Standard hot water boilers are not designed to operate with return water temperatures lower than 140ºF and condensing of combustion gases inside the boiler stack can result in corrosion. Dependent on the buildings BAS capabilities, the hot water temperature schedule could potentially be programmed (reset) based on current building heating load through monitoring hot water supply and return differential, hot water flow/pressure differential, or the average hot water pumps’ variable frequency drive (VFD) electrical load.

“Building owner - oper ator s discussing chilled water and hot water reset strategies, with their C h i e f E n g i n e e r a n d B A S operator, will optimize energy consumption, particularly during relatively lower demand outdoor weather conditions.

Building operators should first consider the HVAC plant’s designed sequence of operations in place for standard HVAC operating modes, e.g., pre-occupancy start up, occupied cooling, occupied heating, unoccupied night set-back, etc. It is within the designed sequence of operations, where the chilled water and hot water reset strategy might already be in place and programmed into the BAS system”.