Reset Strategies

Reset Strategies

Supply Air Temperature Reset is a long name for a simple strategy to cut energy costs.

In an office, school, or other large building with an HVAC system a reset strategy saves energy by reducing the production of something – cold air, hot water, or air pressure – that takes energy to make.

Cooling air at 95°F down to 65°F on a hot day requires energy, and to get the same air to 60°F or 55°F takes more energy. The colder you make it the more energy it takes.

It takes lots of energy to push conditioned air through ducts to offices, classrooms or zones in a facility. The faster you push the air the more energy you use, and pushing a larger volume of air will also drive up energy consumption.

Energy Cost Savings Example 1 – Supply Air Temperature Reset

Let’s say it is 95°F outside and cooling is needed from the HVAC system. A Supply Air Temperature Reset strategy adjusts the supply of cold air to meet demand, balancing comfort versus energy cost. In this case the Building Energy Management System decides when to reset supply air temperature based on the temperature of air coming back from the cooled spaces.

A high return air temperature means the zones are still warm so the HVAC system should supply very cold air until temperatures drop to a satisfactory state. Later, as air coming back from the zones drops in temperature it signals the Energy Management System there is less need for cooling.

The Building Energy Management System then raises the temperature of air discharged by the cooling system, which lightens the load on compressors and reduces their energy consumption.

One advantage of a Supply Air Temperature reset strategy is that it can be applied to both Constant Volume (CV) and Variable Air Volume (VAV) systems.

CV systems are cheaper to install but VAV systems offer energy savings compared to CV systems because they adjust the volume of air distributed to match cooling or heating demand.

Compared to CV systems, VAV energy savings are substantial because the amount of energy required to move air is a cubic function of speed. Looking at key points on a graph of speed and power we see that at 100% speed a fan uses 100% power but an 80% speed cuts power required to about 50%, and at 60% speed you cut power requirements by more than 75% versus full load.

reset1Figure 1

A future article will discuss uses and benefits of Variable Frequency Drives (VFDs). VFDs adjust the frequency of electricity supplied to a motor from 0 to 60 Hz to make a motor run at variable speeds to capture the cubic energy savings described above.

An EMS can display a screen with current status of a Supply Air Temperature Reset-enabled cooling system to help users understand system behavior. In this example screen, building users can click green arrows to adjust the reset parameters within limited ranges, fine tuning HVAC performance and conditions to their liking. The display updates in real-time to help users understand changes in discharge air temperature they may sense.

reset2Figure 2

EXAMPLE 2 – Static Pressure Reset

In a ducted cooling system the central AHU or RTU has to supply sufficient discharge air pressure to ensure that rated airflow is supported at the furthest reaches of the supply duct, and potentially that air also returns to the central unit in appropriate volumes.

If you over-pressurize the system you waste energy. ASI Controls terminal unit controllers automatically adjust duct static pressure in response to airflow demand, reducing energy costs while delivering the freshly conditioned air that keeps people comfortable.

A system that adjusts both air temperature and airflow simultaneously to meet demand is a Variable-Volume Variable-Temperature or VVVT or VVT system.

Other possible reset strategies including Hot Water Supply Temperature and/or Pressure reset using the Heating Coil Valve position as the reset input, or resetting Chiller Cold Water Supply Temperature based on the Cooling Coil Valve position.

ASI Controls can help you run through the numbers and calculate projected savings from an automated EMS using reset strategies.