Heating and cooling a building at the same time doesn't make much sense. The two would work against each other, reducing comfort and wasting energy. Unfortunately, simultaneous heating and cooling is hardly rare. In fact, it's one of the most common efficiency opportunities available in commercial buildings.
What causes simultaneous heating and cooling?
Simultaneous heating and cooling is often the result of unnecessary reheating. In a typical HVAC system, the air handling unit's discharge temperature is designed to be at a constant set point between 53°F and 55°F to account for cooling needs on the hottest day of the year. Supply air temperature reset has become a more common best practice, but it's not universally used.
When a zone needs only partial cooling, the airflow volume in the zone terminal unit is adjusted to meet the demand. If the airflow rate is already at the minimum set point but is still too cold for the zone, reheat will be used to increase the discharge air temperature at the terminal unit. Improper control settings, leaky dampers and sensor malfunction can also cause simultaneous heating and cooling.
Detecting simultaneous heating and cooling
HVAC system types most likely to experience unwanted simultaneous heating and cooling include variable air volume and constant air volume systems with reheat, as well as dual-duct and multizone systems.
Fault detection and diagnostic (FDD) tools can help identify instances of simultaneous heating and cooling. Building automation system trend log data from key points in the HVAC system are used. Such data points may include air handling unit heating and cooling coil valve status, terminal unit reheat coil valve status, damper position and outdoor air temperature.
Trend data from these points is used for FDD analysis. If it is found that the heating and cooling coil valves of an air-handling unit are operating simultaneously, it could mean that the air entering the building is being both heated and cooled. Maintenance staff could then be alerted to check the condition of the coil valves.
Interval metering data can also be used to uncover simultaneous heating and cooling. Whole-building or submetered hourly energy use data is plotted on a chart with corresponding outdoor air temperatures to spot potential instances of heating energy use in summer or cooling energy use in winter. For example, if you submeter your HVAC system and the data shows high natural gas consumption on a warm day, that could be a sign of simultaneous heating and cooling.
Eliminating simultaneous heating and cooling
By eliminating simultaneous heating and cooling, buildings can achieve savings of 5% to 20% in HVAC energy use, according to author and energy engineer Steve Doty. Commonly used methods to minimize simultaneous heating and cooling include:
- Reset air handling unit discharge air temperature based on zone demand or other demand measures, such as outdoor air temperature, time of year, return air temperature or some combination of these.
- Reduce preheat settings in air handling units to eliminate unnecessary recooling when using economizers for free cooling.
- Repair any leaking coil valves or dampers to make sure that no unwanted heating or cooling is taking place when they are closed or stuck open.
- Reduce minimum airflow in variable air volume systems whenever possible. Airflow settings should be based on actual space conditions to satisfy building pressure and ventilation requirements. Energy can be wasted if minimum airflow is greater than required.
If you suspect that your building is experiencing simultaneous heating and cooling, take action to stop wasting energy and money.