Overventilation Wastes Money
March 16, 2009
When someone asks whether a ventilation system is adjusted correctly, who determines whether it’s correct? The leading standard for ventilation system design is American Society of Heating, Refrigerating, and Air-Conditioning Engineers’ (ASHRAE’s) Standard 62.1-2007. While the standard contains many variables, calculations, and dependencies for determining different ventilation rates for specific building situations, the common recommendation is 25 cubic feet per minute (cfm) per person.
When designing or adjusting a ventilation system, there are two rules of thumb. The first is to make sure the mechanical system delivers enough ventilation air to meet the standard. An HVAC system that delivers too little outside air (OA) allows the buildup of potentially harmful indoor air pollutants. The second is that a system that over ventilates or delivers too much outside air causes needlessly high energy bills. To prevent this, size and equip the HVAC mechanical system air-handling units to deliver the proper amount of ventilation air, but not more.
CALCULATED SOLUTIONSMany older air-handling units use a fixed OA damper position - usually 15 percent - to deliver what was once thought to be the correct volume of air. In many instances, however, the damper position is not an actual measurement of outside airflow in cfm. When this happens, the system is conditioning more OA than necessary. To calculate the appropriate percentage of OA in a ventilation system, certain pieces of information must be gathered and plugged into a formula including: return air (RA), mixed air (MA), and OA.
%OA = 100 x RA – MA/RA – OA
These values can be measured as either temperature or CO2 content as well. Using temperature units:
%OA = 100 x TRA – TMA/TRA – TOA
Where TRA is the temperature of the RA, TMA is the temperature of the MA, and TOA is the temperature of the OA.
If you know the desired percent of OA, and you’ve measured the temperatures of OA and RA, adjust the damper to obtain the necessary TMA.
This percentage is typical for an HVAC system that uses outside air for “free” cooling. Fortunately, some new products, such as the Fluke 975 AirMeter™, make these calculations easier. The built-in keypad menu records the temperatures at the unit and calculates the ventilation air percentages. As long as you know the cfm produced by the unit, the measuring tool will determine the percentage and volume of OA.
If you don’t know the volume at the air handler, use the meter to measure air velocity at different sampling points in the duct. Enter the duct dimensions and calculate the cfm airflow of the unit. Now you can determine the percentage of ventilation air.
CALCULATING COSTSAs much as possible, you need to prevent overventilation. It wastes energy. Any extra air introduced into the system must be heated, cooled, humidified, or dehumidified as needed.
Once you know the total heat in Btu, you can determine the tonnage and electrical cost for the unit. Here’s the formula for the total heat absorbed by the cooling coil.
Total heat = cfm x 4.5 x âˆ†H (difference in enthalpy)
Using the above formula, the following scenario and supposed numbers are provided as an application example.
Total heat = 5,000 x 4.5 x (28.8-22.3)
Total heat = 146,250 Btu
Since there are 12,000 Btu/ton of cooling, the coil tonnage due to overventilation of this system is slightly more than 12 tons. For a chilled-water distribution system, the common energy requirement is approximately 0.8 kW/ton of cooling. That means the energy consumed by excess OA is 9.6 kW (12 tons x 0.8 kW/ton).
Now you can calculate the energy cost. If the excess outside air is used for 220 hours per month, the total kWh of energy used is 2,112 kWh of energy/month (9.6 kW x 220 h). If 1 kWh costs 10 cents, the overventilation cost for one unit is approximately $211 per month (2,112 kWh x 0.10). This is just one unit within a building. If the building’s other air-handling systems are operating in a similar fashion, the total overventilation cost easily runs into the thousands of dollars.
In cold climates, you could calculate the heating energy in a similar fashion. Any air that overventilates the space will be very cold and must be heated. As an added side benefit, if overventilation is measured and corrected, the system’s heating coil will not be as likely to freeze up in bitterly cold weather.
Using an air meter to measure ventilation rates and adjust airflow can significantly reduce the energy bill in a typical facility. To document the adjustments and savings for your customer, save the readings, download them to a computer, and create a report.
Publication date: 03/16/2009