So why bring in outside air into buildings? Occupied buildings are full of people and chemicals that could be harmful to human health. This “dirty”, unhealthy air comes from fabrics off gassing, cleaning supplies, Human metabolism, perfumes, colognes, and the list goes on. Less than desirable air can also be exhausted directly from bathrooms, kitchens, equipment rooms, and by the central ventilation system. Clean, healthy fresh air is brought into the building to replace the dirty, unhealthy air. When the outdoor air is cooler than the air temperature inside a building, then it can also be used to cool the building without running mechanical cooling equipment. But this outdoor air is not always ideal for the indoor environment either. It should be filtered, and often needs to be tempered (warmed up or cooled down) to be comfortable for the inhabitants. Introducing the right amount of outdoor air to a facility is critical to comfort and energy-efficient operation. Too little outdoor air can save energy and money but result in an unhealthy or sick building. Too much outdoor air can waste energy and money but provide a fresh facility with improved Indoor Air Quality (IAQ). Outside are optimization is critical to balance a healthy environment and energy conservation.
When it comes to performing Outside Air Optimization the first decision that must be made is will the system complies with ASHRAE Standard 62.1 Ventilation for Acceptable Indoor Air Quality or California Title 24. If so, those standards require extensive knowledge of the area served, and calculations will be required to perform outside air control to those standards. If compliance to those standards is desired, we recommend looking at ASHRAE Guideline 36 High Performance Sequence of Operations for HVAC because it gives some good guidance on what is required and how to achieve compliance for several equipment types.
If compliance with ASHRAE Standard 62.1 or later or California Title 24 is not required, then outside air optimization can potentially be somewhat easy and straight forward.
Air conditioning systems use energy to relocate heat (move the heat from inside to a place we do not care) and there are two types of heat that it relocates. The first is the “sensible heat” and the second is “latent heat”. Sensible heat is the heat that is measured by a typical thermometer and thermostat. Sensible heat is what you feel (heat or lack of heat). The other factor that influences how the temperature feels to you (feels like) is known as relative humidity or the grains of moisture in the air (water vapor). Ever wonder why it can be 72 degrees and you feel hot? Most likely the relative humidity is high. The opposite could occur if its 72 degrees and dry (low relative humidity) you could feel cold because of evaporation of perspiration occurring at your skin. Sensible heat requires less energy to relocate than humidity because of the latent heat released when it goes through a state change to be converted from a gas (grains of moisture in the air or water vapor) to a liquid (condensate) so it can be removed. To convert the water vapor in the air you must condense it out by passing it across a surface which is cold enough (at or below dew point) to cause condensate (water) to form (like on the mirror in the bathroom when you take a hot shower, or a window on a cold winter day). Condensation occurs when the air is cooled causing it to compress to the point where it reached 100% relative humidity and it can no longer hold the grains of moisture (like squeezing a sponge). When a vapor changes to a liquid, it gives up heat. This is latent heat is because energy is required to change the vapor into liquid and not from lowering the temperature. All air conditioning units’ usually effect both latent and sensible heat, assuming the evaporator coil (indoor coil) is below the dewpoint.
Depending on the area of the country (climate zone) where the building resides, minimizing the induction of outside air can save energy because the availability of “Free Cooling” available by using cool outdoor air is minimal. These areas are mostly along the gulf coast where the humidity levels are usually high and since humidity is latent heat an AC system is required to remove moisture. The simplest form of Outside Air Optimization would open the outside air damper to a minimum position and enable any exhaust fans only during the building scheduled occupancy occurs. Some outside air needs to be brought into a building to accomplish at least two things. First if there are any exhaust fans running then the outside air needs to be brought into the building make-up for the air that is being exhausted. The air that the exhaust fans are removing from the building will come into the building from somewhere and it is better to condition the air through the AC system than have it come into the building through leaks in the structure. When the air enters the building through leaks in the structure it is unfiltered and prone to causing condensation at that location and the possibility of leading to mold issues. Once the minimum position of the outside air damper is determined then it can be modulated further open based upon CO2 or VOC (Volatile Organic Compound) sensors which values change in response to the number of people in the space. People exhale CO2 so a CO2 sensor monitors the increase of the CO2 as more people enter the space. A VOC sensor measures air contaminants from other sources besides respiration, such as building materials, cleaners, perfumes along with furniture and carpet off-gassing so both can be used and compared to a setpoint to induce more outside air and the space values increase. Placement of these sensors is very important. If the HVAC unit only serves one area or a few areas that would have the same density of occupation, then one of these sensors could be placed into the common return air plenum of the unit or in the space. If a CO2 or VOC sensor is not in the space it must be in the return air and not the mixed air of the unit to give a fair representation of the contamination concentration in the space. The more areas that the unit serves with varying numbers of occupants with common sensors the less accurate the depiction of individual space conditions becomes because one area could have high CO2/VOC but when mixed with another areas low CO2/VOC in the common return the area with the high reading is not accurately represented. In this case several sensors may need to be located in the spaces instead of a single sensor in the return air so the worst case sensor can be selected to be compared against the setpoint for the outside air control.
If the building resides in an area of the country that allows for the use of outside air for “Free Cooling” then the outside air can be used for more than just CO2 or VOC control. The term “Free Cooling” is commonly used but it’s misleading because the only true free cooling is when someone opens a window, a door, or some other opening so the outside air can naturally draft through the building. Anytime an electrical blower or fan motor is involved in moving the air it is technically no longer free. It can greatly reduce the cost of cooling but it’s not free. Anyway, there are several things that need to be considered when “Free Cooling” is to be used within an AC system and they are:
All the criteria above for non-free cooling control still apply. Minimum OA Damper position, type of sensor, sensor location, and so on.
The “Total Heat” (Enthalpy) needs to be calculated for the indoor environment and the outdoor air that would be used for the free cooling.
Does the space need cooling?
Does the space need additional humidity added and is it available outside?
Is there a mixed air sensor or supply air sensor that can be used for the control?
As you see in the list above there are several more items to consider if free cooling is going to be used and there needs to be validation of the sensor inputs because the “free cooling” can really cost a lot if the outside air is brought into the building when it’s not truly useable as free cooling due to a faulty sensor.
Sometimes the outside air can be used to pre-cool a building or to initiate optimum start. If those opportunities are to be utilized then the temperature-humidity sensors that a being used to calculate the total heat (enthalpy) must be located in the space and not the return air plenum because unless the units circulation fan (blower) is bringing the air from the space across those sensors they are not getting an accurate reading in relation to the space conditions. Quite often those sensors could be mounted in the return air section of a roof top unit and when the roof top unit is not running that air temperature and humidity is totally different than the spaces temperature and humidity.
Based upon the criteria above when the outside air is useable the outside air damper will be controlled by either the economizer control (free cooling) or the CO2/VOC control whichever is needing more outside air to be induced into the building. When the economizer (free cooling) control is in operation it is controlled to supply the cooling, the space requires to remain at its space cooling setpoint. Optimally this would be accomplished by controlling to a mixed air setpoint or a supply air setpoint so the possibility of extremely cold air blowing into the space could be avoided. When either using mixed air control or the supply air control those setpoints would be reset in response to the cooling needs of the space. The additional induction of outside air by the economizer control can improve the Indoor Air Quality (IAQ) assuming the outside air that is being induced is a good quality and does not have a lot of pollen, mold, VOC’s, or undesired gasses in it. In those cases, an automatic or operator-initiated shutdown of the outside air dampers may be needed.