PART ONE: Cross-contamination of air streams
It is the plight of professionals in most industries: You introduce your company to potential clients only to discover their perception of your product or concept has already been influenced by myths or outdated information. It’s no different for the sales force at BPE, Inc. when we explain how Energy Recovery Ventilators (ERVs) are the cost-effective answer for bringing fresh, preconditioned air into buildings and homes so airtight we think of them as Tupperware containers. Therefore, part of what we do involves busting myths that prevent folks from ruling out the very thing that can improve Indoor Air Quality (IAQ) and comfort, heating and cooling bills, and health. Let’s start dismantling some of the myths surrounding ERVs with an oldie but a goodie:
Myth One: “What good is an ERV if cross contamination allows dirty air to pollute the fresh air stream?”
As stale and possibly contaminated air is pulled out of a building through an ERV, a separate flow of fresh outdoor air is pulled in. In ERV designs, the two airstreams often pass close to each other, opening the possibility for exhausted particles, odors, and condensation to return indoors. However, it is important to note that this minimal leakage opportunity does not promote an unbridled contaminant free-for-all. ASHRAE standard 62.1 is quite clear in limiting acceptable leakage or the Exhaust Air Transfer Ratio (EATR) to 5 to 10% depending on the type of air exhausted. So, in properly engineered ERVs, a small amount of leakage is not a disaster and certainly not a reason to throw the baby out with the bathwater. Many ERVs leak a mere 3 to 5% of exhaust into the fresh airstream with others showing results below 1%.
On top of limiting the amount of recirculating air, ASHRAE designates the types of air that may or may not re-circulate in a space:
Class One: Lowest concentration of contaminants. This includes inoffensive odors, and it’s the sort of air you encounter in offices, classrooms, hallways, and assembly rooms.
Class Two: This air can cause a moderate amount of irritation to the senses. It is often found in restrooms, warehouses, dining rooms, locker rooms, and pool areas.
Class Three: Exhaust air from venues such as kitchens, salons, pet shops, and dry cleaning shops can carry a significant concentration of offensive odors and sensory irritants.
Class Four: Dangerous particles, fumes, and gases fall into this air quality category. You will find them in areas prone to trapping aerosol paints fumes, kitchen grease exhaust, and laboratory exhaust.
In handling the exchange of air classifications, ASHRAE is clear:
⦁ Class 1 air may recirculate to anywhere
⦁ Class 2 air may be re-designated as Class 1 air in the process of recovering energy when it is diluted with outdoor air such that no more than 10% of the resulting airstream is Class 2 air.
⦁ Class 3 air may be re-designated as Class 1 air in the process of recovering energy when it is diluted with outdoor air such that no more than 5% of the resulting airstream is Class 3 air.
⦁ No re-circulation or transfer of Class 4 air is permitted.
ERV styles affect the amount of cross-contamination between airstreams as well. While designs using an enthalpic wheel–an older technology–can see up to 10% leakage, the more typical stream transfer of 3 to 5% of the flow rate generally wouldn’t cause occupants to detect fumes. Cross-flow plate units typically see less leakage than enthalpic wheel designs, and counter-flow plate ERVs such as BPE Inc.’s units often test at less than 1% transfer between streams. In fact, ASHRAE (2000 ASHRAE Systems and Equipment Handbook, Chapter 44) states that one of the advantages of a plate exchanger is that, “It is a static device built so that little or no leakage occurs between airstreams.”
Fear of cross-contamination should not prevent anyone from considering an ERV for their ventilation needs. The technology is a cost-effective, quiet, comfortable, and environmentally-conscious boon for turning airtight indoor spaces into physically and fiscally healthy environments.
Interested in superior Indoor Air Quality? BPE, Inc. can help. Give us a call.