Ventilation Techniques. Topic 5 - Audio 53

Transcription

1 Topic 5 - Audio 53 Increasing Fresh Air Increasing fresh air make-up to a building can dilute radon and other indoor air contaminants. Increasing make-up air can reduce the negative pressures within a building, thereby reducing the radon entry. Isolating and ventilating sub-structures such as crawl spaces. Passively adding fresh air make-up to living space. Actively adding fresh air make-up to living space, while removing interior air with some means of heat recovery. Isolating and Ventilating Crawl Spaces Most crawl spaces have vents Model codes: 1 sq. ft. of opening per 150 sq. ft. of crawl floor surface area. Adding vents can have some benefits provided they are not closed and do not cause frozen pipes. Blowing air into crawl space can cause freezing and may force radon into living area. Air Exchange Rate The Air Exchange Rate describes the amount of outdoor air that is exchanged with indoor air, due to either passive leaks or mechanical systems that either add or exhaust air. ACH is a dimensionless term used to quantify the amount of air exchanged. ACH is an abbreviation for Air Changes per Hour. ACH = Volume of air exchanged per hour Volume of house Example: If 40 cubic feet of air enters and leaves a building every minute (40 cubic feet per minute or CFM) and the building has an interior volume of 8,000 cubic feet (1,000 square foot building with 8 foot ceilings), the Air Exchange rate is: 40 CFM x 60 minutes per hour/8,000 Ft 3 = 2,400 CFH/8000 Ft 3 = 0.3 ACH In other words, a volume of air equal to 30% of the house volume is exchanged with the outdoors every hour. Increasing the ACH through additional mechanical ventilation can reduce radon through dilution. 1

2 Topic 5 - Audio 54 Heat Recovery Ventilators (a.k.a. Air-to-Air Heat Exchangers) Reduce radon through dilution. Can help reduce radon from building materials (emanation) Exhausts air from home and replaces with fresh outdoor air. HRV is a means for retaining some heat of exhausted air. Should be installed by competent person. Simultaneously supply and exhaust air from building. Can improve indoor air quality. Very useful as a second phase after Active Soil Depressurization has reduced radon to near 4.0 pci/l. Assume that the radon is evenly distributed through out the home, i.e. equal mixing. Assume that if the normal ventilation of the home was increased, that the radon level would decrease proportionately. Question: If you had a radon level of 8 pci/l and you doubled the ventilation rate into the building, what would the resultant radon concentration be? Graph shows radon reduction in three houses. Numbers at top of each column are the ACH of the house (either before or after ventilation rate was increased). 2

3 Topic 5 - Audio 55 A blower door can be used to determine Initial ACH Determine current radon measurement Determine current air exchange rate for house Make an assumption (0.3 is not unreasonable) Have a blower door test done Choose a target radon level to which you want to reduce the radon Use the following equation to determine the amount of air exchange you want to have in the building after you have added the HRV. Final ACH = Initial ACH x Initial Radon Final Desired Radon Determine the amount of additional ACH needed to be added by the HRV ACH HRV = Final ACH Initial ACH Determine volume of air needed from HRV: HRV flow (CFH) = ACH HRV x Volume of house = cubic feet/hour Convert to cubic feet per minute HRV flow (CFM) =HRV flow / 60 Select an HRV that has an air flow capacity equal to or greater than amount calculated. Example: Initial ACH = 0.3, Initial Radon = 8 pci/l Final Desired Radon = 2 pci/l, House = 2,000 square feet with 8 ft ceiling height (volume = 8x2000=16,000 cubic feet) 1. Final ACH = 0.3 x 8/2 = 0.3 x 4 = 1.2 ACH 2. ACH HRV = 1.2 ACH 0.3 ACH = 0.9 ACH 3. HRV flow = 0.9 ACH x 16,000 cubic feet = 14,400 CFH / 60 = 240 CFM 4. If sizes are 100 CFM, 200 CFM or 300 CFM, choose 300CFM 3

4 Topic 5 - Audio 56 Install as stand-alone system in lower level. Focuses reduction on lower level Supply fresh air to main living level May increase basement levels Supply fresh air into forced air unit Distributes air though home Conditions incoming air EPA Mitigation Standards regarding HVAC Modifications (Section 14.8) Modifications to an existing HVAC system, which are proposed to mitigate elevated levels of radon, should be reviewed and approved by the original designer of the system (when possible) or by a licensed mechanical contractor Foundation vents, installed specifically to reduce indoor radon levels by increasing the natural ventilation of a crawlspace, shall be non-closeable. In areas subject to subfreezing conditions, the existing location of water supply and distribution pipes in the crawlspace, and the need to insulate or apply heat tape to those pipes, should be considered when selecting locations for installing foundation vents Heat Recovery Ventilation (HRV) systems shall not be installed in rooms that contain friable asbestos In HRV installations, supply and exhaust ports in the interior shall be located a minimum of 12 feet apart. The exterior supply and exhaust ports shall be positioned to avoid blockage by snow or leaves and be a minimum of 10 feet apart Contractors installing HRV systems shall verify that the incoming and outgoing airflow is balanced to ensure that the system does not create a negative pressure within the building. Contractors shall inform building owners that periodic filter replacement and inlet grill cleaning are necessary to maintain a balanced airflow. This information shall also be included in the documentation Both internal and external intake and exhaust vents in HRV systems shall be covered with wire mesh or screening to prevent entry of animals or debris or injury to occupants. 4

5 Topic 5 - Audio 57 Fresh air inlet and Exhaust outlet Install screens Air filter on inlet (often an allergen filter) Separate fresh air inlet from exhaust by 10 feet Can be near grade-be careful that fresh air inlet does not pick up exhausts, etc. Interior stale air pick-up and fresh air supply Fit with dampers Place stale air exhaust and fresh air inlet at least 12 feet apart. Balance flows such that fresh air supply = stale air exhaust Use pitot tube to measure and dampers to adjust Ductwork should be insulated to minimize heat loss and condensation within the home. Common Failure Modes Inlet screen clogs Decreases supply air while exhaust unaffected Building goes negative Increased backdraft potential & radon entry Moist interior condenses on heat exchange surface. Condensate drain needed Moisture can freeze on heat exchanger Can damage rotating wheel types, defrost controls for cold climates. Clogged inlet screen Performance Indicators Not specifically required by EPA Radon Mitigation Standards It makes a lot of sense to install an indicator Can be a simple manometer that measures static pressure in air intake or across intake and supply. Helps identify imbalance & need to replace filters. 5

6 Topic 5 - Audio 58 Advice: Practice first before going live with the public. Play with variables of fans and sealing, etc. to get a good feel for what affects radon. Approaches that modify HVAC can supplement a traditional ASD system by reducing radon or radon decay products. Can provide alternate approaches especially when consumer wants to also improve other indoor air quality concerns. Heat Recovery Ventilators (HRVs) Exchange stale indoor air with fresh outdoor air, thereby diluting radon. Have means to transfer heat of exhausted air to incoming air. Appropriate when levels are relatively low, like 4-6 pci/l or as a supplemental technique to ASD. Generally incorporated with an existing forced air unit (FAU). Only effective when FAU is operating Newer FAUs have variable speed motors or they can be retrofitted on existing FAUs. Running a FAU constantly is better for life of unit. Can be drafty if not properly designed. Fresh Air Make-Up Adding fresh air directly into home is not recommended. Adding it to a FAU is good and in fact is code in many areas. Provides dilution and also pressurization of interior to reduce radon entry from soil. FAU needs to run in order for this to be effective. FAU must have capacity to handle this. HVAC Can Work Against You Exhaust systems like range hoods can overpower an ASD system (look at adding fresh air make-up to FAU) Combustion appliances can create negative pressures (sealed combustion appliances, especially those with fresh air make-up can help). Leaky ductwork in crawl space or buried ductwork can be problematic. (Seal ductwork, or perhaps even abandon buried ductwork) Always look at HVAC system to identify challenges. Working With HVAC Contractors Most jurisdictions require a license and insurance to modify HVAC systems Find an HVAC contractor you can work with. 6