Makeup Air For Exhaust Systems In Tight Houses. Tony Jellen Engineering Projects

Transcription

1 Makeup Air For Exhaust Systems In Tight Houses Tony Jellen Engineering Projects PA Housing & Land Development Conference February 23, 2012

2 Effects of Kitchen Ventilation on Residential Dwellings Anthony C. Jellen Engineering i Projects Incorporated 1 Personal Background U.S.N. ( ) BS B.S. in Civil Engineering (2005) PHRC employee during undergraduate training Residential Housing Certificate Engineering Projects Inc. (2006 Present) State College, PA 2 1

3 Outline Section I (Kitchen exhaust fundamentals) IRC requirements for make up air. Building pressure and ventilation effects. Depressurization and why it s a problem. Infiltration as makeup air. Depressurization risk assessment. Section II (Make up air) Need for makeup air. Considerations and requirements. Control. Passive systems. Forced air systems IRC Section M Makeup Air Required Exhaust hood systems capable of exhausting in excess of 400 CFM shall be provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with a means of closure and shall be automatically controlled to start and operate simultaneously with the exhaust system. 4 2

4 Where did this originate? ICC Code Proposal Reasons (As pointed out in the proposal) Kitchens in homes are becoming larger, resembling commercial kitchen environments. No provisions for makeup air provided in the IRC. Exhausting large amounts of air can adversely affect the operation of other appliances in the home. 5 Risks Large exhaust rates coupled with tight building enclosures and little or no makeup air provisions can DEPRESSURIZE a home which could lead to Backdrafting of combustion appliances. Outdoor odors introduced to home. Comfort issues (i.e. Drafts). Radon drive increases. Door operation issues. Enclosure moisture problems. 6 3

5 Depressurization? Air pressure within the house is negative with respect to the outside airpressure. What events can depressurize a house? Natural occurring phenomena. Mechanical systems operation (i.e. Kitchen exhaust). 7 Naturally Occurring Phenomena Building pressure is constantly changing depending on the weather. Primarily two natural phenomena drive pressure changes within a house. Wind Stack effect Temperature differentials across the enclosure. 8 4

6 Naturally Occurring Phenomena 9 Mechanical Systems Operation Combustion appliances Can increase infiltration rates and pressure differences if no combustion air is provided. Central air distribution equipment Can cause variations of pressure between individual rooms if not designed and installed properly. (i.e. leaky ducts, inadequate returns). Ventilation equipment Can cause large pressure differentials if not balanced. 10 5

7 Balanced Ventilation Makeup air (MUA, or replacement air) Airprovided either intentionally or unintentionally thatreplaces air exhausted by ventilation equipment. In our case; the Kitchen exhaust hood. Balanced ventilation Exhaust out = Makeup air in Effects of Ventilation Equipment 12 6

8 Infiltration Air Traditionally assumed to be a makeup air supply for residential kitchenexhaustsystemsand exhaust systems and a combustion airsupply for mechanical equipment. Infiltration rate is a function of pressure differential across the enclosure and leakage area of the enclosure (i.e. cracks, holes to the outside). Quantified by: Empirical Methods Accuracy depends on assumptions (location, weather, construction). Testing Methods Blower door testing (ASTM E779, or CAN/CGSB M86) 13 Building Enclosure Definition The part of the buildingthat physically separates theinterior environments from the exterior environment both above and below grade. Components Base floor systems. Foundation wall systems. Above grade wall systems. Windows and doors. Roof systems. 7

9 Building Enclosure 15 Infiltration Used for Makeup Air Can t really control where it comes from. Can cause unwanted unconditioned drafts. Can come from garage or other areas where undesirable odors may exist. Hard to accurately quantify without testing. Will add heating and cooling load dto central lhvac equipment, which may or may not have been accounted for during equipment sizing. 16 8

10 Infiltration Used for Makeup Air Supply rate is directly proportional to the airtightness of the enclosure (i.e. crack area) Supply rate is directly proportional to the pressure difference across the enclosure. Q = C d x A x (2 P/ρ) n Building Pressure It may not be possible to infiltrate an adequate amount of air at a reasonable pressure to satisfy the exhaust demand. A reasonable pressure differential is below 3 Pa. Typical residential exhaust fans can produce high static pressures and are quite capable of creating large pressure differentials across the enclosure. 9

11 Exhaust Fan Performance Curves 10 Pa (.04 in wg.) = 50 CFM loss Assessing Depressurization Risk 20 10

12 Questions on Section I? When Do I Need Makeup Air? When Kitchen exhaust rate exceeds 400 CFM. Per IRC requirement. When the combination of exhaust rate and enclosure airtightness level suggest that pressure differentials may exceed 2 Pa. If blower door data is available it can be estimated from the chart. Keepin mind that this is only an estimate, fora better indication of potential problems the ASTM E1998 test method should be performed

13 Risk Assessment 2009 IRC Section M Additional Requirements Exhaust hood systems capable of exhausting in excess of 400 CFM shall be provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with a means of closure and shall be automatically controlled to start and operate simultaneously with the exhaust system

14 Additional Requirements Means of closure Usually some type of damper; Motorized or Barometric. Keeps airway closed to the outside when makeup air is not needed. Typically interlocked with exhaust operation. Automatically controlled MUA system operates with exhaust system. Minimizes i i conditioning i i costs of MUA. Automatic Control Current sensing switches. 13

15 Makeup Air Considerations Improper specification or installation of a makeup air system can lead to potential: Mold Issues. Comfort problems. Depressurization. Overstressed or inefficient HVAC. Range hood performance highly dependant on makeup air introduction. Capture and contain (C&C) cooking effluents Fan efficiency 27 Makeup Air Suggestions Always read the manufacturers recommendations. Ideally around 80% should be provided directly in the vicinity of the exhaust fan. The other 20% can be drawn through the residence (This is sometimes difficult). Keep discharge velocities low to eliminate discomfort. Use perforated diffusers vs. 4 way diffusers Question Short Circuit Hood Designs providing greater than 15% 20% of total required makeup air. Deliveredmakeupairtemperatureshould air ideallybe within10 degrees of thermostat set point. Read some literature about makeup air and hood performance

16 Methods of Introducing Makeup Air Passive Methods Engineered Openings. Forced Air Methods Using Central HVAC to distribute. Dedicated Makeup air unit. 29 Passive Methods Similar to infiltration; relies on the pressure differential across the enclosure to draw air into house. Varying degrees of effectiveness. Highly dependent on opening location, room pressure, and access to room. Typically good for small quantities of unconditioned air

17 Engineered Openings Basically a fancy term for a hole in the wall. Airflow across opening depends primarily on the area of the opening(s) and P across opening. Susceptible to weather conditions. Barometric Pressure. Wind. OutsideTemperature. 31 Engineered Openings Supply rate is directly proportional to the airtightness of the enclosure (i.e. crack area) Supply rate is directly proportional to pressure difference across the enclosure. Q = C d x A x (2 P/ρ) n 16

18 Engineered Openings Pressure (Pa) Airflow (CFM) Based on Opening Size 4 in 6 in 8 in 10 in 12 in Engineered Openings 34 17

19 Engineered Openings Pros: Cheapest makeupairair system. Air can be directly introduced to space. Air has the opportunity to mix with interior conditioned air before returning to the central HVAC. Cons Additional load on central HVAC. Varyingdegrees of effectiveness. Dependent on placement and weather conditions. Could introduce drafts. Limited to low supply / exhaust rate applications. Forced Air Methods Central HVAC conditioning and distribution Introduceunconditionedoutside unconditioned outside air directly tocentral HVACunit. Unit condition, filters and distributes air. Dedicated MUA unit conditioning and distribution Provide additional piece of equipment to condition, filter and distribute air. Separate distribution ductworkcancan be provided or designer can make provisions to integrate with central air handler, for distribution after conditioning

20 Conditioning of Makeup Air Manual J Procedures Heating Sensible Load Heating (btuh) = 1.1 x cfm vent x (T room T vent ) x ACF cfm vent = Makeup air rate T room = Thermostat set point T vent = Outside air temperature ACF = Altitude Correction Factor (Assume = 0.97) Example: Heating (btuh) = 1.1 x 800cfm x (70 7) x 0.97 = 53,776 Heating (tons) = 4.5 tons = 15.7KW 37 Conditioning of Makeup Air Cooling Sensible Load Cooling(btuh) = 11x 1.1 cfm vent x (T vent T room ) x ACF Cooling Latent Load Cooling(btuh) = 0.68 x cfm vent x (GR vent GR room ) x ACF GR vent = Moisture content of ventilation air (grains) GR room = Moisture content of indoor air (grains) Assume the GR vent GR room = 26for 50% indoor relative humidity. 19

21 Conditioning of Makeup Air Cooling Example Sensible (btuh) = 11x cfm x (87 75) 75) x Sensible (btuh) = 10,243 btuh =.85 tons = 3 KW Latent (btuh) = 0.68 x 800cfm x 26gr x 0.97 Latent (btuh) = 13,719 btuh = 1.14 tons =4 KW Total Cooling = 0.85 tons tons = 1.99 tons 39 Conditioning of Makeup Air Estimating Cooling CFM Sensible heatingratio (SHR) SHR = Cooling Sensible load / Cooling Total Load SHR = 0.85 tons / 1.99 tons = 0.43 Typical Central HVAC SHR ={0.75,0.85} Low SHR = colder coil = lower cfm 20

22 Integrated System All conditioning is through central HVAC. Good for makeup air rates less than or equal to 10% of design CFM. Air handler fan distributes air through central ductwork. 41 Integrated System 42 21

23 Integrated System Safe quantities of makeup air. Varies with individual equipment. 10% of design air flow; safe rule of thumb. Ventilation air limitations in ACCA Manual S. Problems with excess quantities. Equipment inefficiencies (Not enough capacity) Can t properly p heat, cool or remove moisture. Equipment performance can plummet. Equipment shutdowns (Sensor problems) 43 Integrated System Central Air Distribution Airhandler should be interlocked to operate when exhaust system is in operation. Distributes air throughout residence evenly. Could be problematic if kitchen has limited connectivity to other rooms in house. Closed doors could restrict airflow. Room to room pressure imbalances could arise

24 Integrated System Pros: Relatively inexpensive Conditioning accomplished through central HVAC Even distribution of air throughout residence. Takes advantage of air handler fan. Cons: Additional load to HVAC. Applicable to limited supply / exhaust rates. Susceptible to room to room pressure imbalances. bl 45 Integrated System Dealing with additional makeup air load. Oversized central HVACequipment can lead toproblems. Typically heat pumps in our climate zone can function efficiently up to 125% of heat loss / gain of the house (25% oversize O.K.) Problems with oversized equipment: Short cycling Performance (Inefficient operation) Comfort issues (Decreased latent capacity of air handler) Chilled water and hot water equipment. Low incoming air temperature could be detrimental. Freezing air handling coils. Boiler return water temperature (temperature rise limitations)

25 Dedicated MUA Unit Unit completely dedicated to the conditioning of make up air supply. Interlocked with exhaust operation. Conditioned air can be distributed through central air handler or a dedicated distribution system can be specified. 47 Dedicated MUA Unit Unit completely dedicated to conditioning make up air. Size directly to makeup air load. Individualize systems to suit extreme entering air temperature swings. Greater control over operation and delivery of air

26 Dedicated MUA unit Air distribution options Integrate with central air handler. Dedicated air distribution system. 49 Integrated With Central HVAC 50 25

27 Dedicated Air Distribution System 51 Dedicated MUA unit Pros: Tempered air can be added to central HVAC in greater quantities. Can eliminate comfort issues and local depressurization issues. Eliminates or minimizes load to central HVAC. Cons: Expensive 52 26

28 Closing Statements Depressurization is a concern in modern day residential construction due to the installation of significantexhaust systems coupled with tightly constructed building enclosures. Makeup air is required in many scenarios. For exhaust rates greater than 400 cfm a dedicated MUA unit should be considered. Ventilation design can be complicated, hiring a design professional or qualified HVAC professional might ihtsave you money in the long run. 53 Questions? 54 27