• Increased depressurization of CAZ• Back-drafting of combustion appliances• CO poisoning
Terminology• Pressure –
• A force encouraging flow by virtue of a difference in some condition between two areas.
• The effect of a force applied to a surface• In Wx, usually measured in Pascals (Pa)
• Pressure Balancing –• The process of neutralizing pressure differentials in the
home• Performed by changing building components and/or
mechanical systems to eliminate major pressure differences
Presenter
Presentation Notes
Pressure is the effect of a force applied to a surface. Pressure will be measured in pascals Pressure problems will mostly be caused by forced air heating systems but we can have problems in houses without FA also.
Pressure Principles for Wx’ers
• Airflow is measured in cubic feet per minute, also written as ft3/min, or CFM.
• 1 CFM OUT = 1 CFM IN.• Airflow takes the path of least resistance.• Air moves from high- to low-pressure areas.• Air usually moves from high- to low-temperature areas.
Presenter
Presentation Notes
One cubic foot is about the size of a milk crate
Air Flow
Conditions for Air Movement• Pathways for air
movement (hole)• Pressure difference
(driving force)
High pressure
Low pressure
Airflow Caused by Pressure Difference (P )
Flow is from ______________ to ______________ pressure
For every CFM that _______, one CFM _______
Flow takes the path of _______ resistance
Positive Negative
positive (high) negative (low)
enters exitsleast
Presenter
Presentation Notes
Click ahead to reveal direction of air leakage. Review terms infiltration and exfiltration. Ask students to fill in the blanks before you click ahead. Flow is from positive (high) to negative (low) pressure. For every CFM that enters, one CFM exits. Flow takes the path of least resistance – Air isn’t smart. Air goes where it is easiest to flow, not where you want it to flow. Pressure acts on all sides of the home. Q: What danger is associated with negative pressure in the home? A: Backdraft.
Types of Driving Forces
Wind
Heat: Stack effect, combustion
Mechanical Systems and Fans:HVAC System, exhaust fans, duct leaks, interior doors
Presenter
Presentation Notes
Pressure differences drive air leakage, but what causes pressure differences? The driving forces that induce temperature or pressure differences are: Wind. Heat – Stack effect is an example of heat producing a pressure difference. Combustion produces both temperature and pressure differences. Fans – Exhaust fans, duct leaks, interior doors.
Stack Effect
Wind
MechanicalSystems
Presenter
Presentation Notes
Slide Intent: To explain multiple ways air moves through the house and the positive and negative attributes of each. Talking Points: Use each graphic to further explain the driving forces of air movement and how if affects air pressures within the house. Stack effect can create pressure differences of 1-5 Pascals. Wind can create pressure differences of 10-20 Pascals on the windward side. Mechanical systems can create pressure differences of up to 30 Pascals. Demonstrate with duct house. Picture Captions: Images demonstrate the stack effect, driving forces such as wind, and mechanical systems.
WIND DIRECTION
Wind creates a positive pressure on the windward side of
the building . . .
positive pressure
negative pressure
Which creates a negative pressure on the other sides of the house
Presenter
Presentation Notes
Wind creates a positive pressure on the windward side of the building, which creates a negative pressure on the other sides of the house. The leeward (protected) side of the house has negative pressure that sucks air out of the house. This is why during big storms, people will crack the windows open to equalize pressure and reduce the chances of windows blowing in or out. During blower door tests, the hose taking external readings must be thrown clear of the wind path created by the blower door fan to avoid faulty readings due to the wind effect.
Stack Effect
Warmer air rises and escapes out of the top of the house. . .
Which creates a suction that pulls in outside air at the bottom of the house.
negative pressure
Neutral pressure plane
positive pressure
Presenter
Presentation Notes
Warmer air rises and escapes out of the top of the house, which creates a suction that pulls in outside air at the bottom of the house. Every CFM that escapes one place has to be replaced from somewhere. Click to reveal airflow rising to top and entering through bottom. Q: What is a useful version of stack effect? A: Proper draft of a chimney.
Positive pressure (with reference to outside)
Neutral pressure plane
Photo courtesy of David Keefe, Vermont Energy Investment Corp.
Negative pressure (with reference to outside)
Stack Effect
Presenter
Presentation Notes
Click to reveal the anatomy of the pressure on this building. Air leakage in a building suffering from the stack effect is predictable. Hot air rises, creating a negative pressure in the lower portion of the building. Negative pressure leads to infiltration, or air leaking into the building. The neutral pressure plane is not located halfway up the building, but wherever the force of rising air is neutralized by limited leaks, or points of escape for the rising air. Positive pressure in the top of the building causes exfiltration, or air leakage out of the building.
Combustion Equipment & Exhaust Fans
Exhaust Fan
Negativepressure
Negativepressure
Presenter
Presentation Notes
Click to reveal the anatomy of depressurization in both situations. Combustion appliances and exhaust fans can create negative pressure in the home. This pressure difference increases infiltration. Combustion appliances that take their combustion air from inside the home, e.g., most 80+ furnaces and conventional gas water heaters, can create negative pressure in the home. In a tight home, this could result in unsafe levels of CO. Direct-vented appliances, those that take combustion air directly from the outside, e.g., most 90+ furnaces, don’t cause depressurization. Exhaust fans also cause negative pressure.
Mechanical Systems
Device CFMBath
Range hood
Downdraft hood
“Chef”Hood
Dryer
Air Handler/ ton
50
150
500
1500
200400
Device CFMBath
Range hood
Downdraft hood
“Chef”Hood
Dryer
Air Handler/ ton
Fans
Presenter
Presentation Notes
Duct house – bath fan, no return path, PAV – Example problem – duct leakage affects on house pressure List some other fans and typical flows: Bath 50 cfm Std hood – 100-150 cfm (150) Downdraft – 4-600 cfm (500) Viking – 1000-2000 cfm (like a blower door!) (1500) Dryer – 200 cfm AHU – 400 cfm / ton
Duct leakage can create positive and negative pressures in different areas of the house
The pressures associated with duct leaks can be larger and more important because the driving force is stronger.
All holes are not created equal!
Duct Leakage
Return Supply
Presenter
Presentation Notes
Duct leakage can create positive and negative pressures in different areas of the house. The pressures associated with duct leaks can be larger and more important because the driving force is stronger. All holes are not created equal. In this situation, when the door between the bedroom and main body of the home is open, pressure remains balanced throughout the home. Q: What happens if you close the door to the bedroom in this situation? Click ahead to reveal answer.
16
Closed doors that prevent supply air from getting back to a return cause positivepressures in those rooms with supply vents. . . .
Meanwhile, starving the return for air, causing negative pressure in the zone where the return is located.
16
Duct Leakage
Return Supply
Presenter
Presentation Notes
Closed doors that prevent supply air from getting back to a return vent cause positive pressures in those rooms with supply vents. This situation starves the return for air, causing negative pressure in the zone where the return vent is located.
Leaky Supply Ducts
Presenter
Presentation Notes
Slide Intent: To discuss how leaky supply ducts affect the pressure balance of the house. Talking Points: Discuss how holes in the supply ductwork as shown by the blue arrows, decrease the amount of air that is supplied to the room. Essentially, the supply duct is leaking out the air before it reaches it’s destination, therefore there is not enough air in the supply duct to fill the room. These holes in the supply ductwork, increase the pressure in the basement by putting more air into this space, and decrease the pressure of the supplied rooms because the supply ducts are not replenishing the air at the same rate that the return duct is removing it. Catch phrase for remembering pressure problems with leaky supply’s is: Supply Leaks Suck. They suck extra air out of the living space. Picture Caption: The graphic depicts the pressure imbalance created by a leaky supply duct.
Leaky Return Ducts
Presenter
Presentation Notes
Slide Intent: To discuss how leaky return ducts affect the pressure balance of the house. Talking Points: Discuss how holes in the return ductwork as shown by the blue arrows, increase the amount of air that is supplied to the room. Essentially, the return duct is picking up “extra” air. This extra air is coming from the basement where it picks up dust which is then blown into the house. These holes in the return ductwork, decrease the pressure in the basement by taking air away from this space, and increase the pressure of the supplied rooms because it is filling those rooms with more air than that which is leaving those rooms. Catch phrase for remembering pressure problems with leaky returns is: Return Leaks Blow. They blow extra air into the house. Picture Caption: The graphic depicts the pressure imbalance created by a leaky return duct.
MasterBedroom
Bedroom Bath
Utility Room Kitchen
Living Room
Whole-house return in hallway
Room Pressure Imbalances
Presenter
Presentation Notes
It is easy to predict pressure imbalances based on the locations of return and supply vents throughout a home, even without the use of a blower door. Q: Which rooms of the house will have positive pressure and which will have negative, and will air leak into or out of those rooms? A: Click to reveal answers in this order: Master bedroom: positive, exfiltration. Utility: negative, infiltration. Kitchen: negative, infiltration. Living room: negative, infiltration. Bathroom: positive, exfiltration. Bedroom: positive, exfiltration.� Q: What are some methods for balancing the pressure in this home? A: Here are three: Add returns in bedrooms and bathroom. Undercut doors. Add grates to the doors.
HVAC
R
DHWDryer
Louvered Door
Problems caused by Pressure Imbalances – Case Study
Presenter
Presentation Notes
Slide Intent: To present a case study of combustion safety hazards – true story. Talking Points: This is the first slide in a series – the following slides add detail. Furnace and water heater were gas and atmospherically vented. Homeowners detected strange smell in home and weren’t feeling well, suspected it had something to do with furnace. Called their HVAC contractor, they tested ambient CO and furnace CO. Did not detect elevated levels. Also, furnace flames were blue and looked fine to contractor. Contractor told homeowner there were no problems. About a month later, homeowner called contractor at night with same complaints – were told technicians would be out in the morning. When the technicians arrived the next morning, EMTs were on the scene and entire family were in hospitals with CO poisoning. It is believed that when occupants closed their bedroom doors at night, the furnace came on and a negative pressure developed at the central return and the furnace was backdrafted and CO was sucked through the louvered door, into the return and pumped to the occupants in their bedrooms. Picture Caption: Diagram of house presented in case study.
HVAC
R
DHWDryer
Louvered Door
Presenter
Presentation Notes
Slide Intent: To present a case study of combustion safety hazards – true story. Talking Points: This is the first slide in a series – the following slides add detail. Furnace and water heater were gas and atmospherically vented. Homeowners detected strange smell in home and weren’t feeling well, suspected it had something to do with furnace. Called their HVAC contractor, they tested ambient CO and furnace CO. Did not detect elevated levels. Also, furnace flames were blue and looked fine to contractor. Contractor told homeowner there were no problems. About a month later, homeowner called contractor at night with same complaints – were told technicians would be out in the morning. When the technicians arrived the next morning, EMTs were on the scene and entire family were in hospitals with CO poisoning. It is believed that when occupants closed their bedroom doors at night, the furnace came on and a negative pressure developed at the central return and the furnace was backdrafted and CO was sucked through the louvered door, into the return and pumped to the occupants in their bedrooms. Picture Caption: Diagram of house presented in case study.
R
HVAC blower comes on; with doors closed and exhausts on, pressure imbalances occur
+ +
+ +
-- -
CO
CO CO
COCO
Presenter
Presentation Notes
Slide Intent: CONTINUATION OF PREVIOUS SLIDE Talking Points: When doors are closed, bedrooms are pressurized, hallway depressurized CO created by furnace water heater, etc. negative pressure in hallway draws it into the house Very negative pressure in return vent draws CO into air handler, distributes it to rooms Picture Caption: Diagram of house presented in case study.
Sealed Ductwork
Best approach is to use all sealed combustion appliances (or non-combustion appliances)
RComb. air w/ solid door
Transfer Grilles Minimize Pressure Imbalances
Solid, Weather-stripped Door, Insulated & Air Sealed Walls
Presenter
Presentation Notes
Slide Intent: How to avoid the combustion problems in this home. Talking Points: Best thing to do is isolate or remove combustion appliances. Insulate and air seal mechanical closet. Replace louvered door with solid weatherstripped door. Bring in outside air for combustion. Add transfer grilles on bedrooms to minimize pressure imbalances. Picture Caption: Diagram of house presented in case study.
Testing to determine pressure imbalances
• Dominant Duct Leak Test• All Doors Closed Test• Room Pressure Test / Under Door Pressure Test• Maximum Depressurization Test (Worst Case CAZ
Attractive/Inexpensive option for solving indoor pressure imbalances caused by the HVAC system. At 3 Pascals achieves 250 cfm of air flow. (pressure relief) Durable ABS plastic (paintable) and interior baffle to mitigate noise and light. Comes in White. 24" width X 4" height rough opening, fits most standard size doors.
• Down-size exhaust fan flows• Add ducted supply air to Combustion Appliance Zone• Shift supply air to different areas by adjusting HVAC
system• Interlock supply air fan• Add supply air by bringing in outside air via dampered duct
into return plenum
Pressure Balancing Process
• Perform pre-test (test-in): This will give indications of what types of pressure balancing will be needed
• Complete all air sealing and duct sealing work prior to pressure balancing
• Perform interim testing• Install pressure balancing measures based on testing• Perform final test-out to verify success
Summary
• Pressure imbalances can cause many problems in a home• Pressure imbalances are caused by many factors• Various diagnostic tests may used to identify and
understand pressure problems• Several different methods are available for balancing
