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HERS Rater Field Exam - Pa, CFM, CFM50, CAZ, Draft, Room Pressure

Term
Ambient CO

Baseline
 
Dominant duct leakage

Room pressure

Worst case




CAZ Readings





Draft






Spillage


MVR / BAS





CFM

CFM50


ACH50

Pressure pan

Zonal pressure
Unit
ppm - parts per million
Pa - pressure
 
Pa - pressure


Pa - pressure

Pa - pressure




Pa - pressure





Pa - pressure






No units, pass or fail


Cubic feet per minute - flow rate




Cubic feet per minute - flow rate
Cubic feet per minute - flow rate

No units

Pa - pressure

Pa - pressure
What it tells us
If the homeowner and ourselves are at risk during our tests
Normalizes wind and outside conditions
If there is more leakage on the supply (positive) or return (negative)

If a room is getting too pressurized and the house depressurized
Sets the home to try to make it fail while we are there to alert the homeowner. Done to: 1) get a CAZ reading and 2) perform combustion safety tests
Done under worst case conditions, in the same room as a combustion appliance (furnace, water heater, oven, dryer, central vacuum,  fireplace). CAZ reading taken WRT the outside. 
Tells us if the draft of the flue stack is strong enough to remove flue gases. Done under worst case conditions first, 1-2 feet above diverter, within 60 sec of start-up.


Test if backdrafting of combustion gases occurs.

If a home is too tight according to BPI Standards based on a 0.35 ACHN exchange rate.



The flow rate of air leakage from a home while ramping up to 50 Pa
The flow rate at 50 Pa.  This is our blower door whole house leakage number.
# of times air house air is replaced with outside air
How much duct leakage is from a duct coming out of each register.
Which zones are connected to each other, if a zone is outside or inside.
Threshold
35 ppm or lower

None

Positive pressure = more return leakage
​ 
 -3 to +3 Pa is good


More negative is better for our test, but bad for the homeowner.


See BPI Standards for limits. The more positive, the better.  



See BPI Standards for limits.  The more negative, the better.  Done under worst case conditions first, 1-2 feet above diverter, within 60 sec of start-up.
Pass or fail.  Done under worst case conditions first, within 60 sec of start-up.
If BAS > final CFM50 > (0.7 x BAS) Must recommend mech. vent.
If (0.7 x BAS) > final CFM50 Must install mech. vent.
None

See BPI Standards for BAS

None

Less than 1 Pa, the lower the less duct leakage
0 - 10 Pa Inside
11 - 39 Pa Confused
40 - 50 Pa Outside
Self check - see if you can answer these questions from memory:

  1. How are room pressures are different from worst case?
  2. How is CAZ is different from worst case?
  3. What is the formula for ACH50?
  4. What is the formula for CFM50? (Answer - there is none)
  5. What are the steps to calculate the BAS for a home?
  6. What are the steps to put a home under worst case conditions?

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