Retrotec Residential Pressure & Air Leakage - Manual (Page 29)

Page 29 of 75
Pressure relief may be needed in any particular room that is pressurized or depressurized by 3 Pa or more,
compared to the main body of the house. If combustion appliances are in a depressurized area, even a minor
depressurization can interfere with their ability to draft properly.
4.2 Test how much Ducts Leak to Outdoors
A Blower Door can be used to measure “Duct Leakage to Outside” which we prefer to call “Duct Leakage to
Outdoors” because “Outside” could be taken to means outside a particular zone which may still be inside the
building but the intention here is to measure leakage to the great outdoors.
4.2.1. Measure Duct Leakage to Outdoors by Subtraction for tight houses
Very few State Codes allow this method: Georgia allows it. The entire house enclosure must be finished and
tight.
Inaccuracy of this method
This method works reasonably well if the house is 5 ACH
50
or less and the ducts are fairly leaky because this
method relies on subtracting total house leakage from total house leakage with ducts sealed. Since the house
leakage is often 1000 to 5000 CFM but the duct leakage to outdoors must be less than 100 CFM, repeatability is
extremely important. Repeatability is typically 1 to 3% depending on the wind and how long readings are taken
for. This 1 to 3% translates into 10 to 150 CFM meaning that measuring 100 CFM could be in error by 10 to
150%. Still, if you have a blower door and no DucTester, this method could be useful although adding the
DucTester and using the Blower Door gauge would make it inexpensive to add the DucTester.
“If you want to measure how much a cup a coffee weighs. The method I use is - I step on my bathroom
scale and weigh myself and then re-do with the cup of coffee. I subtract the 2 measurements to
determine the weight of the coffee. How accurate is this method?”
Courtesy of Joe Medosch of the Moultrie Technical College
Modified Blower Door Subtraction Test using a DM-2
1. Setup for a depressurization basic air leakage test.
2. With the air handler off, open all registers, and remove all filters from the HVAC.
3. Seal all exterior combustion air intakes and vents, connected to the ducts.
4. Press
[Set Pressure] [50] [Enter] to depressurize the building to -50 Pa (or manually adjust the fan
speed to reach -50 Pa). Press [@ Pressure] until “@50” is displayed or “@25” if results at 25 are
required to improve accuracy. Note: the value the [@ Pressure] key displays can be changed in
using the Setup key.
5. The measured flow is the Whole House Leakage at 50 Pa or at 25 Pa if “@25” was used.
6. Press
[Exit] to turn the Blower Door off.
7. Seal all supply and return registers with Grill Mask.
8. Depressurize the house to -50 Pa again. The measured flow is the Envelope Leakage at 50 Pa.
9. Measure the pressure in the duct system, either at the return or supply plenum, or behind the Grill
Mask at a supply or return register. This is the Duct Pressure.
10. Use the measured Duct Pressure and determine the correction factor from Table 6.
11. Calculate the “Duct Leakage to Outdoors” by subtracting the Envelope Leakage (step 8) from the
Whole House leakage (step 5), and then multiplying that result by the correction factor.
(Whole House CFM50 − Envelope CFM50) × (Correction factor ) = Duct leakage to outdoors at 50 Pa