Retrotec DucTester 341 - Manual (Page 61)

Page 61 of 83
©Retrotec Inc. 2014
Return Loss Factor = (RLS x RLP x RPM)
Where: RLS is Return Leakage Split (default 0.5)
RLP is Return Leakage Penalty (default 0.5)
RPM is Return Pressure Multiplier (default 1.0)
Return Loss Factor = 0.5 * 0.5 * 1.0
= 0.25
Definitions:
Leakage Split is the estimated fraction of total measured leakage, either on the supply side or the return
side of the duct system
Supply Leakage Penalty: The default value is 1.0. This means that of the supply air leakage, all of its
energy leads to the outdoors (i.e., none of its energy is regained back into the house). This number can
be lowered if some of the energy from duct leakage leads back into the house.
Return Leakage Penalty: The default value is 0.5. This assumes that of the return air leakage, about half
of its energy is lost to outdoors. (i.e., the other half of its energy is regained back into the house). This
number can he raised if you suspect that there's actually more energy loss to the outdoors from the
return side.
Estimate HVAC system airflow
Estimate the total airflow from the system documentation, measured through static pressure/fan curve,
or by measuring system airflow using flow measuring techniques.
Calculate HVAC system loss (%)
% HVAC System Loss =
CFM25 Leakage to outside × (Supply Loss Factor+Return Loss Factor)
Estimated System Airflow in CFM
Example 1:
We conduct a Duct Leakage to Outdoors Test on a 3 ton, 11 SEER heat pump system (supplies
located in the attic and returns in the crawlspace). The owners report a $1300/yr cooling bill
and a $700/yr heating bill. Using 25 Pa as our duct testing pressure, we measure 270 CFM
25
of
duct leakage to the outside. We measure a total system airflow of 1425 CFM using a total flow
measurement method. We will use the default values for Supply/Return Loss Factors.
% HVAC System Loss =
270 CFM × (0.5+0.25)
1425 CFM
% HVAC System Loss = 14%
Annual cooling loss = 14% x $1300 = $182
Annual heating loss = 14% x $ 500 = $ 70
Annual capacity loss = 14% x 3 ton = 0.42 tons
Annual operating SEER = (1 – 14%) x 11 SEER = 9.5 SEER
Example 2:
For the same house measured above, the correct leakage split is determined with the majority
of leakage on the return side at 200 CFM. Assume the average operating pressure for the return
side is closer to 35 Pa because it is close to the plenum takeoff.