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BPI Written Exam - Section 1 Building Science Fundamentals
11. Understand Total Equivalent Length
Total equivalent length (TEL) is used in HVAC duct length measurements to account for bends, fittings (wyes and mixing boxes) and friction loss to find the installed sizes of ductwork. The reason is that if a HVAC system delivers 50,000 CFM of air output, the fact is that 50,000 CFM of air is not the total amount actually delivered to each room. The reason is because the air travels down the ductwork and some of it gets lost due to friction, bends and fittings. There are engineering manuals for mechanical engineers that have the shapes and sizes of ductwork and the associated friction loss with each one. For small commercial and residential purposes, Wrightsoft publishes duct sizing software that automatically accounts for these losses but we need to be aware they exist. Wrightsoft software is not always required though as a ductulator can be a suitable substitute for ductwork sizing. Sometimes utility companies require a Manual D (duct sizing) calculation be done, but if you are working with existing homes with no utility program requirements, go with a ductulator. If you want to see the science and formulas behind the losses, check out this manual for the HVAC industry.
The TEL of a 90 degree square to rectangular elbow for example is the equivalent length of 35 ft of straight duct. Rigid metal ductwork has about half the TEL of flex duct, so every 2 feet of straight flex duct is the equivalent of 1 foot of rigid ductwork. As a best practice, it is better to use two 45 degree bends or better yet a curve, than it is to use one 90 degree bend. The longer the TEL, the more the ductwork resists airflow and the bigger the blower has to be to deliver enough air to the rooms. TEL guidelines can be found in the ACCA manual.
The same restriction guidelines go for the PVC pipes in the suction and discharge sides of a pool pump. Long straight runs are much better and require the pump to work less, using less energy, than bends and elbows.
1c. Combustion science
The main topics of the BPI BA exam are listed out below. Click on a link that interests you, or you need some brushing up on to learn more on each subject.
1. Building Science Fundamentals
1a. Basic terms and definitions
1b. Principals of energy, air & moisture
1c. Combustion science
2. Buildings and Their Systems
2a. Building components
2b. Conservation strategies
2c. Comprehensive building assessment process
2d. Design considerations
3. Measurement and Verification of Building Performance
3a. Applied diagnostics and troubleshooting
4. BPI National Standards and Project Specifications
4a. Comprehensive building assessment
5. Analyzing Buildings Systems
5a. Comprehensive building assessment
5b. Appliances and lighting
6. Conduct and communications
6a. Conservation strategies