HVAC Systems, Combustion Safety, and Ventilation
HVAC Systems, Combustion Safety, and Ventilation
This domain tests equipment efficiency, airflow, combustion appliance testing, and whole-house ventilation. Expect questions on AFUE, SEER2, HSPF2, static pressure, worst-case depressurization, CO action levels, and ASHRAE 62.2.
Core rules
- AFUE rates furnaces and boilers; SEER2 rates cooling; HSPF2 rates heat pumps. Higher is better, but installed performance depends on duct design, charge, and airflow.
- Typical target airflow is about 350–450 CFM per ton of cooling. High static pressure indicates restrictive ducts or filters.
- Combustion safety testing includes the combustion appliance zone (CAZ), worst-case depressurization, draft measurement, spillage observation, and CO measurement in ambient air and flue gas.
- Many programs use action levels of 35 ppm ambient CO and 100 ppm air-free flue CO; know your program's thresholds and evacuation procedures.
- ASHRAE 62.2 provides whole-house ventilation rates. For a 2,000 ft², 3-bedroom home, Q = 0.01 × 2,000 + 7.5 × (3 + 1) = 50 CFM.
- Ventilation options include exhaust-only, supply-only, balanced, HRV, and ERV. HRVs/ERVs recover heat; ERVs also transfer moisture.
Worked example
A 2,000 ft² home with 3 bedrooms needs 50 CFM continuous per the 62.2 formula. A 50 CFM bath fan run continuously satisfies the rate but may cause depressurization; a balanced ERV avoids starling the CAZ and is preferable where combustion appliances are present.
Traps
- Testing draft before worst-case depressurization.
- Testing only one appliance when multiple share the CAZ.
- Confusing flue CO with ambient CO.
- Confusing ventilation CFM with infiltration CFM.
- Ignoring duct leakage and return-path restrictions.