Heating Systems and Equipment
This domain covers furnaces, boilers, and the controls that keep combustion safe. Know the difference between induced-draft, natural-draft, and condensing appliances, and why condensing units achieve higher efficiency by recovering latent heat from flue gas (flue temperatures low enough to condense water vapor). Understand AFUE as the annual efficiency metric for furnaces and boilers, and that steady-state combustion efficiency differs from AFUE.
Combustion fundamentals are heavily tested: the three legs of the fire triangle, stoichiometric air (roughly 10 cubic feet of air per cubic foot of natural gas for complete combustion), excess air, and the products of complete versus incomplete combustion. Carbon monoxide is the signature of incomplete combustion - insufficient air, poor draft, blocked flue, or flame impingement. Know draft, spillage, and why a blocked heat exchanger is a red-tag condition.
Boiler basics: operating pressure, expansion tanks, air elimination, low-water cutoffs, relief valves, and the difference between steam and hot-water systems. Hydronic concepts like primary/secondary piping and circulator placement show up.
Traps: equating high efficiency with high flue temperature (it is the opposite); forgetting that a dirty filter can cause CO by starving combustion air; and confusing AFUE with SEER. Alaska-specific traps involve freeze protection, glycol systems, and combustion air in tightly built homes.
Worked example: a furnace rated 80,000 Btu/h input at 80% AFUE delivers about 64,000 Btu/h output. If the home needs 60,000 Btu/h, that unit is adequate - but always check that the model is approved for the altitude and venting configuration.