Heating Systems: Gas, Oil, Electric and Hydronic
Gas
Gas furnace questions usually test combustion basics and manifold pressure. Natural gas manifold pressure is typically 3.5 in. w.c.; LP is usually 10-11 in. w.c. after the appliance regulator, and LP conversion requires the correct springs and orifices. Input is derated above sea level, roughly 4 percent per 1,000 ft for natural gas, by adjusting manifold pressure or changing orifices. Category I appliances are naturally drafted and use metal vent. Category IV condensing appliances vent in PVC and must slope back to a condensate trap. Flame proving is done by thermocouple, thermopile, or flame rectification measured in microamps.
Oil
Oil burners atomize fuel at about 100 psig pump pressure through a nozzle rated in gallons per hour. Electrodes are set to the manufacturer's gap, commonly around 5/32 in., and the transformer supplies roughly 10,000 volts. Check draft and smoke number before calling the burner good.
Electric
One kW equals 3,413 BTUH. Sequencers stage the elements, limits protect against overtemperature, and airflow must be verified before elements energize.
Hydronic
Boilers use aquastats, low-water cutoffs, circulators, expansion tanks, and air elimination. A diaphragm expansion tank is charged to the system fill pressure. Design temperature drop is commonly 20 degrees.
Traps
Confusing supply pressure with manifold pressure, venting a 90 percent furnace with single-wall metal, omitting combustion air, skipping altitude derating, and assuming a thermocouple is the only kind of flame sensor.
Worked example
A 10 kW electric furnace delivers 10 x 3,413 = 34,130 BTUH, roughly the output of a 34 MBH strip set.