How to tell if you have one
Look at where the furnace vents. A high-efficiency, condensing furnace usually has two plastic pipes, white or gray, running out through a side wall or the roof: one brings air in, one takes exhaust out. An older furnace vents hot exhaust up a metal flue. The rating label inside the furnace door will also give its efficiency.
Fault one: the water has to leave
A condensing furnace pulls so much heat out of its exhaust that water condenses, and that water drains away through a trap, a hose and often a small condensate pump. Slime grows in the damp line and blocks it. When the water backs up, a float or pressure switch stops the furnace.
What you see is no heat, and sometimes a puddle near the furnace that looks unrelated. They are very often the same fault. Mop it up, note whether the water is clear, and say so when you call.
Fault two: the vent pipes end at the wall
The intake and exhaust pipes usually end low on an outside wall, where snow drifts, leaves, mulch, grass clippings and insects can block them. A blocked pipe holds the pressure switch open and the furnace will not fire. In a hard freeze, frost can also build at the tip of the exhaust.
Check from outside: are both pipe ends clear? Clearing snow or debris away from the terminations is safe. Do not push anything into the pipes.
Fault three: the pressure switches
A condensing furnace watches its venting and its drain through pressure switches and their small hoses. A cracked hose, water in a hose, or a partly blocked vent will hold a switch open. Before anyone quotes a control board for a furnace that will not fire, it is fair to ask whether the switches, the hoses, the drain and the vent ends were checked first.
Venting rules in Idaho
Idaho's amendments to the fuel gas code require all plastic pipe within a dwelling used for venting flue gases to be tested at five psi for fifteen minutes on an inspected installation. If a vent run is replaced or extended, that test is part of doing it to code.
What else is the same
The igniter, the flame sensor, the blower and the heat exchanger work as they do on any gas furnace. See igniter and flame sensor and furnace running but not heating.
No heat? Tell us what the furnace is doing.
Gas, electric and high-efficiency furnaces across Caldwell, Nampa, Middleton and Parma. Start with the symptom and your ZIP code.
You'll be asked for your ZIP code first, so your call reaches a technician who covers your area.