â–¶How do I diagnose a gas furnace that won't start or shut down early?
No start: Check thermostat (is it calling for heat?), then power (is the unit receiving 24V control power?), then gas supply (is gas flowing?), then ignition (electronic ignitor or pilot light). A multimeter can verify voltage; a combustion analyzer can confirm gas and flame. Early shutdown (short cycling) is usually a limit switch or thermostat issue—the furnace shuts off prematurely when the heat exchanger temperature reaches a limit. Check that the limit switch is clean (dust blocks heat sensing) and operating at the correct temperature. Also check airflow (is a filter clogged?)—restricted airflow causes rapid heat buildup and premature shutdown.
â–¶What is combustion efficiency and how do I test it?
Combustion efficiency is how completely the fuel (gas or oil) burns, converting to usable heat. Perfect combustion produces CO2, H2O, and N2 with no leftover oxygen or unburned fuel. In reality, furnaces run at 75-90% efficiency due to flue losses and imperfect combustion. A combustion analyzer measures flue gas (CO2, O2, CO, NOx) and calculates efficiency. High CO (carbon monoxide, >100 ppm) indicates incomplete combustion (bad for health, the furnace is wasting fuel). High O2 means excess air. The analyzer guides adjustment: if O2 is high, restrict air intake (close the air damper); if O2 is low, increase air. Modern furnaces have 90%+ efficiency; older units may be 60-70%.
â–¶What is the difference between a forced-air furnace and a hydronic boiler?
A forced-air furnace heats air in a heat exchanger and blows it through ducts to heat the building. It is fast (room temperature rises quickly) but noisy and prone to uneven heating (some rooms hot, others cold). A hydronic boiler heats water, which is pumped through pipes to radiators or baseboard convectors (or radiant floor loops). It is quiet, even, and efficient but slower to respond and more complex. A boiler may also provide hot water for showers; a furnace does not. Boilers are popular in older eastern U.S. buildings; furnaces are standard in new construction and western U.S.
â–¶What is a limit switch and why does it shut down the furnace?
A limit switch is a temperature sensor that shuts off the furnace when the heat exchanger exceeds a safe temperature (usually 140-150°F for air, to prevent the furnace from overheating and damaging itself or the ductwork). If the switch is stuck on, the furnace will run continuously. If the sensor is dirty (dust blocking the bimetallic strip that senses temperature), it may read falsely and shut off prematurely. Clean a limit switch by gently brushing the sensor; never scrape or bend. Replace a faulty limit switch (costs $20-100). Frequent limit switch trips usually indicate poor airflow—a clogged filter is the most common cause.
â–¶How do I clean a furnace and change the filter properly?
Annual furnace maintenance: (1) Turn off power and gas. (2) Remove and inspect the filter (1-inch or 4-5 inch thick, depending on furnace); if it looks dirty, replace it (cost $10-50 depending on size and quality). (3) Vacuum the blower wheel and housing (dust reduces airflow and efficiency). (4) Clean the heat exchanger (a vacuum or soft brush works; never scrape metal parts). (5) Inspect the burner (look for corrosion, yellow flame indicating incomplete combustion, strange noises). (6) Check the draft (a draft gauge measures negative pressure in the flue; adequate draft means gases are being exhausted safely). (7) Test the thermostat (call for heat, listen for ignition, feel for warm air). (8) Turn power and gas back on.
â–¶What is a thermocouple and why does a pilot light go out?
A thermocouple is a temperature sensor that generates a small voltage (millivolts) when heated by the pilot light. This voltage keeps a gas valve open; if the pilot blows out, the thermocouple cools, the voltage drops to zero, and the gas valve closes (a safety feature preventing gas leakage). A dirty or misaligned pilot light won't heat the thermocouple sufficiently, causing it to drop out and the gas to shut off. Solutions: clean the pilot (blow out dust), adjust its position so the flame fully heats the thermocouple, or replace the thermocouple ($20-50). Modern furnaces have electronic ignition (no pilot light, no thermocouple), which is more reliable.
â–¶How often should a furnace be serviced, and what does a full service cost?
Furnaces should be serviced annually, typically in fall before heating season. A full tune-up includes filter replacement, burner inspection and cleaning, heat exchanger inspection, limit switch check, thermostat test, electrical safety checks, combustion efficiency test, and documentation. Cost ranges from $150-300 depending on equipment complexity and region. A service call for emergency repair (no heat in winter) costs more ($300-500+, often with emergency surcharge). Regular maintenance extends furnace life (15-20 years vs. 10-12 without), improves efficiency (saving 5-10% on heating bills), and prevents dangerous situations (gas leaks, carbon monoxide production).
