Furnace Troubleshooting and DIY Repair

How to Inspect Your Furnace

You can repair your furnace as a homeowner. This guide will help you identify furnace problems by sight, sound, and touch.

There is nothing worse than a furnace breaking down during a cold Massachusetts winter in Boston. This guide will help you identify furnace issues that require professional attention.

Your furnace does its job year after year. The furnace turns on when the thermostat signals it. It heats up and blows warm air throughout your home. It does this every day. It may run 24 hours a day in the winter. It may also be running continuously in cold areas, from late September through early April. This is a lot of asking for a machine. When was the last time you looked at it and had your heater furnace inspected?

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What you will see

The floor and walls around the furnace will be clean. The furnace’s dust will not be considered typical basement or furnace debris, such as mouse droppings, dust, and cobwebs. It will not be sooty or a by-product of combustion. The flue should be straight and unaltered, without any gaps or holes. If you look at the pilot lamp and flame (for gas furnaces), there will be a steady blue flame. The fan belt in the motor area will be firm and flexible (not stiff and cracked).

What you will touch

To determine temperature drop-off, you can feel the warm air duct sideways if you have access. Your return air duct side will always feel cool to your touch. Flip the kill switch to turn off the circuit breaker or switch off the furnace’s electricity. The furnace fan belt should feel firm when you reach into the motor and press down on it. It should depress approximately 1/4 inch.

What you will smell

In a perfect world, you won’t smell anything. It is not uncommon to smell singed powder, particularly during the first heating cycle in the cold season or after a long vacation. The heating element burns off any dust that has collected, creating a pungent odor. This usually dissipates in less than 20 min.

A furnace can also smell musty. Moldy filters can cause a house to smell musty. This happens when the air passes through them.

What you will hear

If a furnace is working correctly, there will be lots of sounds. If your furnace makes sounds, it means that it is operating correctly. However, these must be the right sounds. You will most often hear the low, continuous howl of the draft inducer. To clear out the combustion gases from the previous heating cycle, they will be heard before the burners are fired for about one minute.

Popping and banging can be attributed to the furnace housing’s metal or metal ductwork cooling off and contracting. This is similar to the car’s hood after it has been turned off. Although metallic rattling can be heard from some areas, it is not common in the furnace. Heating registers are known to rattle when air is forced through them.

Identify by sight

Visual inspection can diagnose many furnace problems. Before touching the furnace, make sure to:

• Choose the correct flame color flame should be pure, uncolored, and not streaked with yellow/orange. Any other color than blue is considered partial combustion.
• Properly-Drawing Flute flue is a metal or PVC chimney that transports potentially toxic, partially-combusted gases from the furnace to the outside. The furnace flue must be clean, undamaged, and free from vermin activity.
• Regular Debris, not Soot: Do not place ashes or gravely-cinder-type soot near the furnace’s combustion area. Any debris found near furnace areas should be of the same type as usual, like cobwebs or mouse droppings.
• Relatively clean filter: Furnace filters can get dirty and collect Debris. If your furnace filter has been changed recently, it may become too clogged.

Identify by Touch

Ducts After the system has been running for 10 minutes; you can touch the return and warm air ducts if you have both accesses. The return air should be cool. The warm air duct, however, should be warm near the furnace.

Register Identify the location of your furnace’s heating registers. Temperatures will be asymmetric between registers closer to the stove and those further away. This is a concern. You can combat this by adding insulation around warm air ducts, innovative vents that open or close according to schedule, adding a second furnace, and blocking rooms that are not used often.

Motor Fan Belt Turn off the electricity and let the fan motor pully belt cool down. It should not depress more than 1/4 inch. Repair assistance is available if it presses more than 1/2 inch.

Smell to Identify

Cool air is drawn in by furnaces from your central intake. It is then pulled through return-air conduits to heat it. The warm-air tube and ducts then transport it back into your house. Normal odors come from all this air exchange.

Rotten Eggs

Natural gas is odorless, so utility companies add mercaptan to homeowners to tell if there is gas leakage. Mercaptan smells like rotten eggs. This is a sign that you should leave your home immediately and contact the gas company to get assistance.

Metallic; Burning Plastic, or Rubber

The fan belt or motor may be faulty and cause a chemical-type, metallic, or burning rubber or plastic smell around the furnace. It could also indicate that an electrical component has to arc. These are serious concerns that should be addressed immediately by a furnace technician.

Foul, Pungent

If the smell is strong, it could be indicative of rotting meat. The smell won’t go away for very long, so it is essential to remove the animal and sterilize the area.


If your pet’s urine smells strong when the furnace turns on, it could be a sign that they have urinated near or on the intake vent.

A dusty, lingering scent

A persistent smell of burning dust shouldn’t last beyond the first or second heating cycle. This is why a constant dusty smell can be a concern. A dusty smell could be caused by mold or fungus growth.


Although not common, sewer smells when the furnace is on can be annoying and hard to identify. Water that has become stagnant smells musty and should be treated with organic material, such as kitchen waste or toilet sewage.

Identify by Sound

Furnaces can make a lot of noise, even the most efficient models with AFUE 97%. What level of noise should you tolerate?

Rushing/whooshing Furnaces are designed to blow air so expect a sound like rushing air. If the noise is loud, it could be due to the heating registers or the furnace unit. The rushing sound is caused by the fact that some heating registers can be closed partially or entirely. Turn on the heating system by opening all heating registers. If the noise is less frequent, you will have found the source.

A high-pitched whistling sound usually indicates whistling Airflow restriction. Air cannot freely flow from the vents, so it must find other escape routes, such as through the gaps in the conduits. Also, a clogged filter can produce a whistling sound close to the furnace.

Scraping sound is aware of any sound that sounds like metal scraping metal coming from your furnace. The furnace should be turned off immediately. Furnaces are mainly used as a significant component of a central heating system. . It could be that the blower shaft has become loose or that the blower wheel is worn out from years of usage. The motor mount may have become more flexible, allowing it to continue turning while striking the furnace housing’s sides.

Booming or backfiring: This is often what is known as delayed ignition. Your furnace can build up gas for a brief time before the flame ignites. This can be dangerous and alarming.


1. Visual Inspection Guide – https://www.trane.com/residential/en/for-owners/maintenance-tips/gas-furnaces.html
2. Install Furnace Flue – https://www.doityourself.com/stry/how-to-install-a-flue-pipe-in-a-furnace
3. Furnace Smells – http://www.roth-heat.com/blog/2014/11/25/furnace-4/
4. Gas Odorization – https://naturalgasodorization.com/gas-odorization-history
5. Sewer Gas Smell – https://www.askthebuilder.com/sewer-gas-smell
6. Common Furnace Sounds – https://glendaleheating.com/common-hvac-sounds-and-what-they-mean
7. https://en.wikipedia.org/wiki/Furnace

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