Basic information for inspecting a forced air furnance:

  1. Make sure you turn the furnace off using the thermostat o rthe safety switch which is on the outside of the furnace.
  2. Once the furnace has been turned off, check the burners for signs of scorching, rust, dirt, drywall dust and corrosion.
  3. Turn the unit back on, remember to stand back a minimum of 5′ and off to the side of the furnace to watch for any problems in the igniting process, such as long lasting ticking sound, roaring noise smoking, flame rollout or any type of vibration.
  4. The heat exchanger of your furnace is by far the most critical area of the furnace.  It is located above the combustion chambers and the burners.  If this area either has heavy rust or cracks it could cause serious health and safety issues (even death) in your home.
  5. Remove the lower access panel to view the blower, you will need to hold the safety switch in to run the blower.  Listen for any rattling or squeaking sound coming from the blower.  Release the safety switch and inspect the blower to see how dirty it may or may not be.  If very dirty we recommend to have it cleaned.
  6. Inspect the flu pipe.  The flu pipe should be checked very carefully for small or large rust holes.  If you see a small rust area you should lightly tap it with a screw driver and see if it is rusted all the way thru.  If the furnace is not a high efficency unit, check to make sure the flu has a positive air flow leading to the chimney liner.  The chimney liner in the chimney is normally made of clay or metal.
  7. gas_furnace_2

    Cold air returns should be connected to side of the furnace and be found in most every room of the home.

  8. Heat runs should be found in almost every room of the home, however it is common in some older homes that no cold air returns were installed in small bathrooms.  Most homes with a basement which were finished off by a non-professional will not have proper heat runs or cold air retuns installed.  This is very common and will cause improper heating in the basement.
  9. Furnace filters are generally located between the furnace and the return air (the air flow would be from the return air to the furnace).  Dirty furnace filters will cause the furnace to run less efficiently, causing an increase in energy usage and can be damaging to the furnace.  Filters should be cleaned depending on the home once every other month

With the exception of a dirty furnace filter (which may have just been neglected by the homeowner), if you find any issue from the above inspection list, we recommend calling a licensed heating contractor.


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We never recommend having a furnace installed sitting directly in the dirt of a crawl space, we always recommend to have it lifted off the ground.

Rusted furnace

Rust from moisture leak in the exhaust. Very common. If you find a leak, call a licensed professional.

heating 1 photo 2

Filthy furnace covered in drywall dust. May cause problems with operation of the furnace, this furnace should be cleaned.


This is a old gravity air furnace, some of these furnace just don’t want to go bad.  The problem with these units they are very inefficient and they are very costly to remove when having a new furnace installed because of all the asbestos found on them. (Possible FHA Concern)

forcer air furnace exhause

If you ever see a forced air furnace with a blower vent exhaust on it and it is not a high efficient unit than call a professional heating and cooling contractor to inspect for safety.


Always pull the bottom panel of the furnace cover off, when pulling this one off we found the electrical mother board was seriously rusted due to the basement flooding,  This is 2 year old home that was a foreclosure. Foreclosure can be great but they should be check very closely.



This is a photo of asbestos insulation that has been used to wrap over water lines fora boiler heating system.


This asbestos is in very poor condition, it is what you call friable asbestos, this is a SAFETY HAZARD, we recommend to have all this asbestos removed by a professional.


This asbestos is in very poor condition, itis what you call friable asbestos, this is aSAFETY HAZARD, we recommend to haveall this asbestos removed by a professional.




This furnace exhaust has holes in it and is not safe. (Safety Hazard)

Heating 1 photo 6

Exhaust line has hole in it allowing carbon monoxide back into the home.(Safety hazard) (Interior)


Hole in exhaust line. (safety hazard)(Interior)


This is the furnace exhaust and has been properly installed.

furnace exhaust

The furnace exhaust is missing an elbow as the photo on the left shows.


This furnace exhaust has not been properly installed, as this furnace is exhausting it is sucking the exhaust back into the home through the fresh air intake. (Safety Hazard)


More Furnace Issues


Forced air furnace with a properly installed cold air return.


This cold air return has no cover, we recommend to have a cover installed.


When looking at the cold air returns or heat duct and you see some small round black plugs, these are installed because the ducts have been cleaned at some point and time.


This cold air return is from a home that is only10 years old and is very dirty, this in a sure sign it is time to clean your air ducts.  Dirty cold air returns will restrict air flow back to the furnace and in return keep the furnace working harder than it was designed to. With the furnace working harder it will cost you more money with operating the furnace, and put more stress on the furnace.


This heat duct found in a home has white rust spots on the it, this is caused from high moisture levels generally found in abasement.  We recommend that the majority of homeowners that have a basement usea dehumidifier to keep the humidity levels down. This goes for newer and older homes.

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Properly installed Electrical wire feeding this furnace.


Properly installed electrical wire  feeding the furnace.  This wire was just run off to the side of the furnace.

Furnace electrical problem

Electrical wire should be contained by a metal conduit. This is a safety hazard



This furnace gas line has been properly installed the next photo to the rightindicated improper installation.

Gas line issue

Gas line is not properly installed. See to the left for proper installation


Gas line is not properly installed. See above for proper installation.


Furnace Filters


This is a photo of a electronic air cleaner when they are operating they work great.As a Home Inspector we find more of these not operating than we do operating, it is very important to always push the test button on the cleaner when the furnace is operating and listen for a snapping, this will indicate that the unit should be operating.  It is also important to keep the inside of these units clean, if it is not clean we recommend to have a heating contractor come out and clean it.


This photo is of a furnace air filter, there is several different types of filters on the market today we don’t recommend one from another,we only recommend that you keep them clean.  Approx. 25% off all furnaces we see doing Home Inspection have dirty furnace filters, this cuts down on the efficiency of your furnace, it makes your furnace work much harder than designed to and at the worst it will shut your furnace off and you will end up calling a heating contractor.


Even if you have one of these larger filters that they say it should only be changed once a year if you have dirty air ducts you may still need to change it every 6 months.


Humidifiers and Thermostats


Flow thru Humidifier.  This type of humidifier is most commonly found in homes today


We recommend that all homes have a programmable thermostat installed for energy savings.

humidifier thermostat

Humidifier thermostat normally located some were on the furnace or next to the furnace thermostat on the main floor of your home.  We recommend that you set the seat a moderate to low setting

humidifier filter

Flow thru Humidifier filter dirty.  The filters should be inspected every month during the winter months and cleaned if they are dirty

humidifier drum unit

Drum unit Humidifier.  When finding a furnace that has a drum unit humidifier we recommend that you a extra cautious on keeping the unit cleaned




Heating duct problem

Heating duct has evidence of asbestos. We recommend having all asbestos removed by a professional.

condensate pump

Condensate pump, used to remove the condensation from the furnace and AC unit.



Air Conditioning Unit Inspection

  1. We recommend that the AC is turned on if exterior temperatures are over 65 degrees.  If temperatures are not over 65 degrees, you risk damaging the unit.  If you live in a cold climate area that AC cannot be tested in the winter.
  2. We recommend running the AC to make sure it is cooling down the home properly (external temperature permitting).
  3. Look at the line set and make sure you see no signs of freezing.
  4. Make sure the compressor is running smooth and not vibrating, make sure it is clean and it is clear of all vegetation / plants and also the unit is sitting level.

AC Unit

We recommend to always have AC level.


All Air Conditioning systems should have a main electrical shut off on the exterior of the home near the compressor.

Frozen Line Set

Frozen line set inhibits coolant flow and causes a drop in efficiency


The AC fins are damaged, this generally happened when a home owner clean the unit.

AC unit with vegetation too close

Vegetation must be kept clear of AC unit


Example of a dirty AC unit it is very important to keep these clean, caution when cleaning you don’t want to damage the fins we recommend to call a heating contractor to properly clean.


We recommend that the AC line be wrapped in a foam insulation.