Furnance Basics

(Almost) Everything you always wanted about Furnaces
- but were afraid to ask

Safety First

Always ensure that each floor of your home (inclusing the basement level) is equipped with both Smoke and Carbon Monoxide detectors - it may save your life!

Have you ever wondered what that large metal noisemaker in your basement is? You've probably walked by it a thousand times, but you may not really know what it actually is or does. Well, it's your furnace for goodness sake. Without it, you'd be rather cold in the wintertime. Furnaces heat most new homes in addition to many older homes. That's why it's a good idea for the homeowner to know the major ins & outs about furnace systems, at least the basics. So if you're the type of person who only knows the furnace by its occasional rumbling noises, stay tuned.

How does a furnace heat my home?
  • In the simplest illustration, a furnace creates heat and then circulates this energy to the other parts of the house. How it creates this heat depends on the make of the furnace. Different furnaces burn different materials - some burn gas, others oil or coal. Newer furnaces create heat with an electrical resistance element.  No matter what type of heating system your home has, always ensure that you have sufficient smoke and carbon monoxide detectors installed in your home.  It may save your life one day (please see our articles on each under health & safety)
How do I maintain it?
  • Although furnaces are meant to run predominately maintenance free, ones with gas or oil burners require occasional cleaning. Ideally, this should be taken care of in the fall before the heating season kicks in. Fortunately, furnace maintenance is not tool reliant - a good flashlight and vacuum is all you need.
  • You should perform a visual inspection on furnace's exterior before opening it. You're looking for loose connections wherever two pipes join, at elbows, and where the pipe joins the chimney. Push back together any you might find. Also check for corrosion around the bottom of the pipes. This often occurs there because of condensation buildup, which suggests a maladjusted burner.
  • Any loose or crumbling cement in the area where the pipe enters the chimney should be repaired immediately.
  • Check the condition of the air filter and replace it if necessary
  • You will most likely find the air filter in the return air duct system, usually at the bottom of the furnace where the large duct enters the furnace. You should know if your furnace uses a permanent foam-type filter or a replaceable one. The latter kind usually needs to be replaced annually (more often if you have central air conditioning) but check the particular specifications for instructions. They are readily available from just about any hardware store or home center. Changing a filter is very easy. Simply slide the old filter out and replace it with a clean filter. Note the markings on the filter, which shows you which side should face the incoming air.
  • Less common furnaces have a U-shaped filter called a hammock filter. It is located at the bottom of the furnace below the blower assembly. In order to change this type of filter, you need to remove the inspection panel from the side of the furnace - lift the wire frame off of its rack and push it together, then pull the filter directly out of the furnace.
  • This type of filter element is usually made of a fiberglass batting, which comes in rolls. Again, remove the old filter, cut a new piece of filter material from the roll, and then wrap it around the wire frame.
Clean the filter element

Regardless of whether your furnace uses a flat or hammock filter, they both have a permanent foam filter element, which should be cleaned and reused. Check with the manufacturer instructions first, but most elements require you to vacuum it followed by a wash in warm water with a mild detergent.

Check on the condition of the furnace blower
  • Before doing any cleaning on the blower, be sure to shut down the power to your furnace at the main circuit panel. Otherwise, someone may turn up the thermostat upstairs, engaging the blower and entangling your head in the belt - very dangerous.
  • Furnace blowers come in 2 varieties: some are belt driven and should be serviced every year; modern furnaces use direct drive blowers. They are typically found in the bottom of an upright furnace, directly behind the furnace filter. Either kind is what forces the air through the heating system throughout your home and both systems require occasional cleaning and lubrication. In most models, there is an access panel on the side of the furnace that conceals the blower assembly.
  • On most belt driven blower assemblies, both the blower motor shaft and the fan shaft should be lubricated with oil annually. Some units might even have visible oil spouts with little covers on them. In any case, 20-weight machine oil is good. Be sure NOT to overfill the oil compartments because this will lead to a difficult mess. You have to use some discretion here, as some direct drive units may not require oil. It most wise to consult the manufacturer's manual for specific directions. Oiling a furnace that does not require it is an admirable notion, but it will only lead to problems. Be sure to visually inspect the belt for cracks or frayed areas and replace it if it looks questionable. The cost of doing so is completely irrelevant in comparison to the hefty cost of emergency service! These belts will eventually fail if left unchecked.
  • Push the belt in the center between the pulleys to check for tension. It should have at least " give on it.
  • To remove the belt, you will probably have to loosen the hold-down bolts that secure the motor assembly and then push the motor towards the blower to slacken the belt. If this scenario does not pertain to your furnace, refer to your manufacturer's instructions on how to change the belt. They are usually printed on the inside of the removable panel door.
  • Once you've removed the belt, take it to a hardware store or home center and purchase the replacement. Reinstall the belt, reversing the steps you took to remove it. Remember to allow about " give on the belt; failure to do so will result in excessive wear on the bearings.
How about electric furnaces?

There is much less maintenance to worry about in the case of electric furnaces. Not only are they extremely efficient, but the actual heating elements have no moving parts to service. Sounds too good to be true? Well, keep in mind that while electric furnaces are efficient, they can also be rather expensive to maintain, especially when your system is backed up with excessive dirt and dust. The registers and air vents throughout your home should be cleared and vacuumed on a regular basis.

Gas Furnaces
  • Gas furnaces are renown for their cleanliness. As a result, their burners require minimal upkeep. They are very cost effective long term.
  • Major tune-ups of a gas burner assembly should be left to a professional. This type of service should be done every 2 years.
  • A little vacuuming inside the unit will keep it running well however.
  • Again, be sure to shut the power to your gas furnace at the main circuit panel before doing ANYTHING with the furnace.
  • In order to expose the burner, you will have to remove the front panel. Read the manufacturer's instructions if you cannot figure it out. This will expose a row of horizontal tubes, which are part of the burner assembly.
  • Along the top of these tubes are the burner heads with small holes for the gas air mixture to come out and be burned. Here is where you may want to vacuum up any loose and flaking rust particles, which often block the holes and hurt the efficiency of the unit. Please resist any temptations to fiddle with the exposed control knobs.
Forced air furnace checklist
  • With the furnace power switch off, remove the service panel to the fan compartment located directly below the burner or at the back of the furnace.
  • Vacuum dust from the fan compartment. If the motor has a fan belt that looks worn, have it replaced. Newer direct drive motors do not have a fan belt.
  • Lubricate the fan motor according to manufacturer's instructions if the fan motor requires oiling by hand. Do not try to oil sealed bearings.
  • Clean or replace the furnace filter if it looks dirty. Follow the manufacturer's instructions to clean an electronic filter.
  • Make sure warm air ducts are not loose or dislodged. Use furnace duct tape to seal joints and seams. Forced air systems have manual dampers that are used to balance airflow to rooms - ensure these dampers are not closed.
  • Replace all service panels and switch on the furnace. Check for warm airflow.
Oil Furnaces
  • Oil furnaces are the most complex and require a great deal of attention.
  • Again, like their gas cousins, any serious maintenance should be left to a professional on a yearly basis. This goes for adding oil, changing the filter, or adjusting any sensors.
  • You can safely perform some cleaning tasks to maximize performance, and in the case of oil furnaces, they will perform much better if you do so regularly.
  • Similar to gas furnaces, the burner on oil units should be kept as clean and free of dirt as possible. This is an ongoing job with oil furnaces, because oil burners suck in a lot of air for combustion.
  • It is critical to keep these air holes clear and keep the burner housing clean for optimal performance of an oil furnace.


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