Furnace, The Basics
A furnace is designed to automatically add heat to your home. By understanding
some basics, you can help them to do this efficiently. This will help you to keep some of your hard earned
money in your pocket. To explore your options in heating the garage, we have a separate
page for that. There are several types of units and they are usually grouped by the type of fuel they use.
Some of the common types include gas, oil, wood, biomass, and electric.
A special type is used in mobile homes. They use special
venting and duct systems and are certified for use in manufactured homes.
How a gas-fired unit works
We will start by looking at a typical residential gas fired forced air heating system. These units burn
either natural or liquified petroleum (lp) gas.
The heating cycle starts when the thermostat senses that the room temperature is below the setpoint. On
newer units, this causes an inducer fan to pull air through the unit establishing adequate combustion
airflow. After proper airflow is verified, the gas valve is opened allowing gas to flow. The gas flows
through the burners where it is ignited by a spark or the heat from the ignitor. (Older units have what is
called a standing pilot which is a small flame that burns constantly to light the gas.)
After the unit senses the flame is lit, the spark or igniter is turned off. Normally, the gas will burn
for approximately two minutes before the blower is started. This prevents blowing cold air out the vents
at the start.
After the preset time or at a set temperature, the blower motor is energized and air is blown over the
heat exchanger. The air is heated and supplied to the duct system.
When the thermostat senses that no more heat is required, the gas valve is deenergized and the gas is shut
off. The blower motor usually runs for another few minutes to cool off the heat exchanger. This can be
controlled by either a timer or a temperature switch.
Size & Efficiency, Why Does It Matter?
A gas furnace has an input rating that is expressed in btu per hour. This is how the size of the unit is
expressed. Common sizes are 40000, 60000, 80000, 100000, and 120000 btu/hr. It is absolutely critical that
your unit is the proper size for your home. A load calculation is performed to
find the right unit for your specific home.
Not all of the heat that is available from the gas is put into the home's air. The ratio of the heat
output of the unit to the heat content of the gas supplied is the unit's efficiency or afue. AFUE is short
for annual fuel utilization efficiency and is expressed as a percentage. Some common efficiency ratings
are 80%, 90%, etc. A higher efficiency unit requires less gas to heat the home and therefore your utility
bill is lower.
There are several classes of gas furnaces. Most manufacturers still produce non-condensing models. These
units have only one heat exchanger. These units are the least efficient at about 80% and are sometimes
referred to as mid efficiency units. These units use metal pipe to exhaust the combustion products from
the home. With the price of fuel these days, it is not a good idea to purchase one of these units.
The next class is referred to as high efficiency units. There is no standard as to when a manufacturer can
use this phrase to describe their furnace. Therefore, you have to look at the details. In general, for a
unit to be considered high efficiency, it should be a condensing furnace. These units have two heat
exchangers to get more heat from the unit. Their efficiency rating (afue) is at least 90%. These units
extract heat from the exhaust products of the furnace. This produces water which is called condensate.
They collect this water and deliver it to a drain system outside the furnace.
The last class is where the units have added features built into them. One such feature is called a
variable speed blower motor. In these units a sensor monitors the speed of the blower and adjusts the
power supplied to it to provide a desired speed. These blowers are usually quieter and more energy
Another added feature is where the unit has two gas input rates. These are called two stage furnaces. On
the first stage the unit burns less gas and this helps to more closely match the heating load. On the
coldest days, the unit can increase it's output by switching to the second stage firing rate. An advantage
of these units is that you normally have longer run cycles which can provide more uniform temperatures
throughout the home. These units usually have a slightly higher afue rating. Most of the manufacturers
combine this feature with the variable speed blower.
Some advanced units, called modulating gas furnaces, have variable gas input rates. This allows them to
very closely match the heating requirements of the home. Usually the units have a built in control that,
after a preset amount of operation, increases the gas input. This is continuously done to match the load.
The most advanced units are part of a CHP system and produce electricity while they
heat the home.
Next, we will look at oil fired furnaces. The heating cycle for these systems starts with the thermostat
sensing the need to add heat. This energizes a burner motor which pulls fuel oil from a tank and combines
it with the proper amount of air. At the same time a spark is generated by an ignition transformer. The
oil passes through a nozzle which has a very small hole and a tiny screen in it. This breaks up the oil
and causes it to vaporize so it can be ignited by the spark.
After a delay to allow the heat exchanger to warm up, the blower motor is energized. This draws air into
the unit and passes it over the heat exchanger where it is heated. The air is then sent into the duct
system to be distributed throughout the home. The blower is normally turned on and off by a temperature
switch. This switch also will shut the unit off if the unit overheats.
After the thermostat is satisfied, the burner motor is deenergized and oil flow is stopped. The blower
motor continues to run until the heat exchanger is sufficiently cooled.
Corn burning furnaces
These units burn shelled corn to produce the heat. They usually have a small flame burning constantly
once the unit is lit for the first time of the season. When the thermostat senses a need for heat, two
augers and a combustion blower are energized. These augers transfer the shelled corn from a storage bin to
the furnace. The combustion blower establishes draft and provides the proper amount of air to burn the
After the heat exchanger is warmed up, a temperature switch turns the blower motor on. This pulls air into
the unit where it is warmed and transferred to the supply duct system.
When the corn is burned it produces ashes. Typically it produces one pound of ash for each 100 pounds of
corn burned. These ashes fall into a pan where they are collected.
When the thermostat is satisfied, the augers and combustion blower are shut off. The blower motor runs
until the furnace is cooled down. Then the temperature switch shuts it off.
These units produce heat from electricity flowing through special wires called elements. Unlike the
other units, these units are 100% efficient. All of the electricity is used to produce heat. The size of
these units is expressed in kilowatts instead of btu/hr.
When the thermostat senses a need for heat, a relay energizes the element and the blower motor. Some units
use more than one heating element. These units have other relays to turn on the other elements.
When the thermostat is satisfied, the relays are deenergized. After a short time delay, the elements and
fan are shut off.
Some units are combined with the air conditioning system and are referred to as packaged air
conditioners. Units that combine a gas furnace with the air conditioner are commonly called gaspacks. The
main advantage of them is that all of the equipment is located outside the home. Therefore, no valuable
floor space is used for the hvac system. The main disadvantage of them is that they are usually less
efficient. Typically the gas furnace section has an AFUE of 80%.
Other units combine electric heating elements with the air conditioning system. They have the same
advantage as gaspacks without the loss in efficiency. The main disadvantage is that, in some areas, an
electric heating system may cost more to operate than gas.
Some units combine a heat pump with a gas furnace and they are called hybrid systems. They can be more
cost effective to operate.
Other related pages
Shopping for a unit can leave you dazed and confused. But, with our brand ratings, you can shop with
confidence and save money at the same time. The best unit in the world is only as good as the quality of
work installing it. Our buying guide will show you what to
look for to ensure you get a quality job. That could save you a lot of money down the road.
If you prefer to do it yourself, our installation guide will help you
through the process. You can typically save $500-$1000 or more on labor costs. You will save even more if
you purchase the unit yourself and avoid the dreaded dealer markup!
No matter what type of system you have, they all require some basic maintenance. You can purchase a service
agreement from a dealer for around $100 per year to do it for you. But, with our maintenance guide you can do it
yourself (and keep that money). A unit that is not maintained properly is more likely to release carbon
monoxide into the home.
Even the finest built units don't always work. But, many times they are really easy to fix. With our troubleshooting
guide, you get step by step diagnosis and repair tips. You can watch a professional explain how to
repair a gas furnace with our video.