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Furnaces 101

What is a furnace?

A furnace is a combustion heating appliance in a forced air heating system which heats air and distributes it through ductwork. The heating capacity of a furnace is measured in BTUs (British Thermal Units) and the efficiency is measured in AFUE (Annual Fuel Utilization Efficiency).  So, what is a BTU? Here’s the technical definition; a BTU is the amount of heat required to raise the temperature of 1 pound (0.454 kg) of liquid water by 1°F (0.56°C) at a constant pressure of one atmosphere. In simple terms, it’s a measure of heat energy and in a furnace it refers to the amount of heat it will produce in a specific time period. Average BTU ratings range from 40,000 to approximately 120,000. AFUE refers to the ratio of output energy of the furnace to the input energy. Energy.gov defines it as “AFUE = how efficient the appliance is at converting the energy in its fuel to heat divided by the annual fossil fuel consumed by the appliance.” Old low-efficiency furnaces have a rating of 56 - 70%. Mid-efficiency furnaces are rated 80 - 83% and high-efficiency are 90 - 98%.


How do furnaces work?


There are four main components to a furnace: burners, heat exchanger, blower and flue. The burners heat up air and send the hot air to the exchanger. The heat exchanger transfers heat to the home’s air and the blower pushes it out through the ducts. The flue vents out toxic air from inside the exchanger. The whole thing happens when the thermostats tells it to. Let’s take a closer look.


Control & Ignition
The thermostat controls the temperature settings, telling the furnace when to turn on and off. They can be powered either by the furnace’s control board or by batteries. For proper temperature control, the thermostat needs to be centrally located, out of direct sun light. When the temperature in the room drops below the thermostat setting, the thermostat sends a signal to the furnace control board telling it to turn on. The control board then sends a signal to the draft inducer motor which, in turn, starts pulling air into the burner assembly either from around the furnace or from outside the building if a fresh air intake pipe has been installed. When the draft inducer fan reaches its maximum speed, the vacuum switch closes, preventing gases from escaping into furnace and then into air being circulated into the house. The closing of the vacuum switch allows voltage to flow through the limit switches, back to the furnace control board. The control board then sends voltage to the igniter. Some furnaces use electric spark igniters while others use glow bar igniters.


The control board now sends voltage to the gas valve causing it to open and allowing the gas to flow over the igniter which creates a flame. A flame sensor then detects the presence of a flame and the flame roll out limit switches (mounted on the burners) ensure the flame is drawn through the burners. Now the furnace control board sends a current to the circulation blower fan which pulls air into furnace through the return air duct. The air is heated as is flows over the heat exchanger before being pushed out through the duct to be distributed throughout the home. When the room reaches the set temperature, the thermostat sends a signal to the furnace control board telling it to shut off.


Air Circulation
The furnace has two air circulation systems: an air intake system for heating & combustion and an air circulation system to deliver heated air to the home. The air circulation system pulls air in through the return air vent, through the furnace (over the heat exchanger) and pushes the conditioned air back out through the ductwork.  The air intake system pulls air into the furnace, through the heat exchanger and out the flue. Because the condensation builds up in this exhaust pipe, either a condensate pump or a floor drain will be installed. To keep air moving freely, the room vents should be kept free from blockages by things such as furniture. Free air movement for the intake system is just as important. Air is pulled into the furnace, through the exchanger and out the flue. If the inducer fan cannot draw in enough air, combustion will not happen. For this reason it is important to make sure the furnace can breathe. The furnace room should be free from debris since this is where most furnaces draw their air. The furnace also has a filter which must be checked monthly and replaced as needed. Allowing the filter to become too dirty limits the air that can pass through it and will cause the system to overheat and the gas will be cut off.