The Building Thermostat senses temperature, turns on the furnace burner: As building temperatures drop a wall-mounted thermostat in the occupied space senses the temperature drop and in response, calls for heat at the furnace, causing the furnace oil or gas burner to turn "on".
The Gas or oil burner: The heat source such as a gas or oil-fired burner will turn on in response to the thermostat, and it will continue to run until either the high temperature or "FAN LIMIT" temperature is reached inside the warm air plenum or until the building thermostat inside the occupied space senses that the desired temperature there has been reached. At that time the burner is turned off but the blower will usually continue to run for a bit longer for reasons we'll explain in more detail below.
Heat exchanger: Hot combustion gases produced by the oil or gas burner circulate inside of the furnace's metal heat exchanger causing it to get hot. Combustion gases leave the inside of the heat exchanger and flow through a flue vent connector to a building chimney where they are vented safely outside.
The furnace blower inside the furnace blower compartment draws returning cool air from the living area and blows it across the outside of a steel "box" called the heat exchanger, sending the now-warmed air onwards into the furnace's output side or "supply air" plenum where it is sent into the building warm air duct system for delivery to the occupied space.
Air ducts connect and permit movement of cool air from occupied space through furnace and deliver warm air back to occupied space: The building air duct system includes return or cool air ducts and warm air ducts. Cool air ducts carry air from the occupied space into the furnace return air plenum, possibly through an air filter, and into the furnace blower compartment. Warm air supply ducts connect to the supply air plenum and carry warm air into the occupied space where it flows out of floor, wall, or ceiling warm air registers or diffusers.
Combination Fan & Limit Control: This control turns the furnace blower on and off at the proper times. The FAN ON setting on this control makes sure that the blower fan does not turn on (even though the building thermostat has asked for heat) so that the furnace wont' blow cool air into the occupied space.
Furnace FAN ON: When an adequate warm temperature has been reached inside of the furnace warm air plenum chamber the FAN ON switch turns on the furnace blower to deliver warm air to the occupied space.
Furnace FAN OFF: The furnace combination fan and limit control FAN OFF setting lets the furnace blower continue to run for an interval after the furnace burner has turned off, but will shut the blower off after the heat exchanger has been cooled down and the heat it contained has been sent to the occupied space.
Furnace LIMIT: The LIMIT indicator setting is a safety control that will turn off the oil or gas burner if temperatures inside the warm air plenum exceed a safe level.
When does the furnace blower turn OFF in normal operation?
When the thermostat has been satisfied and turns off the oil or gas burner at the furnace, the fan limit switch will cause the blower or fan unit to continue to operate until the temperature at the supply plenum has reached or dropped below the "cut-in" or "fan-on" lower limit on the switch.
When does the furnace oil burner, gas burner, or other heat source turn OFF in normal operation:
On most heating systems the burner or heat source will continue to run all of the time that the building thermostat is asking for heat, and will stop running as soon as the thermostat is satisfied. If the furnace oil or gas burner is very high capacity, or if the furnace fan/limit controls have been set to cause this effect, the burner may on some systems cycle on and off periodically while the warm air blower continues to run.
How is heating furnace efficiency or economy measured? What does Furnace AFUE mean?
Each furnace model is assigned an AFUE number. AFUE is an abbreviation for Annual Fuel Utilization Efficiency. In short, the AFUE tells you, for each dollar you spend on energy for heating by gas, oil, or another fuel, just how much of your dollar shows up inside the occupied space of your building as heat. Higher AFUE is better. If your furnace has an AFUE rating of 90, that means that for every dollar you spend on fuel, 90 cents worth of heat is delivered into your building. The remaining 10 cents is lost in inefficiency such as heat that escapes up the chimney along with the products of combustion.
AFUE is not the whole story of heating cost efficiency. A high-efficiency heating system that has not been cleaned and serviced may be running poorly and wasting money. In fact, an 85% AFUE heating furnace that has not been cleaned might be running at an efficiency much lower, perhaps 65%.
Furthermore, if your building is drafty or poorly insulated, you may be delivering heat at high efficiency but losing it from the building much faster than necessary. These articles can help with a more complete approach to saving money on heat:
Signs of Improper Warm Air Heating Furnace Operation
Furnace Oil or Gas Burner Cycling On and Off Frequently
If a furnace oil or gas burner cycles on and off frequently while the furnace blower continues to run, the heating system probably needs service, inspecting, testing, or adjustment.
• An over-sized or gas burner that is putting heat energy into the furnace too fast for the blower to extract it can cause this symptom.
• A dirty or blocked air filter or a dirty or blocked blower fan can cause this symptom because it is reducing the rate of air flow across the heat exchanger. If you want to save money on heating costs be sure the warm air flow is not reduced or obstructed by dirty or improper air filters.
• Improper furnace control settings can cause furnace oil or gas burners to cycle on and off too often; check with your heating service technician.
Reduced or no Air Flow at Heating Supply Registers
If you are getting some but not much air flow out of heat supply registers there is probably a problem with:
• the air filter(s) - dirty or damaged
• the blower fan is dirty or has a loose fan belt
• the duct system itself, such as collapsed, blocked, or disconnected duct work
• if the system is a combined heating and air conditioning system, the cooling coil may be blocked by dirt if the system has always delivered only poor air flow at the supply registers, the blower may be dirty or defective, or the return air supply may be inadequate
If you are getting no air flow whatsoever but the furnace burner seems to be running there is probably a problem with:
• the blower fan motor or fan itself has failed
• the duct system has become totally blocked or disconnected
• the furnace is not running at all