Forced warm air
Because it is economical to install and is versatile, the forced warm-air heating system is found in more homes than any other central heating system. The basic difference between this system and the gravity warm-air system is a blower in the heat exchanger that circulates the warm air. Since the warm air is distributed under a draft, comfort heating can be achieved at lower furnace temperatures with lower fuel consumption. In addition, the supply and return ducts need not be as large as those of a gravity system.
There are three basic controls for this system: a thermostat, a fan control, and a high-temperature limit control. The thermostat has been discussed earlier in this chapter. The purpose of the fan control is to prevent the fan from circulating cool air around the house. The fan control is a temperature-sensitive switch that turns the blower on and off at preset air temperatures. It is independent of the thermostat. When the thermostat calls for heat, only the burners should fire. The fan should not begin to operate. If it does, the fan control is either faulty or in need of adjustment. After the heat exchanger warms up to a temperature of about 110° F to 120° F, the fan should begin to operate. When the temperature setting on the thermostat is satisfied, the thermostat will shut off the burner but not the fan. The fan will continue to operate until the temperature in the heat exchanger drops to about 85° F.
If the heat exchanger gets too hot, the high-temperature limit control will shut off the burner. The limit control is usually set at about 175° F. For proper operation, the fan should begin to operate before the burners are shut off by the limit control. Otherwise, the temperature of the air discharging from the registers will be too high for comfort heating.
Because of the need to conserve energy and the escalating cost of fuel after the energy crunch of the 1970s, the heating industry developed a new generation of gas-fired forced-warm-air furnaces. These furnaces have been able to achieve an overall operating efficiency of 90 to 97 percent, whereas many of the conventional furnaces found in most homes today have an overall operating efficiency of around 60 to 65 percent. Because the U.S. Department of Energy set a standard in 1992 that requires furnaces to have an overall efficiency in the 80 percent range, conventional furnaces are no longer installed in new homes or as replacement units.
The new furnaces are called condensing furnaces. The increase in efficiency is the result of the installation of a secondary heat exchanger that extracts heat from the exhaust gases that normally flow up the chimney with conventional furnaces. During this process, the temperature of the exhaust gases drops to a point where the water vapor in the exhaust gases condenses, thereby releasing additional heat. With the conventional furnace the temperature of the flue gases is about 450–550° F. With the high-efficiency units, it’s about 120–130° F.
To achieve the increased efficiency, it was necessary to incorporate additional components into the overall furnace package. Included with the furnace are a power vent fan (also called an induced draft blower), plastic piping to vent the flue gases through a side wall or the roof, condensate drainage piping, and an intake air duct for those units that have a sealed combustion system using outside air.
A power vent fan is needed to overcome the additional resistance to the flow of the exhaust gases caused by the secondary heat exchanger. Also, with the low temperature of the exhaust gases, it is not necessary to use the conventional chimney.
Another type of high-efficiency furnace is the pulse-combustion furnace. As with the condensing furnaces, the high efficiency of a pulse unit results from the extraction of heat from the exhaust gases until the associated water vapor condenses. However, the combustion process is completely different. Whereas the heat in a condensing furnace results from a continuous burning of fuel, in a pulse combustion furnace, it results from sixty to seventy tiny explosions of a gas-air mixture per second in the combustion chamber.