Know your Central Heating Systems
By Matt Weber
Home buyers and home builders should know the ins and outs of their heating system. Here’s a cheat-sheet that covers differences among the standard central heating options, from fuel types to system setups.
A furnace heats a home by pulling cool indoor air into the unit, heating it and then distributing the warmed air throughout the home via ductwork. Today, most furnaces today use natural gas, oil or electricity, and each system works a little differently.
With natural gas heating, the gas is mixed with air inside a burner and ignited inside a combustion chamber. In a forced-air system, a blower pulls cool air from the rooms through the air ducts and into the furnace. The air is heated by a heat exchanger connected to the combustion chamber, and the warm air then flows back into the house through ductwork. Exhaust gases from the burners are vented outside through the roof, or in some cases, a side-wall vent.
“Gas furnaces are the most common winter heating units in North America,” says Jason Ingram of Ingram’s Water and Air Equipment, a wholesale equipment supplier based in Paducah, Kentucky. “Dollar for dollar, they’re still the most effective option for many homeowners.”
An oil-fired furnace operates in a manner similar to gas, but the oil is atomized and then burned. Air absorbs heat in the exchanger, and a blower returns the warm air back through the house via the ductwork. Emissions are then vented outside.
Some homes are equipped with gravity furnaces that do not rely on forced air or blowers. Gravity furnaces are typically installed in basements and warm the home as heat naturally rises through the ducts.
An electric forced-air furnace uses a blower to move air over electrically-heated coils. The warm air is then distributed through the through ductwork. These units can be used with heat pumps or central air conditioners, and they require no venting.
Boilers heat water that transports heat to radiators, baseboard heaters, hydronic radiant floor systems and more. The hot water of a boiler can heat an entire home or just certain zones, as well as provide consumable hot water to
a kitchen, bathroom, swimming pool or hot tub. A boiler can even provide the hot water for a radiant system installed beneath a driveway or sidewalks to melt ice and snow.
In a boiler, water is typically heated to a pre-set temperature between 160- to 180-degrees F. The hot water is then circulated by pumps through pipes in the home. This heated water then warms radiators and other fixtures installed throughout the home.
As the cool air in the room flows through the hot radiators, the air is warmed. As the heat from the water inside the radiator is released into the air, the water inside the radiator is cooled and returns back to the boiler to be reheated.
The primary energy source for modern residential boilers is oil or natural gas. Heat is created during the combustion process when air mixes with the fuel and is burned. Residential gas boilers typically use a pilot to ignite the flame, while oil burners are ignited using direct spark of the oil flame. In all cases, the burned fuel is exhausted to the outdoors.
The efficiency of both a boiler and furnace can be determined by its AFUE rating (Annual Fuel Utilization Efficiency). The Federal Trade Commission requires new furnaces or boiler manufacturers to display the system’s AFUE so consumers can compare heating efficiencies of various models.
“Remember, when it comes to AFUE, higher is always better,” notes Ingram.
The Department of Energy’s “Energy Star” qualified boilers have annual AFUE ratings of 85 percent or greater. The minimum efficiency level for furnaces currently manufactured in the U.S. is 80-percent AFUE. A rating of 80 percent means that for every dollar you spend heating your home, 80 cents are actually applied to the generation of warmth. The other 20 escapes up the chimney or elsewhere. For enhanced energy efficiency, you should consider a 95-percent AFUE unit.
Air-transfer heat pumps can provide both heating and cooling to maintain the indoor comfort of your home. In the summer, a heat pump functions as a central air conditioner. But in the winter, the heat pump reverses its action to create warm, comfortable air for your home.
A heat pump operates by transferring heat from air inside the home to the outside air. It has three main components: a compressor, a condenser and an evaporator coil. These parts are responsible for converting a chemical refrigerant from a gas to a liquid and back again. During the summer the indoor unit, which contains the evaporator coil, removes indoor warmth and humidity from the air. The outdoor unit, which contains the condenser and compressor, rejects the heat that was captured indoors.
A fan draws outdoor air through louvers surrounding the outdoor cabinet and blows air across the hot coil. As the air blows across the coil, it transfers the heat to the outside air and cools the refrigerant inside the coil.
When an air-source heat pump is heating your home, the cooling cycle is reversed. In the outdoor unit, the heat pump evaporates the low-temperature refrigerant. As the liquid evaporates, it absorbs heat from the outside air. (Even cold winter air contains heat.) After the gas is compressed in the outdoor unit’s compressor, it passes into the indoor coil and condenses, which releases heat inside the house.
“An energy-efficient heat pump is a great home HVAC system,” says Ingram. “Depending on where you live, it could be the only cooling and heating system you need.”
Some manufacturers offer a “package” heat pump. These all-in-one systems supply both cooling and heating equipment in a single “package” that sits on the ground or rooftop outside of your home, economizing space.