A forced-air

central heating A central heating system provides warmth to a number of spaces within a building from one main source of heat. A central heating system has a Furnace (central heating), furnace that converts fuel or electricity to heat through processes. The he ...

system is one which uses

air An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

as its

heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...

transfer medium. These systems rely on

ductwork Ducts are conduits or passages used in heating, ventilation, and air conditioning (HVAC) to deliver and remove air. The needed airflows include, for example, ''supply air'', ''return air'', and ''exhaust air''. Ducts commonly also deliver '' ve ...

, vents, and plenums as means of air distribution, separate from the actual heating and

air conditioning Air conditioning, often abbreviated as A/C (US) or air con (UK), is the process of removing heat from an enclosed space to achieve a more comfortable interior temperature, and in some cases, also controlling the humidity of internal air. Air c ...

systems. The return plenum carries the air from several large return grills (vents) to a central

air handler An air handler, or air handling unit (often abbreviated to AHU), is a device used to regulate and circulate air as part of a heating, ventilating, and air-conditioning (HVAC) system. An air handler is usually a large metal box containing a Centr ...

for re-heating. The supply plenum directs air from the central unit to the rooms which the system is designed to heat. Regardless of type, all air handlers consist of an air filter, blower, heat exchanger/element/coil, and various controls. Like any other kind of central heating system, thermostats are used to control forced air heating systems. Forced air heating is the type of central heating most commonly installed in

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

. It is much less common in

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

, where hydronic heating predominates, especially in the form of hot-water radiators.

Types

Natural gas/propane/oil/coal/wood

*Heat is produced via combustion of fuel. *A heat exchanger keeps the combustion byproducts from entering the air stream. *A ribbon style (long with holes), inshot (torch-like), or oil type burner is located in the heat exchanger. *Ignition is provided by an

electric spark An electric spark is an abrupt electrical discharge that occurs when a sufficiently high electric field creates an Ionization, ionized, Electric current, electrically conductive channel through a normally-insulating medium, often air or other ga ...

, standing pilot, or hot surface igniter. *Safety devices ensure that combustion gases and/or unburned fuel do not accumulate in the event of an ignition failure or venting failure.

Electric

*A simple electric

heating element A heating element is a device used for conversion of electric energy into heat, consisting of a heating resistor and accessories. Heat is generated by the passage of electric current through a resistor through a process known as Joule heating. He ...

warms the air. *When the thermostat calls for heat, blower and element come on at the same time. *When thermostat is "satisfied", blower and element shut off. *Requires very little maintenance. *Usually more expensive to operate than a natural gas furnace.

Heat pump

*Extracts heat from the environment, using either the ground or air as the source, via the

refrigeration cycle Thermodynamic heat pump cycles or refrigeration cycles are the conceptual and mathematical models for heat pump, air conditioning and refrigeration systems. A heat pump is a mechanical system that transmits heat from one location (the "source") a ...

. *Requires less energy than electric resistance heating and possibly more efficient than fossil fuel fired furnaces (gas/oil/coal). *Air source types may not be suitable for cold climates unless used with backup (secondary) source of heat. Newer models may still provide heat when coping with temperatures below 0 °C (32 °F). *A refrigerant coil is located in the air handler instead of a burner/heat exchanger. The system can also be used for cooling, just as any central air-conditioning system. *See

Heat pump A heat pump is a device that uses electricity to transfer heat from a colder place to a warmer place. Specifically, the heat pump transfers thermal energy using a heat pump and refrigeration cycle, cooling the cool space and warming the warm s ...

Hydronic coil

*Combines

hydronic Hydronics () is the use of liquid water or gaseous water (steam) or a water solution (usually glycol with water) as a heat-transfer medium in heating and cooling systems. The name differentiates such systems from oil and refrigerant systems. ...

(hot water) heating with a forced air delivery *Heat is produced via combustion of fuel (gas/propane/oil) in a boiler *A heat exchanger (hydronic coil) is placed in the air handler similar to the refrigerant coil in a Heat Pump system or a Central AC. Copper is often specified in supply and return manifolds and in tube coils. *Heated water is pumped through the heat exchanger then back to the boiler to be reheated

Sequence of operation

# Thermostat calls for heat # Source of ignition is provided at the boiler # Circulator initiates water flow to the hydronic coil (heat exchanger) # Once the heat exchanger warms up, the main blower is activated # When call for heat ceases, the boiler and circulator turn off # Blower shuts off after period of time (depending on the particular equipment involved this may be a fixed or programmable amount of time)

Self-balancing mechanism

The basis of any CAV regulator is the self-balancing mechanism. It is the design of this mechanism that determines the accuracy of maintaining the set flow rate, noise level, minimum regulator resistance, flow range and other parameters. There are different designs of the self-balancing mechanism that largely determine the technical characteristics of CAV regulators: *Self-balancing mechanism based on a silicone adjustment diaphragm that changes its volume depending on the air pressure in the duct, thereby increasing or decreasing the air flow area. *Self-balancing mechanism with overlapping section. The self-balancing damper with spring automatically closes the remaining part of the cross-section depending on the duct pressures. *Self-balancing mechanism with connector for flow adjustment. Typically, the regulator damper is made of lightweight aluminum, and the self-balancing mechanism consists of plastic levers and transmission, a steel spring and silicone vibration dampers, which are necessary to prevent auto-oscillation.

CAV and VAV

An alternative to a constant air volume system is a variable air volume (VAV) system. Variable air volume systems are generally more complex than their CAV counterparts because they must utilize temperature control and control the actual volume of air blown into each room. Although more difficult to design and implement, a VAV system is more energy efficient than a CAV system because the components of a variable airflow design operate only as needed.

Advantages and disadvantages

Compared to water, air masses have a lower heat capacity. This means that they cool down faster, but they also raise the room temperature in a short time. Low thermal inertia allows literally in a few minutes to heat different in volume buildings. At the same time, all the heat goes only to heat the rooms.

Systems with air-heating units

Disadvantages: high noise level, disperse dust, each unit requires a supply of heat transfer fluid and electricity, have a high gradient of air temperature over height. Advantages: does not require large cross-sectional ducts, has a long spray range

Air heating systems combined with supply ventilation

Disadvantages: require ducts with large cross-sections, it is necessary to reserve the supply unit and pump in the piping assembly, have a high gradient of air temperature over the height, have a small range of the jet. Advantages: presentable from a design point of view (only the grilles are visible), inexpensive (considering the combination with the ventilation system).

References

{{DEFAULTSORT:Forced-Air Heating, ventilation, and air conditioning