Forced-air
This article needs additional citations for verification. (July 2014) |
A forced-air
Forced air heating is the type of central heating most commonly installed in North America.[1] It is much less common in Europe, 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, 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 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.
- 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 pumps
Hydronic coil
- Combines hydronic (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.[2][3] 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.[4][5]
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.[6] 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.[7] 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.[8][9] Low thermal inertia allows literally in a few minutes to heat different in volume buildings.[10] At the same time, all the heat goes only to heat the rooms.[11]
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.[12]
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.[13]
Advantages: presentable from a design point of view (only the grilles are visible), inexpensive (considering the combination with the ventilation system).
See also
References
- ISBN 9781118977996.
- ^ "Constant Air Volume – CAV". theengineeringmindset.com. Retrieved 2023-10-27.
- ^ "US7582009B1". patents.google.com. Retrieved 2023-10-27.
- ^ "Constant Air Volume-CAV". theengineeringmindset.com. Retrieved 2023-10-27.
- ^ "What is a constant volume of air?". www.mrductcleaning.com.au. Retrieved 2023-10-27.
- ^ "VAV Vs CAV In HVAC". airfixture.com. Retrieved 2023-10-27.
- ^ "What is the difference between constant air volume and variable air volume systems?". knowledgeburrow.com. Retrieved 2023-10-27.
- ^ "What has a lower heat capacity than water?". greed-head.com. Retrieved 2023-10-22.
- ^ "Specific Heat Capacity and Water: Heat vs Temperature, Facts, Formula, SI Unit". www.collegesearch.in. Retrieved 2023-10-22.
- ^ "What Is Gas Ducted Heating?". www.comfyhome.com.au. Retrieved 2023-10-22.
- ^ "Thermal Inertia, Comfort and Energy Consumption in Buildings: A Case Study in São Paulo State -Brazil". www.researchgate.net. Retrieved 2023-10-22.
- ^ "9 Types of Air Conditioning System (AC) – Advantages and Disadvantages [Complete Guide]". engineeringlearn.com. Retrieved 2023-10-22.
- ^ "Heat recovery by means of a ventilation unit". www.researchgate.net. Retrieved 2023-10-22.