Air Handling Unit Components: Heat
There are many different types and arrangements of heating systems for air handling units and these types of heating systems and arrangements of heating systems vary from commercial to residential. Some air handling units do not have any heat source inside them at all but have remote heat for zoning. The air handling unit provides the air flow and the remote heaters provide heat for zoning. Sometimes the air handling unit is a VAV air handling unit and other times it is just to provide air for a duct heater whether the duct heater is gas fired or electric. For air handling units which some have heating systems inside the five main ways of providing heat for air handling units are:
- Electric Heat Strips
- Gas Heat
- Oil Heat
- Hot Water Coil Heat
- Heat Pump (Process of Refrigeration Heating)
Air Handling Unit Components: Electric Heat
Electric heat is probably the most used type of heating systems in commercial systems for VAV boxes and in some residential especially for the back-up heat source for heat pumps. Electric is 100% efficient but it costs more to produce the same amount of per BTU as compared to other ways of converting fuels to heat. Electric heating uses resistance electrical strips or metal to generate heat. When electricity passes through this metal it heats up. This metal is often an alloy composed mainly of nickel-chromium. The voltage which feeds these electric heat strips is high voltage of 240V for residential and 277V for commercial systems. The electric heat strips pull high amps and it is necessary to provide the electric heat circuit with the proper circuit protection and wire size to handle the higher amp draws which occur in these circuits. For each KW the BTU output will be 3415. For a 10KW (34,150 BTU/h) electrical heating system which utilizes 240V for main power the amp draw will be (10,000 watts/240 volts = 42) 42 amperes. There are many different areas in the National Electrical Code (NEC) which are applicable to different configurations of electrical heating elements inside of air handling units which the manufacturer engineered into the system. Outside the air handling unit it is necessary for field wiring to the proper size. Circuit protection and wire size for electrical resistance type heating needs to meet the NEC code for safety purposes. Consult with an electrician or electrical engineer for proper sizing requirements. Safety switches in the air handling unit control wiring provide additional fire hazard protection.
Air Handling Unit Components: Remote Electric Duct Heaters
Electrical duct heaters mounted remotely from the air handling unit have different ways of proving air flow before the electrical heating can be energized. Older models have sail switches which close when air flow is present in the ductwork. When the blower energizes to provide airflow the sail switch closes and allows the electric heat control circuit to energize if necessary. Other types of switches for proving air flow before the electric heat is allowed to energize are pressure switches which work on a pressure differential, current switches which detect current on the blower. When the air handling unit blower current is detected a switch inside the current switch closes and allows the electrical heating control circuit to energize as necessary. Other types of switches or air flow proving methods may be employed to ensure there is airflow before allowing the remote duct mounted electric heater to be energized.
Air Handling Unit Components: Gas Heat
There are many different types of gas furnaces used to provide heating. The basics of gas furnace components are covered in detailed components of gas furnaces in the gas furnace section. Whenever dealing with any appliance which burns fossil fuels it should be noted that carbon monoxide detectors should be present in the spaces where heat is provided by this appliance. That being said, many manufacturers provide the best engineering to ensure safety of their appliances but accidents do happen. Heat exchangers and flues have miniscule failure rates but these failures do occur. Machines break down and accidents happen and it best to take proper precautions to protect the safety of all occupants of the dwelling where heating is provided by these appliances. Natural gas is the most popular form of gas heating followed by propane. Natural produces approximately 1000 to 1100 BTU’s per cubic foot while propane produces 2600 to 3200 BTU’s per cubic foot. Check with your local gas company or propane provider for specific BTU’s. Natural gas is available only where provided by a pipeline from the gas company while propane is portable and can be delivered to a storage tank via truck. Natural gas is lighter than air while propane is heavier than air and precautions against leaks should be addressed. Both gases have an odorant additive so that leaks can be sensed by smell. This odorant smells like rotten eggs and if you smell this smell around any gas equipment call the gas company or your local fire department immediately.
An air handling unit which uses gas for heating comprises of gas heating controls including the gas valve, a manifold, and gas burners. The gas burners are inside the heat exchanger and heat the heat exchanger to nearly 2000° F. The heat exchanger is designed to withstand these temperatures and keep the combustion chamber isolated from the air which passes over the combustion chamber. Gas safety controls which are included in the safety circuit include a temperature sensing device which prevents too much heat from building up in the combustion chamber. If the temperature inside the heat exchanger reaches a high limit set point then the gas safety circuit disengages the burners and calls for the fan to run to dissipate the heat before allowing the burners to reignite. High-efficiency furnaces or two stage heating furnaces typically have an induced or forced draft motor which pulls or pushes the combustion gasses through the combustion chamber(s) and forces the combustion gases out to the flue for ventilation. Depending on the flue temperatures and the efficiency of the air handling unit furnace will depend on the type of flue used. Additionally, considerations for the type of flue used are the vicinity of combustible materials near the flue. Consult the Fuel Gas Code, the Mechanical Code, or you local Mechanical Inspections for these specific considerations.
Air Handling Unit Components: Remote Gas Duct Heaters
There gas furnaces which are remotely located from the air handling unit which are installed in the duct system. These are called gas duct heaters and are completely reliant on air flow from the air handling unit before the can fire and provide heating. The same methods employed for remote electric duct heaters are the same for gas duct heaters. There must be a way to prove air flow before the furnace can fire. See electric heating for various methods employed to prove air flow.
Oil is used for many different heating systems including in forced-air air handling systems. The heat exchangers and safety precautions as noted with gas furnaces apply to oil furnace air handling units as well. Oil provides 140,000 BTU’s per gallon. Oil heating controls vary depending on the manufacturer and the age of the equipment. The oil burner components consist the burner housing, a burner control, a blower motor for primary air, an oil pump, a step up transformer, igniters, the oil gun, and a nozzle. All these components combined and operating properly cause a flame to be produced inside the combustion chamber. Most oil furnace air handling units get their fuel from a remote tank with copper tubing or steel pipe connecting the tank with the oil burner. Considerations for tank location and distance from the burner should be made according to manufacturer’s recommendations. For environmental considerations and for proper mechanical operation this oil delivery line should not have any leaks. Additionally, there is an oil filter located in this line usually near the furnace. This oil filter needs to be changed from time to time otherwise, it will become clogged and the burner may not operate properly. As air on the outside of the heat exchanger passes over the heat exchanger it is heated up and delivered to the spaces where the oil furnace air handling unit is designed to heat through the ductwork. The oil furnace also utilizes a flue and the same precautions noted about the flue for the gas furnace air handling unit apply to the oil furnace air handling unit. The only difference is that the oil flue may have a barometric damper and it is necessary for proper ventilation that this barometric damper is adjusted properly.
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