Table of Contents
Air Handling Unit Components 2: 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 airflow 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 2: Electric Heat
Electric heat is probably the most used type of heating system in commercial systems for VAV boxes and in some residential especially for the backup heat source for heat pumps. Electric is 100% efficient but it costs more to produce the same amount per BTU as compared to other ways of converting fuels to heat.
Electric heating uses resistant 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 that 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) that 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 need 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 2: Remote Electric Duct Heaters
Electrical duct heaters mounted remotely from the air handling unit have different ways of proving airflow before the electrical heating can be energized. Older models have sail switches that close when airflow 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 airflow 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 airflow 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 2: 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 the safety of their appliances but accidents do happen. Heat exchangers and flues have minuscule 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 BTUs per cubic foot while propane produces 2600 to 3200 BTUs per cubic foot. Check with your local gas company or propane provider for specific BTUs.
Air Handling Unit Components 2 | Gas Safety
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 that uses gas for heating comprises 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 that are included in the safety circuit include a temperature-sensing device that prevents too much heat from building up in the combustion chamber.
Air Handling Unit Components 2 | Temperature Limit Switches
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 your local Mechanical Inspections for these specific considerations.
Air Handling Unit Components 2: Remote Gas Duct Heaters
There gas furnaces that are remotely located from the air handling unit are installed in the duct system. These are called gas duct heaters and are completely reliant on airflow from the air handling unit before they 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 airflow before the furnace can fire. See electric heating for various methods employed to prove airflow.
Oil Heat | Air Handling Unit Components 2
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 of 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.
Air Handling Unit Components 2 | Oil Storage Tank
Considerations for tank location and distance from the burner should be made according to the 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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