What’s Wrong with Your Electric Air Duct Heaters?
The need for precise temperature control or auxiliary heating is a typical application, but not all duct heaters are created equal. Placing a heat source in a duct ventilation system can be problematic. Each heating source requires heating elements to create and transfer the heat efficiently to the air stream.
Everything electric is causing issues with America’s faltering electrical grid. This winter more and more homeowners are are depending on electric heaters to supplement heat-pumps. Ensuring that your home and commercial electric heating systems are safe and efficient is more important today than ever.
According to NFPA fixed or portable space heaters are the major cause of fires in homes, but central heating is still responsible for 12% of the fires & 7% of the property damage. Estimated average of 44,210 home structural fires, 1,370 civilian injuries, 480 deaths, and $1 billion in direct property damage each year due to heating equipment fires.
Electric duct heaters are a hidden source of heat that can contribute to the statistics.
Types of electric duct heaters.
There are four primary types of heating elements used in duct heating applications.
- Open flame
- Resistive wire
- Finned or matrix heat exchanger
Each of these types has numerous variations to support the efficient transfer of heat into the air stream. Most have multiple resistive circuits connected in parallel to produce heat evenly across the surface of the heat exchanger.
To control the airflow temperature, sensors and controllers are needed to monitor the desired temperature and the flow of electricity/current to the heating elements. The system needs an sensor to detect when there is no air flow and shut off the heating elements to prevent overheating.
Applications for electric element duct heaters
- Pre-heaters & post-heaters for HVAC & ventilation systems with heat recovery (low-energy buildings)
- Enclosure heaters
- Air dryers & dehumidifiers
- Railway heating systems
- Passenger compartment heater for special-purpose vehicles
- Cockpit heaters
Four things to consider about electric duct heaters.
- Safety of electric duct heaters should be a concern.
- Distributed watt density & efficient heat transfer saves electricity
- Air Flow or pressure drop
- Ease of maintenance
Safety is a primary concern in electric duct heaters.
Electric duct heaters are placed inside air flow ducts out of sight and out of mind unless on a preventive maintenance schedule. The location and lack of visual appearance contribute to potential safety concerns.
Airflow ductwork usually has some sort of filter to trap dust and debris from accumulating inside the ducts. Air filters also need to be on a preventive maintenance schedule but often aren’t. Build up of dust on the surface of the filter can restrict airflow. Cracked or broken filters can let the dust settle inside the ductwork or on the electric heaters themselves.
Either condition can create a potential fire hazard from contact with open resistive heaters.
Electric resistive wires have long been the cheap standby for resistive heating in home and commercial HVAC systems. Unfortunately, to produce heat, the resistance to current in the wire produces a glowing filament that can burn dust particles and create a fire in the presence of a build-up of dust.
Newer electric heating elements are enclosed in a protective covering preventing the heating element from coming into contact with dust or other flammables. The safest of these types of heating elements are the PTC, positive temperature coefficient devices. No combustion occurs in the creation of heat, and no potential for igniting flammable materials.
The PTC heating elements also do not heat beyond a set temperature, preventing overheating of the elements and damage to surrounding materials. Learn more about the evolution of PTC heaters.
Air heaters should be electrically insulated from surrounding materials and human contact. More powerful heaters have higher voltages up to 600 V AC. Any heating elements from 24 Volts up the maximum available need to be electrically isolated.
Transfer of heat and watt density
Many homes and commercial supplementary heating systems could be more efficient. Resistance wire coiled across the airflow inside a duct transfers heat to the air passing over the elements inefficiently.
Tubular heaters are used because the tubes can be bent to improve the heat transfer to the passing air. Although better and safer than resistance wire, they still aren’t an efficient way to transfer heat to the air passing through the duct.
Heat transfer structures such as fin or aluminum matrixes where the air flows through a cross-section of a heated surface are much more efficient. Heating elements are embedded in these heat transfer configurations so that heat is evenly distributed across the entire surface.
Smaller solid-state devices like PTC heating elements have a high watt density for their size and are typically mounted in multiple locations for effective heat transfer.
The more effective the heat transfer from the heating element to the surface of the heat transfer configuration, the better heat transfer to the air flowing across the assembly.
The electrical theory behind PTC elements & connections
Heat is created when current meets resistance in an electrical circuit. The voltage in heating circuits is constant with the amount of heat generated based on the resistance to the current flow. Most heating elements have a fixed resistance, and unless the voltage is varied, the current will remain constant, as will the heat produced by the circuit.
Resistance is added or subtracted from the circuit to vary the heat produced, or voltage is changed or shut off. For this type of circuit to work, there must be a temperature sensor and controller. The temperature sensor senses the temperature of elements or output air of a duct system and controls the input voltage.
It is Important to Understand Series or Parallel Circuits for Heating
Both series and parallel circuits are used for duct heating. Each type of circuit requires similar sensors and controls, but current and output wattage can vary considerably.
A series circuit has current flowing through the resistance along one path regardless of the AC/DC circuit. The path may have multiple resistors to create heat, but the total resistance is constant and equal to the individual resistance added together.
If 3 150 ohm resistive heating elements are connected in series, the total resistance is 450 ohm. In this configuration, the current is constantly based on ohms law or the formula. E = IR or I = E/R. If the voltage is 240 V and the resistance is 450 ohms, the current will be 240/450 = 0.533 amps.
If only one 150 ohm element exists, the current will be 240/150 = 1.6 amps.
One 15-ohm resistor will draw 16 amps.
Resistors connected in parallel react much differently to current. The total resistance of elements connected in parallel is less than any single resistor in the circuit.
1/RT= 1/R2 + 1/R2 + 1/R2 | 1/150 = 0.0066 | 1/RT = .02 for a total resistance of 50 ohms.
The current in the parallel circuit is 240/50 = 4.8 amps.
So what is the difference between the two circuits? The power output.
The power output in the first series circuit is P = E x I or 240 V X 1.6 Amps = 384 Watts.
The power = 240 V x 4.8 Amps = 1,152 Watts in the parallel circuit.
More watts = more heat output.
What does watt density have to do with electric resistive heaters
Watt density is usually expressed in watts per square inch of surface area. The higher the watt density, the more heat is produced per square inch of surface area. Electric heating elements in ductwork have limited space to produce heat and transfer it to the air stream. Heating elements with higher watt densities can produce more heat in the limited space available.
Depending on the type of heating element, higher watt densities can shorten the life of the heating element by producing too much heat and deteriorating the heating element over time, shortening its life span. Different types of metals used in heating elements can produce and withstand higher watt densities.
PTC, solid-state heating elements produce high watt densities with little or no deterioration over time. Their compact size contributes to the high watt densities per square inch. These micro heating devices are embedded into heat transfer mediums such as aluminum fins or honeycomb structures that transfer the thermal energy into the air passing through them.
Since the PTC elements are tiny, multiple elements balance the heat across a series of fins or honeycomb structures. One point of heat would not efficiently or evenly distribute heat across a large area needed inside a heating duct.
A typical heating element placed inside ductwork may have six or more PTC heating elements connected in parallel. FAQs on air heaters.
What you won’t learn from electric resistive heating formulas.
The dynamics of electrical circuits can be more than meets the eye with formulas.
If one resistor fails when several are connected in series the circuit is interrupted and none of the heating elements will work.
If one resistor fails when several are connected in parallel the remaining elements will continue to work and supply heat.
If one resistor connected in parallel has a lower ohm value than the others it will carry more current and potentially create a spot hotter than the others. This can cause uneven heating or even damage the resistive heater.
PTC heating elements connected in parallel dynamically adjust the current and balance the heat output. Since each heating element resistant and current changes with temperature the elements will continually balance the current load and watt density output of the heater.
Air resistance and pressure drop due to heating elements
A challenge of electric duct heating is efficient heat transfer with low air pressure drop. The design must allow for maximum air flow and maximum heat transfer.
Tubular and electric resistive wire heaters offer little resistance to air flow but have poor heat transfer characteristics. Aluminum fin and honeycomb structures offer excellent heat transfer but higher resistance to air flow. This is especially true if dust and other particles are allowed to build up on the finned matrix.
Understanding and designing for the lowest pressure drop and best heat transfer across the surface of the heating element is essential. Talk with a experienced thermal engineer is you have questions about your application.
Ease of maintenance
Modern PTC solid state heating elements are durable and require little or no maintenance. However when used in duct or pre-heating applications where there is potential for dust and dirt in the air stream periodica cleaning is advised.
The PTC heaters are sandwiched between aluminum fins in a typical air duct heat exchanger. The heating module can be removed for periodic cleaning and inspection with ease. More sophisticated systems may use air flow instrumentation to identity changes in the air flow calling for inspection of the heating elements or changes of air filters.
Review typical PTC specification sheet or all for assistance.
Call to speak with a thermal management engineer.