AutomatedBuildings.com Column - Electric Duct Heaters

Electric heating coils (duct heaters) are constructed of electric resistance heating elements, which are placed in the airstream (of an upstream air handling unit). When an element is given power, it heats up and glows. The heat from the element is transferred to the air passing over it. Electric heaters are typically staged equipment, with each stage consisting of one (or more) heating elements. A stage of heat is accomplished by giving power to the heating element(s) that make up the stage, by means of a contactor. When the contactor is energized, power is allowed to the element(s), and the air passing over is heated up accordingly. A simple two-stage electric duct heater is made up of two electric resistance heating elements, two contactors, and a control transformer and terminal strip. The terminal strip serves as the point of interface for external control of the heater. A simple two-stage heating thermostat can be wired to the terminal strip, to operate the heater. Other components found in a typical electric duct heater are a control circuit fuse, an air proving (pressure) switch, and a high limit thermal cutout switch. The air proving switch prevents heater operation unless there is sufficient airflow through the heating coil. The high limit cutout switch prevents the heater from getting dangerously hot.

Electric duct heaters are sized to increase the temperature of the air entering the heater to some calculated value. In sizing and selecting an electric heating coil for an application, the mechanical designer will run a load on the zone to be served, and come up with the BTUH requirement, as well as the CFM. By plugging these values into the “airside” equation for BTUH, which is BTUH = 1.08 x CFM x delta T, the total required delta T can be found. Delta T simply refers to the increase in temperature that the heater can impart to the air passing through it. Delta T’s can range, depending upon the application, from 15-30 degrees, as in the typical electric duct heater zoning application, to upwards of 80 degrees or more, as in a make-up air application where the entering air temperature can be below zero.

Electric heater capacities are not normally categorized in units of BTUH. The term used to describe electric heater capacity is kilowatts (thousands of watts). The formula for converting BTUH to KW is KW = BTUH / 3410. For the sake of putting some numbers to all of the theory going on here, let’s imagine that an engineer has performed a load calculation on a space, and found that 30,000 BTUH is required to heat the space under worst case conditions. Using the above formula, we find that the KW of the required heater is 8.79. Since heaters are typically offered in discreet sizes, he will likely specify the next largest size, assume 10 KW. Now per his design criteria, the engineer has also specified that the volume of air to be delivered into the space is to be a constant 1000 CFM. In order to find the actual delta T of the heater, we must convert kW back to BTUH, and then plug this value, along with the CFM value, into the aforementioned “airside” equation. This yields a delta T of approximately 31.6 degrees.

Determining the number of stages of electric heat requires looking at the total delta T of the particular heating coil, as well as considering the degree of control required. The more stages the merrier! But seriously folks…practically speaking, for precise space temperature control, the delta T per stage should be generally no more than 5 degrees. For mediocre control, the delta T per stage can be in the range of 6-12 degrees, and for coarse control, 13 degrees and up. The rule of thumb is, 10 degrees per stage, for mediocre control. Note that we’re talking about space initiated temperature control. For discharge air control of an electric heating coil, the temperature control process generally needs to be a bit more precise. This of course depending upon the application (we’ll leave it at that!).

So anyway, for the above-mentioned example, selecting a three-stage electric heater would yield a per-stage delta T of 10.5 degrees, which puts us in the middle of that “mediocre control range”. For a more critical application demanding more precise control, the heater should be specified to have more stages. For instance, a six-stage heater would yield a per-stage delta T of a little more than 5 degrees, which lands on the upper cusp of that “precision control range”.

The whole origin of the “mediocre control” rule of thumb stems from the notion that the larger the delta T per stage, the more noticeable the difference in supply air temperatures is when staging takes place. With a 10-degree delta T per stage, the occupant may be likely to feel the difference when a stage of control is added or dropped out. The larger the difference is, the more likely he/she is to feel the difference. This is normally an undesirable and often an unacceptable condition for HVAC comfort control. For more critical applications, the delta T per stage should be lessened, by increasing the number of stages.

It is sometimes mistakenly perceived that the larger the electric duct heater the more stages of control are needed. From what has been presented above, we see that the number of stages required is solely a function of the total delta T and the desired delta T per stage. A 20 KW duct heater handling 3,150 CFM of air yields the same delta T as a 3 KW heater handling 475 CFM of air: 20 degrees. Thus, given the same application, both of these heaters can be supplied with simple two-stage control.

[an error occurred while processing this directive] Now for some “hard and fast” rules for controlling the stages of an electric duct heater:

For stand-alone control up to two stages – a simple heating thermostat will do the trick, with the appropriate number of heating stages required for the job (one or two).

For stand-alone control with more than two stages – a factory-furnished step controller is the best option, unless you can find a multistage thermostat or temperature controller to suit your needs that has the staging capability that you require (there are some practical limitations to this). Step controllers are multistage temperature controllers or “sequencers”, requiring an input signal from a space temperature sensor. The step controller monitors this signal and determines the appropriate number of stages of heat to energize.

For direct digital control (DDC) – a factory-furnished step controller is not required, as long as your digital controller has enough binary output points available to accommodate all of the stages of electric heat. This may not be a very economical use of binary points, depending upon the number of stages. It may be more cost-effective to buy the heater with a multistage step controller, utilize an analog input on the digital controller as a space temperature input, and utilize an analog output on the digital controller to feed the step controller. Though in this case the digital controller isn’t performing the actual staging of electric heat, it at least allows for the heating setpoint to be established (and processed) via the Building Automation System (BAS).

For high-precision applications – SCR (Silicon Controlled Rectifier) controllers are proportional type electronic temperature controllers. An alternative to the step controller, the SCR controller offers precise temperature control for those applications that demand it. Like the step controller, the SCR controller requires a signal from a space temperature sensor, or from an analog output off a digital controller. Unlike the step controller, the SCR controller actually varies the average power output of the electric heater, as a whole, in proportion to the deviation in space temperature from setpoint. SCR controllers are available as factory installed control options for electric duct heaters.

Tip of the Month: Control a three-stage electric duct heater using only two binary outputs from a digital controller. Have one output (output #1) “fire off” one stage of electric heat, and have the other output (output #2) “fire off” the other two stages, both at once. For one stage of heat, activate output #1. For two stages of heat, activate output #2, and de-activate output #1. For three stages of heat, re-activate output #1. This works on two conditions: 1-the type of programming can accommodate the required logic. 2-there is no restriction on which stage of electric heat is first, second, etc. If these criteria aren’t met, then you need to provide the appropriate “outboard” logic with relays and additional wiring…a more tedious task indeed.

To control three stages of electric heat with a two-stage heating thermostat, incorporate a time delay relay for the third stage. On a call for stage one heating by the thermostat, stage one electric heat is activated. On a call for stage two heating by the thermostat, stage two electric heat is activated, and the time delay relay is energized, thus initiating the time delay period. After the time delay relay “times out” (providing that the thermostat is still calling for stage two heating), stage three electric heat is activated. When the call for stage two heating by the thermostat is satisfied, both stages two and three of electric heat are cycled off, and the time delay relay resets. Simple, yet effective!

Steve Calabrese earned his BSEE degree in 1990 from the University of Illinois at Chicago (UIC). He has since spent much of his professional career working for a mechanical contracting company, in various roles including mechanical systems design, control systems design, project management, and department management. Currently employed by a large Chicagoland controls company, Steve couples his broad mechanical knowledge and experience with a strong background in the area of electricity and electronics. His control systems expertise includes electrical and electronic stand-alone controls, as well as microprocessor-based direct digital controls (DDC) and networked Building Automation Systems (BAS). You can visit his website at www.pcs-engineering.com.

In 2003 Steve’s book, Practical Controls: A Guide To Mechanical Systems, was published. Geared toward the HVAC professional, the book details practical methods of controls and defines the role of HVAC controls in an easy-to-understand format. Steve brings his mechanical and controls contracting experience to this writing, and offers practical approaches to control systems issues.

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