Inline Electric Duct Heaters
Specifying duct heat for a new building, adding electric heat to an air handler or make-up air unit, or heating process air? We handle it.
We supply standard and custom electric duct heaters, open coil or finned tubular, slip-in or flanged, matched to your kW, voltage, duct size and controls.
We size it, quote it, and return a selection ready for submittal. Our offices are in Houston and Dallas-Fort Worth, serving Texas and the Gulf Coast, and we ship nationwide.
Have questions? Need a quote?
Key Takeaways
Electric Duct Heater Types
We supply six types of electric duct heater, from standard open coil units for HVAC ducts to explosion-proof and marine duty heaters.
| Type | Built for | Range |
|---|---|---|
| Open coil, slip-in and flanged (Indeeco QUA, QUZ) | Standard HVAC ducts and air handlers | Up to 456 kW, through 600 V |
| Finned tubular duct heaters (Indeeco TFXU, TFZU) | Wet, dusty or high-voltage ducts, and heaters near grilles or access doors | Sheathed elements up to 600 V |
| Open coil, round and rectangular (QMark) | Round duct and smaller commercial ducts | Round 6, 8 and 10 in, rectangular up to 24 x 18 in, 120 to 480 V |
| Marine duty duct heaters | Ships and offshore vessels | UL Listed, ABS Approved, meets U.S. Coast Guard requirements |
| Explosion-proof duct heaters | Classified hazardous locations | Class I and II, Division 1 and 2, up to 1,000 kW |
| Custom duct heaters | Large air handlers and process air | 0.1 kW to over 900 kW in one heater, multi-section to 2,000 kW, face areas up to 40 x 14 ft |
Common Questions
How do I size an electric duct heater?
An electric duct heater is sized from airflow and temperature rise. The kW equals CFM times the rise in °F, divided by 3,193. For example, 1,000 CFM heated by 30°F needs 9.4 kW. Pick the next standard rating up, then check that the air velocity in the duct meets the heater’s minimum.
How much airflow does an electric duct heater need?
An electric duct heater needs a minimum air velocity, set by the kW per square foot of duct and the entering air temperature. A 10 kW open coil heater in a 24 x 12 in duct with 65°F air needs at least 310 FPM. At 1,000 CFM that duct runs 500 FPM, which passes.
Does an electric duct heater need clearance from combustible materials?
The Indeeco and QMark duct heaters we supply are UL Listed for zero clearance to combustible surfaces, so no minimum distance applies around the heater or its duct section. The terminal box still needs service access. The NEC requires a clear workspace at least 30 in wide and 42 in deep in front of it.
Inline and Duct Mounted Heaters
An inline electric duct heater sits inside the ductwork and heats the air the system fan moves. Unlike a unit heater, a duct heater has no fan of its own.
Inline Duct Heaters
Inline duct heaters and inline air heaters go into the supply duct of an air handler, make-up air unit or rooftop unit, for primary heat or reheat. A duct with no fan needs a fan forced heater.
Slip-In and Flanged Duct Heaters
Slip-in heaters slide through an opening in the side of the duct and suit ducts up to 4 ft wide. Flanged heaters bolt between duct flanges for larger ducts.
Round Duct Heaters
Round duct heaters come in 6, 8 and 10 in diameters for flex duct.
Built-In Controls and Safety
An electric duct heater needs two overtemperature protections and an airflow interlock under UL and NEC rules, and the duct heaters we supply ship with them built in.
Since June 2009, UL has also required disconnecting contactors on every open coil duct heater.
- Thermal cutouts – an automatic reset cutout for primary protection and a manual reset cutout for backup
- Airflow proving switch – a differential pressure switch that keeps the heater off until the fan is running, or a fan relay interlock
- Contactors – magnetic or disconnecting contactors for each heater stage
- Disconnect switch – built-in, snap-acting and door interlocked, so the heater cannot be opened while it is live
- Control transformer and fusing – 24 or 120 V control circuit, with fuses on heaters over 48 amps
- SCR control – proportional temperature control in place of staged contactors
With the controls in the heater’s terminal box, the heater arrives as one listed assembly with less wiring to do on site.
Related Heating and Air Handling Equipment
- Make-up air units – Tempered outdoor air for kitchens, plants and warehouses
- Finned tubular heaters and elements – Sheathed elements for ducts, ovens and dryers
- Explosion-proof heaters – Electric heat for classified hazardous locations
- Industrial heating elements – Replacement and OEM elements for heaters and process equipment
- Door heaters – Warm air at dock doors and building entrances
- Industrial immersion heaters – Heating for water, oil and process liquids
- Tank and pipe insert heaters – Heaters that install through a tank wall or pipe fitting
- Industrial heating specification guide – What to define before you specify a heating system