Electric Air Duct Heaters

Sheathed element air duct heaters provide a clean and safe method of air heating, which is economical to install and operate, especially if an efficient control system is employed.

The heaters described here are intended for ventilation / air conditioning applications with air leaving temperatures below 60 °C.

Electric Heater



The following heaters can be manufactured from ex-
stock components, offering a wide range of duct sizes, kW loadings and number of steps with short delivery times.

Stab in: Insertion heater comprising a terminal box with element bank sized to suit exact duct dimensions (within practical limits).

Circular duct heater: Made to suit ISO Standard spiral ducting from 100mm to 400mm, kW rating 0.5 to 18 as required.

Rectangular duct heater: Flanged duct with "stab-in" type heater section to enable removal of elements without dismantling ductwork.

These heaters can be mounted in a vertical or horizontal position. Please state position when ordering so that the thermal cut-out can be selected to suit the application.


Heaters are designed and constucted to give a considerable service life when used in mainland Europe.

The following tests are employed:

  • Elements are hot flash tested during manufacture and are stored to prevent ingress of moisture.
  • Insulation resistance - after forming bends etc.
  • Circuit continuity and resistive tolerance (-7.5% ).
  • Operation of cut out ( sample ).
  • Various mechanical inspections.
  • Records are kept of all heaters made so that replacement parts can be supplied.
  • Test certificates available on request.

Continued development of the product may result in changes to the content of this page.



  • Mesh guard on one or both faces.
  • Drilled flanges.
  • Mez 120 or 130 pre-formed flanges.
  • Weatherproof terminal box.


  • 10V proportional Controller
  • 6 Step Electronic Thermostat
  • Fan Run on Timer
  • Airflow Proof Switch
  • Thyristor Slave Units
  • Complete Control Panel for Electronic Heat Applications



Forced air duct heater comprising sheathed electric resistance elements, mounted in a pre-galvanised steel casing.

Elements consist of a nickel/chromium resistance wire, spirally wound, insulated by compacted magnesium oxide powder and fitted within a incoloy ( AIS1321 titanium stabilised ) tube. The connection bushes of each element are1/4" PSP brass crimped on.

Elements are return bent and mounted in the terminal box with airtight brass fixing glands and insulating washers.

Heaters are designed to have an element suface temperature of 400°C at an air velocity of 2m/s.

Element tails are either fitted with saddle washers for cable connection or linked with copper busbar.

Terminal boxes are made with 1.2mm pre-galvanised steel and generally conform to IP40. 25mm conduit holes and earth stud are provided.

Every heater is fitted with a high temperature safety cut out ( manual reset ). This usually comprises a bi-metallic disc which is non-current sensitive. The switch contacts are normally closed and can withstand 13A at 240V ( this cut out should be connected into the control circuit to break the supply to the heater load ).


Selection Chart


Determine loading required using chart or use the formula:

kW = Air volume ( m³/s ) x ( tout - tin ) °C x 1.23

Select duct size to ensure that air velocity is greater than 1.5m/s, read off air pressure drop from chart.
N.B. For heaters with + 25°C temperature rise use top line, for 5°C rise use bottom line

Decide upon the number of control steps ( max 5°C/step).


Airflow Chart