Duct Heaters

A duct heater — also called an inline heater, in-duct heater, or electric heater battery — is a compact electric heating element fitted inside a round ventilation duct to warm supply air before it enters the occupied space. We stock the Systemair CBMT/Q range of circular inline heater kits from 100mm to 315mm duct diameter, in outputs from 0.6kW up to 5.4kW, all single phase. Each kit ships ready to install with integrated thermal cut-outs, a dedicated controller connection and spigot connections to match standard circular duct sizes. Used on supply air runs for MVHR frost protection, commercial air handling, warehouse and industrial tempering, and any application where fresh air needs warming before delivery. In stock with next-day delivery available.

Commercial Heaters / Air Curtains / Radiant Heaters / MVHR Units

What is a duct heater used for?

A duct heater is fitted inside a circular supply air duct to warm the air stream before it's delivered to the room or building. The heating element sits across the duct cross-section, and air flowing past the element is warmed. Typical uses include MVHR frost protection in cold weather, tempering fresh air supply in commercial and industrial buildings, pre-heating intake air on AHUs, warming supply air on warehouse and production ventilation, and spot heating on plant room intakes. They are not standalone room heaters — they work in conjunction with a ventilation fan that pushes air across the element.

What sizes and outputs do you stock?

We stock the Systemair CBMT/Q range in 6 duct diameters and 9 rating variants covering the full residential and light commercial range. Diameters: 100mm, 125mm, 150mm, 200mm, 250mm and 315mm. Outputs: 0.6kW (100mm), 1.2kW (125mm), 1.5kW and 2.7kW (150mm), 3.0kW and 5.4kW (200mm and 250mm), and 3.0kW (315mm). All kits are single phase 230V. For three-phase, higher-output or bespoke duct heaters, contact us on 0161 818 2050 — we can source larger units to order.

What's the difference between a duct heater, inline heater and heater battery?

All three terms describe the same product — an electric heating element mounted inside a ventilation duct. "Duct heater" is the most common UK technical term. "Inline heater" emphasises that the heater sits in line with the duct run. "Heater battery" is the traditional HVAC engineering term, borrowed from the older meaning where a "battery" referred to a bank of heating coils in an AHU. You'll see all three used interchangeably in specifications, product catalogues and installer drawings — they're the same product.

What size duct heater do I need?

Sizing depends on three things: the duct diameter (the heater must match the duct it sits in), the required temperature rise across the heater (how much you need to warm the air), and the airflow volume. As a rough guide, 0.6kW warms about 100m³/h of air by 15°C, and 1kW warms about 170m³/h by the same amount. For MVHR frost protection on a typical 3-bed house, 1.2kW to 1.5kW at 125mm or 150mm is usually sufficient. For commercial air handling, sizing should be calculated from the design airflow and required supply temperature. We're happy to help size the correct heater — email hello@efans.co.uk with your airflow and temperature rise requirement.

Do I need a controller with a duct heater?

Yes. Duct heaters require a compatible controller to regulate output and provide the safety interlocks that prevent the element overheating when airflow stops. Systemair CBMT/Q heaters are designed to work with the Systemair range of heater controllers (typically the REGO series) that provide temperature control, airflow-proving interlocks and integration with the fan or AHU. The heater must not be powered up without airflow — both thermal safety cut-outs inside the heater and the upstream controller interlock are there to enforce this. For MVHR frost protection, some MVHR units have built-in heater control outputs that can drive the CBMT/Q directly.

Can a duct heater be fitted to any direction of airflow?

Duct heaters must be fitted with the airflow direction marked on the casing matching the actual duct airflow. The element is oriented for specific airflow direction to ensure even heating and proper operation of the thermal safety cut-outs. Fitting a heater backwards will cause it to trip on overheat protection or, worse, fail without tripping. The CBMT/Q has a clear arrow on the casing — install with the arrow pointing in the direction of airflow. Minimum straight duct runs of around 500mm upstream and downstream are recommended to ensure even airflow across the element.

Are duct heaters safe to use?

Yes, when installed correctly. Every Systemair CBMT/Q duct heater includes two independent thermal cut-outs: an automatic reset cut-out that trips at a lower temperature (typically 50°C overheat) to protect against temporary blockage, and a manual reset cut-out that trips at a higher temperature (around 120°C) and requires physical reset after a fault. The heater must be installed with the airflow interlock connected so it cannot energise without the fan running. Installation must be by a qualified electrician and comply with BS 7671 (IET Wiring Regulations). Duct heaters in fire-rated ducts or compartments also need appropriate fire damper protection.

Can a duct heater be used on an MVHR system?

Yes — MVHR frost protection is one of the most common applications for a duct heater. In cold UK winter conditions, the extract air leaving the MVHR heat exchanger can drop below freezing, causing condensate to freeze and block the exchanger. A duct heater fitted on the MVHR supply inlet warms incoming fresh air above the frost threshold (typically around 1–5°C), keeping the heat exchanger operational through cold weather. Many MVHR units have a dedicated pre-heater control output that drives the duct heater automatically based on outside temperature — check your MVHR unit specification for heater compatibility.

How to choose the right in-duct heater

Selecting the correct in-duct heater starts with duct size, airflow rate, and required heating output (kW). The heater should match the duct dimensions to avoid unnecessary pressure drop and ensure even heat transfer across the supply air stream.

Key considerations include:

• Airflow volume – the heater must perform efficiently at the system’s designed airflow rate

• Voltage and phase – single-phase or three-phase depending on the application

• Control compatibility – thermostats, sensors, BMS, and time controls

• Safety features – overheat protection and airflow proving switches

• Correct specification ensures consistent supply air temperatures, lower running costs, and reliable HVAC performance. The right choice should also align with the wider commercial heater setups used throughout the building, especially where multiple heating methods are used across different zones.

Frequently Asked Questions

Are in-duct heaters energy efficient?

Yes. When correctly sized and integrated with a ventilation system, in-duct heaters provide targeted heating with minimal energy waste. They work best with modern controls and zoning strategies, reducing energy costs compared to standalone heaters.

Can in-duct heaters work with air curtains or other commercial heaters?

Absolutely. In-duct heaters complement air curtains, radiant panels, and other commercial heaters in multi-zone HVAC systems. They provide base heating within the duct network while other units address localized or entrance-specific needs.

What is the lifespan of an in-duct heater?

A well-maintained in-duct heater can last 10–15 years in commercial use. Lifespan depends on operating hours, airflow conditions, maintenance, and quality of the unit. Choosing high-quality components ensures reliability in demanding environments.

Are in-duct heaters suitable for high-traffic commercial spaces?

Yes. In-duct heaters are ideal for high-traffic areas such as shopping centres, warehouses, and office buildings. Because they are installed within the ductwork, they provide consistent heating without obstructing floor space and can handle large airflow volumes required in busy commercial environments.

Can in-duct heaters be integrated into existing HVAC systems?

Yes. In-duct heaters are designed to integrate seamlessly into existing ductwork and air handling units. When retrofitting, it’s important to match the heater’s airflow capacity, duct size, and control compatibility with the existing system to ensure even heating, minimal pressure drop, and optimal energy efficiency.