Active airflow trench heating uses fans to push air through the heat exchanger, rather than relying on natural convection alone. SPC’s new Metropolitan HC range applies this principle with crossflow fans driven by direct-coupled brushless DC (EC) motors, giving higher output, faster response and heating and cooling flexibility from the same footprint as a traditional trench.
Why Metropolitan HC has been added to the range
Older natural-convection trench heaters rely on the stack effect (sometimes called the chimney effect) to move air through the coil. The heating coil warms the air in the trench unit; that warmed air becomes buoyant and rises out of the top grille, and cooler room air is drawn in through the lower intake to replace it. It is a passive, silent process, but the driving force is small. Low air velocity through the heat exchanger means a lower convective heat-transfer coefficient, so for a given coil size and water temperature, output is modest.
SPC’s Metropolitan HC range is built to overcome this limit. By adding fans to push air across the coil, it moves significantly more air than buoyancy alone allows, increasing the rate of heat transfer without increasing the size of the trench.
How does Metropolitan HC deliver active airflow?
Metropolitan HC uses crossflow fans driven by direct coupled brushless DC (EC) motors to actively push air through the heat exchanger. This design holds output consistent along the length of the run and responds predictably when fan speed is adjusted via a 0-to 10V signal. The design incorporates an innovative separation plate to keep the inlet and outlet separate and prevent short-circuiting (where outlet air is drawn straight back into the fan intake).
Key specifications:
- Copper tube/aluminium finned coil heat exchanger
- 15mm copper flow/return connections
- Maximum hot water flow temperature of 85°C, minimum chilled water flow temperature of 5°C
Performance tested to EN 16430, the European standard for fan-assisted radiators, convectors and trench convectors.
What are the benefits of active airflow trench heating?
- Higher output from the same footprint. Moving more air across the fins increases the rate of heat transfer, so Metropolitan HC can deliver more output than an equivalent natural-convection unit without taking up more floor space.
- Faster response. Forced airflow means a room reaches its set point more quickly than it would with buoyancy-driven airflow alone.
- Low-temperature compatibility. Because active airflow increases airside heat transfer, Metropolitan HC can meet design loads even when supplied with lower-temperature hot water (LTHW) from a heat pump or condensing boiler.
- Heating and cooling from one trench. Active airflow allows Metropolitan HC to work as a chilled water emitter as well as a heating one, something natural-convection trench heaters cannot do.
- Predictable control. Fan speed is adjustable via a 0 to 10V signal, allowing output to be modulated and integrated with BMS.
Where does active airflow trench heating perform best?
Active airflow trench heating suits spaces where natural convection struggles to keep up: offices and reception areas with large expanses of glazing, leisure centres and atria with high ceilings that create strong cold downdraughts, and education or healthcare buildings that need both heating and cooling from a single trench rather than two separate systems. It also suits projects where the space available for the trench is restricted but the required kW output is high.
Frequently Asked Questions
Active airflow trench heating uses fans, rather than the stack effect, to move air through the heat exchanger. This increases air velocity across the coil, raising the rate of heat transfer and allowing higher output from the same trench size.
Metropolitan HC uses crossflow fans driven by direct-coupled EC motors to actively push air through the coil, rather than relying on warm air rising by buoyancy. This keeps output consistent along the run and allows the fan speed, and therefore the output, to be adjusted via a 0 to 10V signal.
Metropolitan HC can be used with a heat pump. Because active airflow increases airside heat transfer, it meets design loads even at the lower flow temperatures typical of heat pump and condensing boiler systems.
Metropolitan HC provides cooling as well as heating from the same unit: hot water heating up to a maximum flow temperature of 85°C, and chilled water cooling down to a minimum flow temperature of 5°C.
Metropolitan HC is performance tested to EN 16430, the European standard covering fan-assisted radiators, convectors and trench convectors.
Specify Metropolitan HC on Your Next Project
Full performance data by size, length and fan speed is available in the Metropolitan HC brochure. To discuss specification for your project, or to request the brochure, contact the SPC team: