EN
Cold starts are a routine winter problem for truck, bus and equipment fleets. Low temperatures increase oil viscosity, reduce battery performance and slow the engine’s warm-up process. The result can be harder starting, longer idling time and greater disruption at the beginning of a shift. A diesel-fired coolant heater, also called a hydronic heater, preheats the engine cooling circuit before the engine starts and can reduce this cold-start burden without requiring a mains electrical connection.
Why Cold Starts Create Problems for Fleet Operations
When an engine is very cold, the starter and battery must work harder while lubricating oil circulates more slowly. Combustion quality is also poorer until the engine reaches a suitable operating temperature. In a fleet, a few extra minutes per vehicle can become a significant amount of lost operating time across an entire winter season.
Many operators compensate by idling engines before departure. That uses fuel, adds engine hours and can conflict with local anti-idling requirements. Preheating the engine before the shift is therefore not only a comfort measure; it can also be part of a fleet’s cold-weather operating strategy.
How a Diesel Coolant Heater Works
A diesel coolant heater burns a small amount of vehicle fuel in its own combustion chamber. Heat is transferred through a heat exchanger into the engine coolant, while an electric coolant pump circulates the warmed fluid through the engine circuit. Depending on the installation, the same heated coolant can also pass through the vehicle heater core to support windscreen defrosting and cabin heating.
The heater normally uses the vehicle’s 12 V or 24 V electrical system for the controller, pumps, fan and glow pin during startup. It is therefore different from an electric immersion or block heater that plugs into a 120 V or 230 V mains supply. This independence from shore power is one of the main reasons diesel coolant heaters are useful for trucks, buses and equipment parked away from fixed facilities.
Diesel Coolant Heaters vs. Glow Plugs and Electric Block Heaters
Glow plugs: Glow plugs help a diesel engine start by heating the combustion area during the starting sequence. They do not preheat the complete coolant circuit or engine block in the same way as a hydronic heater.
Electric block or immersion heaters: These use an electric heating element to warm the engine block or coolant. They can be effective where a suitable mains power supply is available, but they depend on the vehicle being parked close to an electrical connection.
Diesel coolant heaters: These use the vehicle’s own fuel and low-voltage electrical system to heat and circulate coolant. They can be started by a timer or remote control and are well suited to vehicles that operate from depots, yards or remote sites without shore power.
Fleet Benefits of Preheating the Coolant
• More reliable starting in low temperatures by reducing the thermal load on the engine at startup.
• Faster warm-up and windscreen defrosting when the coolant circuit is integrated with the vehicle HVAC system.
• Less need for extended warm-up idling before the vehicle enters service.
• Reduced cold-start stress on lubricated engine components compared with starting from a very low temperature.
• More consistent departure times across a fleet during winter operations.
Choosing the Right Heater for a Fleet
Heater output should be matched to the engine size, coolant volume, ambient temperature and required preheat time. A 5 kW hydronic heater is a common choice for light commercial vehicles and smaller coolant circuits, while 10 kW and larger systems may be more suitable for heavier vehicles or greater coolant volumes. These are only starting points; the complete thermal system must be checked before selection.
Also confirm 12 V or 24 V compatibility, coolant type, hose size, pump capacity, fuel connection, installation position and altitude requirements. If the vehicle is intended for EU road use, the relevant UN R122 and R10 approval scope should be checked for the exact heater configuration.
Installation Considerations for Mixed Fleets
Standardizing one heater family across a mixed fleet can simplify spare parts, controls and service training, but the installation kit should not be assumed to be identical for every vehicle. Different engines may require different coolant connection points, hose routing, mounting brackets, fuel pickup arrangements and control interfaces.
For fleet projects, the best approach is to standardize the heater platform where possible and then validate the installation for each vehicle group. Lavaner can support diesel hydronic heater selection and installation-kit planning for commercial and fleet applications.