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CTMS RM4

Grayson’s RM4 Complete Thermal Management System (CTMS) is the multi-function rooftop-mounted reversible heat pump system developed for electric city buses and coaches operating in moderate-to-warm climates.

Delivering integrated cabin HVAC, battery thermal management, and optional power electronics cooling in a smaller rooftop footprint, the RM4 provides efficient, intelligent, multi-loop control without unnecessary mass or capacity. High-voltage compatible and CAN-controlled, it offers OEMs a smart solution for vehicles requiring complete thermal management.

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Overview

The RM4 is the smallest and lightest member of Grayson’s CTMS rooftop platform, designed for OEMs seeking efficient multi-loop thermal management in mild-to-warm ambient conditions.

The system combines reversible heat pump HVAC, battery thermal management (active cooling, optional passive cooling, and heating), and optional traction motor/power electronics cooling within a single integrated module.

Optimised for moderate ambient conditions, the RM4 delivers up to 30kW cooling at 35°C and up to 30kW heating through its reversible heat pump. With CAN J1939 control, R454C low-GWP refrigerant, and Grayson’s intelligent thermal control software, the RM4 maintains precise set-point performance while minimising energy consumption.

Weighing just 235kg and using a two-fan/four-blower configuration, the RM4 suits 8-12m buses and applications where mass, efficiency, and simplicity of integration are key.

Features

This product brings together features that support multiple operational use-cases.

  • Complete Multi-Loop Integration: Combines battery thermal management, driver/passenger HVAC, and power electronics cooling within a single intelligent module.
  • Reversible Heat Pump for Heating & AC: Efficient thermal control across HVAC, battery, and powertrain systems.
  • Battery Thermal Management Modes: Maintains stable battery temperatures in varied duty cycles via heating and active cooling as standard, and optional passive cooling.
  • Grayson Intelligent Thermal Controller: Developed and validated by Grayson for precision regulation.
  • Optional Coolant Loops for Power Electronics & Traction Motors: Protects critical drivetrain components from overheating.
  • Passive Cooling Options & Heat-Pump Efficiency: Lower energy consumption during mild ambient conditions.
  • Optional External PTC Assist: Supports heat pump output in cold-ambient conditions.
  • CAN J1939 Communication: Seamless integration with pumps, heaters, and sensors.
  • Dynamic Control Software: Continuously adjusts operation to match vehicle thermal demands.
  • High-Voltage Compatibility: up to 900V DC, meeting the needs of next-generation electric platforms.
  • Low-GWP Refrigerant Option: R454C compatibility, supporting reduced environmental impact and sustainable operation.
  • Flexible for Global Markets: Suitable for 8 m – 24 m bus lengths and LHD/RHD compatible.
Technical

Every technical element plays a role in delivering reliable and predictable system performance.

  • Nominal Cooling Capacity: 28kW @ 35°C
  • Maximum Cooling Capacity: 30kW @ 35°C
  • Nominal Heating Capacity (Heat Pump): 24kW @ 0°C
  • Maximum Heating Capacity (Heat Pump): 30kW
  • Operating Temperature: -20°C to +35°C
  • High Voltage Range: 540–900V DC (Nominal 800V)
  • Low Voltage Range: 18–32V DC (Nominal 28V)
  • Refrigerant: R454C
  • Dimensions (L × W × H): 2450 × 2000 × 365 mm
  • Weight: 235 kg
  • Communication: CAN protocol: J1939
  • Ingress protection: IP67 (controller box)
Cooling Capacity

Up to 30kW @ 35°C ambient

Efficient cooling for moderate climates.
Heating Capacity

Up to 30kW (Heat Pump)

Reversible heat pump provides strong heating without excessive HV demand.
Voltage Range

540–900V DC

Compatible with modern electric bus architectures up to 900V DC.
Airflow Capacity

4500 m³/h blower output

Fast cabin cooling in warm conditions.
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Compatible Applications

Product Benefits​

See why leading OEMs are selecting the RM4 CTMS for next-gen electrified vehicles.

Lightweight, Space-Efficient Design

Smaller footprint and lower weight helps maximise range.

Optimised for Multi-Loop Thermal Control

Manages cabin, saloon, battery, and drivetrain loads in a single integrated module.

Efficient & Intelligent Operation

Advanced control logic reduces energy consumption and maintains system stability.

Simplified Integration

Rooftop packaging, CAN control, and standardised architecture.

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Discover More About Grayson

For over four decades, Grayson Thermal Systems has provided innovative heating and cooling solutions to vehicle operators worldwide. Our expertise ensures dependable operation and long-term system efficiency.