The five-constant system hardware is divided into four main parts: the five-constant air source heat pump main unit, water system heat exchange terminals, fresh air handling unit, and intelligent dew point control system. The hardware components are deeply connected through water pipes and electrical control lines, forming a complete closed loop.
Temperature and humidity decoupling
The five-constant main unit uses a four-pipe output, allowing it to simultaneously supply two different temperature mediums. One line of medium-temperature water is sent to the floor radiant terminals, while one line of low-temperature water goes to the fresh air dehumidifier and fan coil terminals, so temperature and humidity loads are handled separately.
Ground Radiation Terminal
Ground radiation heat exchange doesn't rely on blowing air; it relies on thermal radiation exchange between the ground surface and people or walls to regulate room temperature. From the piping diagram, you can see that each branch of the manifold is connected in parallel to the four-pipe system of the main unit. Upstream of each branch, there are valves, a buffer storage tank, and a motorized three-way valve. The buffer tank helps stabilize the branch water pressure, absorb water hammer impacts, prevent frequent starts and stops of the main unit, and ensure stable flow in the radiation water circuit.
Fresh Air and Dehumidification All-in-One Unit
The fresh air and dehumidification all-in-one unit is the core of the whole system for handling latent heat loads (humidity). The blueprints have strict installation requirements: the unit itself must be installed in the kitchen or bathroom ceiling and cannot be installed in bedrooms or living rooms.
Many traditional Five-Constant projects use standalone refrigerant-based dehumidifying fresh air units, which require separate power and maintenance. In this Five-Constant plan, the fresh air coil uses a water system, and all the heating and cooling sources come from the Five-Constant heat pump main unit, so no extra cooling compressor is needed, making the system more integrated. The fresh air unit's water circuit is also equipped with an electric three-way valve, and the control system can adjust the water flow into the fresh air unit based on indoor humidity signals to regulate dehumidification capacity. When humidity is high, the flow is increased; once humidity reaches the target, the water flow is reduced.
| Comparison Dimension | Dual control dual supply | Four-Control Five-Constant System |
| Water supply capacity | Only one water temperature at a time | At the same time, it outputs both medium-temperature water and low-temperature water. |
| Summer ground cooling achieved | If the underfloor heating uses cold water, dehumidifying and cooling will conflict, making condensation very likely. | Medium-temperature water is used for radiant temperature control, and low-temperature water is dedicated to dehumidification, operating independently from each other. |
| End adapter | The floor heating is used for warming in winter and left idle in summer, with cooling relying on fan coils. | Radiation, fan coil, and fresh air can all run independently at the same time |
| Load distribution | All the temperature and humidity loads are handled by the air handling unit. | Sensible heat goes to radiation, latent heat goes to fresh air, temperature and humidity are decoupled |
1. The five-constant heat pump main unit starts in cooling mode with dual-circuit synchronous output: one circuit sends medium-cold water to the manifold branches; the other sends low-temperature water to the fresh air unit and the living room fan coil branches.
2. The medium-temperature chilled water is distributed to each room's floor heating pipes through the manifold. The floor surface creates a radiant cooling effect that absorbs indoor sensible heat, lowering the room temperature. Room electric actuators open or close the branch water circuits based on thermostat signals to achieve individual room temperature control.
3. Low-temperature chilled water enters the fresh air coil. Outdoor high-humidity air passes through and condenses the moisture, producing dry air that is delivered to each room, reducing overall indoor humidity and lowering room dew points to create safe operating conditions for floor radiation.
4. When the living room's instantaneous heat load is too high, the fan coil branch is activated, allowing low-temperature water to flow through the coil, and convective cooling handles the excess heat load. Once the load drops, the fan coil reduces speed or stops.
5. All end return water is returned to the heat pump main unit via two return pipelines, completing the water circulation. Branch buffer tanks stabilize the hydraulic conditions.
Using a four-pipe water system to achieve dual-temperature water supply is the hardware foundation, decoupling temperature and humidity is the theoretical core, and multi-stage dew point anti-condensation interlock protection is the system safety baseline. In implementing high-end residential HVAC projects, it's important to strictly follow the design schematics, carefully select equipment, construct piping correctly, plan wireless communications, and fully commission the system. Only then can the five-constant system deliver its human-friendly benefits like no draft, controllable temperature and humidity, and low noise, while avoiding the engineering risks of condensation and water leakage in ground cooling radiation mode.
