Direct expansion system planning

In a direct expansion system, the working fluid from the heat pump circulates in the ground collector as a heat transfer medium and evaporates there. This direct expansion produces very high performance figures and excellent operational reliability, because there is no need for an intermediate heat exchanger and brine circulation pump. In a direct expansion system the heat source system is designed as a horizontal collector. The collector is also referred to as the evaporator circuit. It is made from 75-metre continuous copper piping sheathed in polyethylene (PE). The installation may have to be registered or may require consent, depending on local regulations.

The area of ground to be used for installation of horizontal evaporators must be undisturbed and not banked up on one side. Otherwise any settlement of the ground could damage the pipes. The higher the moisture content of the soil, the higher and better the heat transfer. So in damp, clayey soil a smaller area of ground will be needed for heat extraction than in dry, stony soil.

The area of ground chosen for heat extraction must be undeveloped but may be planted. Rainwater is important for regenerating the soil. Backwater and flooding (e.g. on sloping sites or clayey soils) must be avoided because of the risk of ground instability and structural damage. Drainage must be provided if necessary. Allow the ground to settle before starting heat extraction.

The size of the heat source system is determined by the cooling capacity of the heat pump and the specific heat extraction capacity of the soil. The laying depth is normally around 1.2 m, but may be lower in some regions. The spacing between the pipe loops must be calculated. For standard collector tubes it is normally between 40 and 60 cm.

The pipes are generally laid on level, horizontal subsoil or with at most one lateral slope (sloping site). On sloping sites the ground collector tubes must always be laid perpendicular to the slope.
Sloping site: The difference in height between the highest and lowest point of the collector must not exceed 5 metres.

Make sure that the pipes are not kinked or compressed. Before covering with soil, protect the ground collector tubes with a bed of sand. Care must be taken when replacing the soil. Warning tape should be laid about 50 cm above the collector tubes.

With large-scale collector systems or if the collector system is not directly adjacent to the wall duct, a collecting chamber must be provided. This holds the distributor and the collector. The collecting chamber can be built conveniently using commercial concrete rings.
The connecting line between the building and the collecting chamber must be laid inside a penetration sleeve and be thermally insulated. This penetration sleeve must be laid straight and with a slight incline towards the collecting chamber to allow any condensation to drain away. The maximum connection length is 20 m.

• The max. height difference between the distributor in the collecting chamber and the heat pump is 5 m
• The collecting chamber must be at the lowest point

A minimum clearance of 1.5 m from the water supply mains is required. A minimum clearance of 1 m from drains is required. If pipes are laid parallel to buildings, a safety clearance of around 1.2 m should be allowed. (Prevent frost damage.)
If these safety clearances cannot be complied with, special measures must be taken: Insulate the structure to be protected with a closed-pore thermal insulation material.
Avoid building on top of the installation if possible, or otherwise insulate the pipes generously in the affected area (again with a closed-pore thermal insulation material). The point at which the collector tubes enter the house must also be insulated.