After checking the ground heat exchanger for tightness, installers should take the following testing steps:
- Flush debris and purge air from the ground heat exchanger;
- Verify pressure drop and flow rate;
- If pressure or flow do not check properly, look for possible flow restrictions;
- Fill the system and add antifreeze if required; and
- Pressurize the ground loop piping system (40 psi minimum). In summer, the pipe expands faster than the fluid, creating a loss of pressure or even a vacuum. The minimum pressure is 15 psi - upon startup, loops should be pressurized to no less than 40 psi to allow for temperature changes.
Flushing and purging
Before operating the system, it should be flushed with water to remove debris, such as the pipe shavings, dirt or other foreign objects. While they may not harm the piping, they can damage the pumps or restrict the water flow.
The ground loop system must be purged of any trapped air, which can corrode metallic components and cause failure. Excessive air can also block water flow in some branches.
A water flow of two feet per second in a properly designed piping system will completely remove any trapped air. The flow rate to achieve that velocity will depend on the size of the pipe used.
In parallel systems, the header flow rate decreases as the flow is branched down each loop. Each section of the header system should be checked to ensure that it satisfies the required velocity condition.
Required flow rates for typical pipes sizes and types
The circulating pump used in residential systems will not provide enough power to reach the required flow rate for purging. A temporary external high volume, high head pump unit should be used for this.
Some water pumps are supplied as modules, designed to facilitate flushing and purging. The purging unit is connected to the loop using flexible rubber hoses and clamps.
The purging unit is connected so the loop can be flushed independently of the heat pump. This reduces the chance of debris from the loop getting into the smaller tubing in the heat pump.
The pressure gauge and flow meter is used to verify the pressure and flow rate. The system is purged until air bubbles no longer appear in the reservoir.
If antifreeze is used, it is added at this point. Use only the amount recommended. More is not better. It increases the pumping cost and decreases system efficiency.
When flushing and purging is complete and the loop is filled with antifreeze, the system is pressurized. The first valve that discharges into the reservoir is closed and the hose removed. This causes pressure in the system to increase as the flush pump continues to operate.
The second valve is closed and the flush pump stopped when the system pressure reaches the desired level. Initial system pressure must be no less than 40 pounds per square inch.