Water Treatment and Corrosion

In a ground water source heat pump system, the water flows through the system once. A recharge well, a surface lake, pond or stream may also be used for water disposal. Since large quantities of water are used, it is not usually economical to treat the return water. If water treatment is required, it would be more cost effective to use a ground-coupled closed-loop heat pump system.

The principal concerns with a ground water source system are corrosion, scaling, encrustation, and erosion. The quality of the source water, at installation and in the future, impacts the well performance and the life of the heat pump heat exchanger.

Before using this system, a water sample should be analyzed by a reputable water quality laboratory, and the results interpreted for pH, high undissolved solids, iron, calcium, and other minerals. The lab can also calculate the Langelier Saturation Index, which is a measure often used as an indicator of how corrosive water is.

It is also important to insure that the water is free of sand or other particulates, since these could prematurely erode the heat pump heat exchanger. Proper screening or filtering can reduce the number of particles that pass through the heat pump's heat exchanger, and thus minimize erosion. Use a well screen with a fine enough mesh to filter out known sand-sized particles found when drilling. A filter should also be placed between the pressure tank and the heat pump intake to trap any possible particles. A replaceable cartridge type, with the cartridge replaced by a screen, is suggested. Some manufacturers offer a cupro-nickel heat exchanger as an option. Cupro-nickel has a higher resistance to abrasion than copper.

If any solenoid valves are used in the water line, they should be of the slow closing type to avoid the valve sticking open due a small particle that managed to get by the filter. This will also minimize water impedance.

Scaling is the process where minerals precipitate out of the water and build up or scale on the inside surfaces of the pipes and water-to-refrigerant heat exchanger in the heat pump.

Scaling reduces heat transfer and increases pumping costs. The minerals which combine to form scale are normally present to some degree in well water. If the water temperature rises suddenly and/or the water pressure drops suddenly, the suspended minerals will be released and form a carbonate scale.

The first step to prevent scaling is to keep all water lines under pressure. The second step is to limit the water temperature rise to 20°F in the cooling mode. Most heat pumps don't raise the water more than 10 to 12°F when cooling. If a larger rise is noted, adjust the water flow rate. Scaling typically does not occur in the heating mode.

Galvanic Corrosion
While scaling is one type of corrosion, another type is galvanic corrosion. This occurs when dissimilar metals are used together. For example, do not use iron or galvanized pipe together with the copper pipe used in the heat pump. Acceptable materials are copper, PVC, polyethylene, Polybutylene, and rubber.

Another problem is encrustation. This occurs primarily in return wells. It is a build up of a slimy orange-brown deposit caused by iron bacteria. This can clog the system as easily as scale. Keeping the water lines pressurized and free of contact with air inhibits the growth of this bacteria. If it is found, you can remove it with periodic cleaning using a chlorine solution, such as Clorox.

Consistency of Delivery
There is another risk inherent in using well water. A well that provides an apparently good source of water one year may become a poor source the next year. A review of the probable impacts on the water source may give a clue to future problems.

In all cases, discuss the planned system with a competent well driller possessing local knowledge before designing the system.