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
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.
Encrustation
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.
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