Open-loop ground water heat pumps: how they work
A ground water heat pump pulls water from a well, passes it through a heat exchanger and returns it to the ground or a surface outlet. It can be efficient and cheaper to install than a loop field, but it needs a good water supply, suitable water quality and permission under provincial water rules.
How does an open-loop system work?
A pump in a supply well sends groundwater to the heat pump. In winter the unit takes heat from that water and passes it to the house, and in summer it puts heat from the home into the water, which gives cooling and indoor comfort without an outdoor air unit. The water, now a few degrees cooler or warmer, goes to a return well, a pond or another approved discharge point. Ground water stays at a fairly steady temperature through the year, close to the local average, which is why this geothermal source works for both heating and cooling. The idea is explained with other types in horizontal and vertical ground loops.
The water does not mix with the refrigerant or the house system. It passes through a heat exchanger, and nothing is added to it. The cycle in the heat pump is the same as in other ground source geothermal systems, as described in how ground-source heat pumps work.
What must the well and the water provide?
Two things matter: quantity and quality. The well must deliver a steady flow for as long as the heat pump runs, and the flow needed grows with the size of the unit. A designer works it out from the capacity and the temperature drop across the exchanger, and a pumping test on the well confirms what it can supply. A well that serves the house at a modest rate may be too weak for a heat pump.
Quality decides how long the equipment lasts. Hard water, iron, manganese and dissolved minerals can foul the heat exchanger and the return well, and some waters corrode metal parts. A water analysis before design is standard, and some installations use a separate heat exchanger, often of corrosion-resistant material, to protect the unit.
What water rules apply in Canada?
Water is regulated provincially, and rules differ across the country. Drilling a well generally needs a licensed driller, and using groundwater for heat pump supply, or discharging it, can need a permit or an approval from the provincial water authority. Some jurisdictions require returning the water to the same aquifer, and some restrict surface discharge. The rules also change, so confirm the current requirements with the provincial ministry that manages water, and with the municipality, before you design anything.
Also check the rules for well spacing, setbacks from septic systems and the treatment of any existing well that will be used. A contractor with local experience should know the process, as the drilling contractor checklist suggests asking.
What are the advantages and risks?
Open loops can cost less than a closed loop field, since a supply well and a return well may cost less than many boreholes or long trenches, and the water carries heat directly. Heat transfer is good, and the unit can be smaller in some cases. The main risks are fouling, a well whose output drops over time, pump energy for the well and the work needed if the return well clogs. A closed loop has none of those risks, so many designers choose it unless the site offers a reliable aquifer. The comparison with the buried closed loop is in air source versus ground source.
Maintenance includes testing the flow, cleaning the exchanger when needed and checking the well pump. The tasks are listed in geothermal heat pump maintenance.
How does this relate to aquifer storage?
An open-loop heat pump takes heat from groundwater and does not store anything on purpose. Aquifer thermal energy storage is a larger idea that pumps warm and cold water into separate parts of an aquifer to keep them for later in the year. It is explained in aquifer thermal energy storage. Drake Landing chose boreholes, which keep the fluid in closed pipes and avoid the water-quality and permitting issues of an open system, and the store is described on the borehole storage page.
Quick answers
What is a ground water heat pump?
It is a heat pump that draws water from a well, takes heat from it, and returns or discharges the water. It is also called an open-loop or water-source system.
Do I need a permit to use well water for a heat pump?
Often yes. Water use and discharge are managed by the province, so check with the provincial water authority and your municipality before design.
Can any well supply a ground water heat pump?
No. The well must deliver enough flow continuously and the water must be clean enough not to foul the exchanger. A pumping test and a water analysis show whether it can.
Is open loop better than closed loop?
Not always. It can cost less and transfer heat well, but fouling and permits add risk. Many designers pick a closed loop unless the aquifer is dependable.