5.5 Water-to-Air & Geothermal Heat Pump Systems
Key Takeaways
- PSI lists heat pumps (air to air and water-air) and geo thermal systems in the HVAC Equipment content outline, so ground-source equipment is inside this classification's scope.
- Water-to-air heat pumps are rated to ISO 13256-1: ground-loop heat pump (GLHP) heating is rated at 32F entering water and cooling at 77F entering water.
- Design loop flow is normally 2.5 to 3 gpm per ton of capacity, and the ground heat exchanger must be sized so entering water temperature stays inside the manufacturer's operating window all winter.
- Closed-loop ground heat exchangers in Michigan use high-density polyethylene fused by heat fusion and are pressure tested; socket or butt fusion joints, not clamps or barbed fittings, are the accepted joining method.
- Open-loop (pump-and-dump) systems draw groundwater and therefore trigger Michigan EGLE well construction and discharge requirements in addition to the mechanical permit.
Water-to-Air & Geothermal Heat Pump Systems
The PSI content outline for HVAC Equipment names this material directly: "heat-pumps (air to air and water-air) ... geo thermal systems." A Michigan HVAC Equipment licensee is expected to be able to set, pipe, and commission a water-source machine, not just an air-source condenser.
The physics is the same vapor-compression cycle covered earlier in this chapter. What changes is the heat sink and source. Instead of exchanging heat with outdoor air that swings from -10°F to 90°F, a water-source machine exchanges heat with a fluid loop that in Michigan stays in a narrow band around the deep-ground temperature — roughly 48°F to 55°F across most of the Lower Peninsula, a little colder in the Upper Peninsula.
That stability is the whole value proposition. An air-source heat pump loses capacity exactly when the load peaks; a ground-source machine does not.
Machine Construction
A water-to-air heat pump is a single cabinet containing:
- Scroll compressor (usually), sized 1.5 to 6 tons for residential work.
- Coaxial (tube-in-tube) or brazed-plate refrigerant-to-water heat exchanger. This replaces the outdoor condenser coil.
- Refrigerant-to-air coil and a blower, feeding conventional ductwork.
- Reversing valve to swap the roles of the two heat exchangers.
- Expansion device, typically a TXV or EEV.
- Frequently a desuperheater — a small auxiliary heat exchanger on the compressor discharge line that scavenges superheat to preheat domestic hot water. Desuperheaters are effectively free hot water in the cooling season.
Because the machine lives indoors, there is no defrost cycle, no outdoor fan, and no outdoor coil to ice or to be damaged. Service access, condensate drainage, and water-side isolation valves with P/T ports become the practical installation issues instead.
Terminology that matters on the exam
| Term | Meaning |
|---|---|
| GLHP | Ground-loop heat pump — closed earth loop |
| GWHP | Ground-water heat pump — open loop, pump-and-dump or two-well |
| WLHP | Water-loop heat pump — boiler/cooling-tower building loop |
| EWT | Entering water temperature at the machine |
| LWT | Leaving water temperature |
| ΔT | LWT minus EWT across the coax |
Rating Conditions: ISO 13256-1
Water-source equipment is not rated at the AHRI 210/240 conditions used for air-source equipment. It is rated to ISO 13256-1, and the rating point depends on the application:
| Application | Cooling EWT | Heating EWT |
|---|---|---|
| Ground-loop heat pump (GLHP, closed loop) | 77°F | 32°F |
| Ground-water heat pump (GWHP, open loop) | 59°F | 50°F |
| Water-loop heat pump (WLHP, building loop) | 86°F | 68°F |
This is why a manufacturer's GLHP heating COP looks so favorable compared with an air-source HSPF: the rating point is a 32°F fluid, not 17°F air. It is also why a catalog number is meaningless without the EWT it was taken at. Selection is always done by reading the performance table at the loop temperature the ground heat exchanger will actually deliver in February.
Ground Heat Exchangers
Closed loop
A sealed circuit of high-density polyethylene (HDPE) pipe carrying water plus antifreeze. Configurations:
- Vertical borehole. Typically 4- to 6-inch bores, 150 to 400 feet deep, one U-bend per bore, grouted full length with a thermally enhanced bentonite grout. Grouting the full length is not optional in Michigan — it is what keeps surface contamination out of the aquifer.
- Horizontal trench. Straight pipe or slinky coils placed at 4 to 6 feet of burial depth, below the Michigan frost line. Needs far more land than a vertical field.
- Pond/lake loop. Coils weighted on the bottom of a body of water deep enough not to freeze solid.
Joining method matters. HDPE loop piping is joined by heat fusion — socket fusion or butt fusion — producing a joint as strong as the pipe wall. Clamps, barbed insert fittings, and solvent cement are not acceptable for buried loop piping because a buried leak cannot be found or repaired economically.
Pressure testing. A completed loop is pressure tested and flushed/purged at high velocity to drive air out of the piping. Purge velocity of roughly 2 ft/s in every parallel circuit is the standard target; trapped air in a buried circuit will not come out by itself and will short the loop's capacity permanently.
Open loop
Groundwater is pumped from a supply well, run through the coax, and discharged — to a second injection well, to a surface water body, or to a drain field. It is simple and efficient where water quality allows, but:
- It is a well. In Michigan an open-loop system pulls in EGLE (Michigan Department of Environment, Great Lakes, and Energy) well construction and discharge requirements on top of the mechanical permit from BCC or the local enforcing agency.
- Water quality governs feasibility. Iron, hardness, and suspended solids foul a coax quickly. Cupronickel coax is specified where water quality is marginal.
- Discharge must be legal. Dumping to a storm sewer or a neighbor's ditch is a common and expensive mistake.
Antifreeze Selection for Michigan
A closed loop operating at 32°F EWT needs freeze protection well below that, because the fluid leaving the coax in deep winter is colder still — commonly a burst protection target near 15°F and a freeze point at least 10°F below the lowest expected entering water temperature.
| Fluid | Notes |
|---|---|
| Propylene glycol | The default in Michigan. Nontoxic, widely accepted by permitting authorities. Raises viscosity significantly at low temperature, which increases pumping power — size the circulator for the glycol solution, not for water. |
| Methanol | Better heat transfer and lower pumping penalty, but flammable in concentrate and restricted or prohibited by some jurisdictions. |
| Ethanol | Similar trade-offs to methanol. |
| Ethylene glycol | Effective but toxic; generally avoided for ground loops. |
Never select a glycol concentration from a water table. Glycol at 20% to 25% reduces the fluid's specific heat and raises its viscosity, so both the loop ΔT and the head loss change. Manufacturers publish correction factors; use them.
Flow, ΔT, and Commissioning
The design rules of thumb worth memorizing:
- Loop flow: 2.5 to 3 gpm per ton of nominal capacity. Below about 2 gpm/ton, ΔT climbs, EWT drifts, and capacity falls off.
- Loop ΔT at design flow generally lands 8°F to 12°F.
- Verify flow by pressure drop across the coax using the machine's P/T ports and the manufacturer's flow chart — this is the water-side equivalent of measuring superheat, and it is the single most useful commissioning number on a water-source job.
- Record EWT, LWT, flow, entering and leaving air temperatures, suction and discharge pressures, and superheat/subcooling on the startup sheet. A ground loop that was undersized will not reveal itself until the second or third heating season, and the startup data is the only evidence that will settle the argument.
Heat of extraction and heat of rejection
A ground loop is sized on the energy it must move into or out of the earth, not on the nominal tonnage:
In heating, the loop supplies the building load minus the compressor work, so the loop sees less than nominal capacity. In cooling, the loop must reject the building load plus the compressor work, so it sees more. Michigan's heating-dominated climate means the heating heat of extraction almost always governs vertical bore footage.
A manufacturer's catalog lists a 4-ton water-to-air heat pump's heating capacity at ISO 13256-1 GLHP conditions. What entering water temperature does that rating represent?
A closed vertical ground loop serving a 4-ton water-to-air heat pump is being commissioned. What is the normal design loop flow rate, and how should the installer verify it in the field?
Buried high-density polyethylene ground loop piping is being assembled on a Michigan vertical borehole job. Which joining method is acceptable for the buried portions of the loop?
A Michigan homeowner wants an open-loop (pump-and-dump) ground-water heat pump rather than a closed loop. Beyond the mechanical permit, what additional regulatory exposure does this choice create?