A geothermal heating and cooling system uses a geothermal or ground-source heat pump and a loop system to extract heat from the earth and carry it to a home in the winter. In the summer, the process reverses and the system extracts heat from the home and rejects it to the earth. In either case, the geothermal system delivers efficiencies that exceed those of conventional home comfort systems, offering homeowners savings as high as 70% for heating, cooling and hot water.
To ensure these savings and the added benefits of enhanced comfort, improved indoor air quality, longevity and a smaller carbon footprint, it's important that the system be serviced at least once a year by a trained technician. Although every manufacturer recommends its own procedures for starting, servicing and/or troubleshooting a geothermal heat pump, each checklist includes the following:
1. Prepare the equipment
Turn the heat pump on, and be certain the unit is operating at high speed and factory blower high speed for five to 10 minutes. This ensures that the coil, refrigerant, and the coaxial have reached the proper temperature for accurate readings, and enough time has elapsed to achieve good refrigerant oil circulation.
If the unit is equipped with a hot water assist or desuperheater, the desuperheater should be in the off position. This ensures that measurements capture the BTUs that are going in and out of the ground loop; not those going to the hot water heater.
It's also important to check to see whether the heat pump has a zone control. If it does, be sure that all zone dampers are in the open position. This allows for maximum air flow through the ductwork during any diagnostic checks.
2. Check the flow rate
After the heat pump has run for five or 10 minutes, check the water pressure by inserting a water pressure gauge into the PT port and recording the water pressure in and the water pressure out. Then, subtract the high pressure from the low pressure to arrive at the Delta P.
At this point, refer to the manufacturer’s specification book. Using the book, locate the unit model number and the entering water temperature and Delta P that most closely matches the Delta P arrived at in the above calculation. The manufacturer converts this information to the water flow rate for the unit, measured in gallons per minute or GPM.
3. Take the temperature
Insert the temperature probe into the PT ports to measure the temperature of the water entering the unit and the temperature leaving. Subtract the high number from the low to calculate the Delta T.
The specified Delta T varies from manufacturer to manufacturer, but in most cases, the numbers are close. In any event, the measured Delta T should fall within the manufacturer’s recommendation for the particular unit that is being serviced.
4. Calculate the heat of extraction/rejection
Calculate the heat of extraction/rejection (HE/HR) by multiplying together the Delta T of the water, the GPM and the fluid factor (ÄT x GPM x fluid factor). In an open loop system, the fluid factor is 500. In a closed loop system that uses a mixture of water and antifreeze, the fluid factor is 485. These are fixed numbers used by most of the industry.
The resulting HE/HR measures the amount of heat or the number of BTUs extracted from the ground in the winter and the number of BTUs rejected to the ground in the summer. It can be used to measure the performance of the heat pump by referencing the manufacturer’s specification book. Most manufacturers recommend that the calculated HE/HR be no more than 10% under their specification.
Under normal circumstances, a heat pump with the proper flow rate and Delta T will produce a HE/HR that meets the manufacturer’s specification. A number that doesn’t meet the specification may point to problems with water flow, refrigerant or air distribution. However, at this point in the service checklist, it’s too early to tell. Additional tests are necessary to accurately diagnose the source of the problem.
5. Calculate sensible heat/cooling
Use the same digital probe that was used to calculate the Delta T of the water to measure the air temperature coming into the unit and the air temperature going out. The difference between the two measurements is the Delta T of the air.
To calculate sensible heat or sensible cooling, multiply the Delta T of the air by the CFM times 1.08 (ÄT x CFM x 1.08). The product of these three numbers provides the sensible BTUs of heating or cooling. This represents the number of BTUs blowing through the ductwork and into the house, including the BTUs being extracted from the ground and the BTUs coming off the compressor, pumps, and blower motor.
Compare this calculation to the manufacturer’s specification book. Again, the sensible BTUs should be no more than 10% below the manufacturer’s specification.
6.Measure volts and amps
Most manufacturers provide a range when the unit is running under high speed capacity. Measure the volts and the amps of the heat pump and compare them to the range provided in the manufacturer’s specification book, remembering that amps will increase when the loop temperature increases.
7. Inspect coil and filter
The coil should be inspected for dirt during every start-up or service check. If the coil is dirty, it should be cleaned using a nonacidic coil cleaner and then thoroughly rinsed. It should be noted that some coil cleaners have an acid base and others state that rinsing isn’t necessary. However, acid can eat through the coils, so WaterFurnace recommends using a nonacidic cleaner and always rinsing the coil.
The filter should be replaced by the homeowner at scheduled intervals throughout the year. It should also be replaced by the service technician every time the heat pump is serviced.
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8. Inspect drain pan and trap
It’s important to inspect the drain pan and trap for dirt and sediment, especially following construction. Be sure the trap, drain line and vents are thoroughly cleaned, avoiding any drain pan treatment that includes chlorine. Chlorine can be hard on copper and coils.
9. Turn on the desuperheater
At this point in the servicing process it’s time to turn on the desuperheater and allow it to run with the unit operating. Check the water temperature of the desuperheater with your hand or an infrared thermometer, or clamp a temperature assessing device to the pipes. Typically when a desuperheater is running correctly, it’ll have a Delta T (water in and water out) between 5 and 15 degrees.
If the Delta T is extremely high, more than likely the pump is either not running or there’s a blockage in the heat exchanger – perhaps lime buildup or debris in the water. In the case of the pump, the problem could be as simple as the pump being turned off or a fuse that has burned out. Or, it could mean the pump was not properly burped during start-up, and as a result, the pump burned out.
If the heat exchanger is blocked, the technician may need to flush the unit with a mild acid solution to remove any mineral deposits that may be inside. In any case, it’s important to properly maintain the desuperheater, because a desuperheater can save the homeowner a significant amount of money with respect to hot water bills if the equipment is running properly.
10. Repressurize the ground loop
Most closed ground loop systems lose pressure over time, especially during the cooling mode. So, one of the last things to do when starting, servicing, or troubleshooting a geothermal heat pump is to add water pressure, using one of the many tools available for that purpose. If using house pressure to repressurize the ground loop, locate a faucet in the house (any faucet but the one on the bottom of the hot water heater) or outside, hook up the garden hose and let water run at full pressure through the hose for several minutes to ensure all the air is removed. Then, attach the repressurization tool to the garden hose and allow more water to run through the hose and the repressurization tool.
Next, insert the tool into the PT plug and add water pressure to the ground loop. WaterFurnace recommends a pressure of 40 to 70 PSI. As soon as the appropriate pressure is attained, burp the loop pumps by loosening the burping screw on the end of the pump shaft and allowing the water pressure to push out any air or debris that may be inside the pump. When the air stops spitting and water runs steadily, tighten the screw and repressurize the loop back up to 40 to 70 PSI.
Finally, return the zone control to its normal operating condition.
Servicing a geothermal heat pump is not very different from servicing a conventional heat pump. However, a technician should be familiar with the manufacturer’s recommended service checklist before proceeding to service a particular unit. Doing so decreases the likelihood of callbacks when heating or cooling is needed most. It increases customer satisfaction, enabling the homeowner to enjoy all the comfort, economic, and environmental benefits that a geothermal heating and cooling system offers.
Sonny Hampton has more than 40 years experience in the HVAC industry. He began his career as a plumber with a plumbing company in his hometown and later purchased the business with a partner. He ran the plumbing company as managing partner for several years before he accepted a position at Applied Energy Systems (AES), a WaterFurnace Distributor in Illinois, as geothermal service manager. After 10 years of service at AES, WaterFurnace hired him as a senior field consultant/trainer. Hampton was later promoted to training manager, which he held for eight years. In 2009, Hampton was promoted to director of training.