One of the quickest and most cost efficient ways to install geothermal at your residence is an open loop geothermal system. An open loop system uses a readily available source of water (usually a well, pond, or lake) as the heat transfer media. One of the biggest advantages this provides is savings on installation cost. With an open loop system there is no need for expensive geothermal drilling rigs and a lot of piping. All you need is piping from the heat pump to your water source, and a trench to bury it in.
With an open loop system, you simply draw water from the water source, run it through your heat pump, and pump it back to the source. That’s it! Some people affectionately call open loop systems the “pump and dump” method. Water only stays in the system for the time it takes to run through your heat pump, and then is returned.
How is Open Loop Different from Closed Loop Geothermal?
From inside your home or office, you won’t know the difference. You don’t HAVE to have any bulky “extra” equipment, and you can use the exact same heat pump. The one piece of equipment we do recommend is a small water filter, depending on your water source. If you are drawing potable well water, this can be avoided. However, if you are drawing from a pond or lake, we recommend a filter to protect your heat pump from heat exchanger degradation. The minerals and particulates in an exposed water source with animal and plant life can decrease the efficiency of your heat exchanger. Mineral deposits, acidity, animal secretions, and seasonal lake turnover can all be contributors to harmful water quality.
Other than that, an open loop system can save a substantial amount of installation cost up front and be a viable option for your geothermal choice.
Differences in Piping
There are a few differences in the routing of piping, but no difference in the piping itself (See here for our recommendations of pipe). The pipe going from the water source/sink to the heat pump should still be buried between 4 and 6 feet deep in a trench to prevent freezing. If you are drawing from a well, you can tap off the supply line already installed, and just route an exit pipe from your heat pump outside. Route this at least 10 feet from your house to prevent any possibility of accidental flooding or leakage. Also, make sure the end of the pipe is unrestricted so the used water can be expelled without barrier.
If you are routing pipe to and from a pond or lake, it is important you put the pipes at least four feet below the surface of the water. The deeper the better, as the temperature of the water is more stable the lower the pipe. Dig your trench to the water and secure it, but route the pipe even deeper after you have it submerged in the water. Also, the end of your pipe should have a mesh filter at the very least to prevent large amounts of dirt (or fish) from being pulled in. A blockage in pipe that has already been buried will be difficult and expensive to remove.
The other thing of note in an open loop system is to install the pipe strainer on the supply side of your geothermal line. To prevent confusion, install it BEFORE the heat exchanger. This is the last filter type apparatus left to catch any deposits in the water. We advise to always have the strainer before the heat exchanger, but in many closed loop systems the only critical design factor is that the strainer be before the pump.
Efficiency of Open Loop Geothermal
Although you will save thousands of dollars in up front installation cost by going with an open loop system as opposed to a closed loop system, it’s not without punishment. You will pay a slight efficiency penalty with open loop systems, but the system will still be extraordinarily better than a conventional air cooled heat pump system.
Why? Because the water used in an open loop system never reaches “undisturbed deep earth temperature.” Well water can get close depending on depth, but lake water has a quite a temperature gradient. In order to maximize heat pump efficiency, you would want warm water to be used in heating mode and cool water to be used in cooling mode. Since that can’t happen without supplemental equipment (lowering system-wide efficiency), maximum efficiency is achieved when the water is at a moderate temperature. Around 50 degrees F is a good place to start. This occurs best deep (400 feet or more) in the earth.
Open loop systems geothermal systems are a great way to get a premium efficiency HVAC system in a very cost effective manner. Some piping and trenching is involved, but as long as the water is filtered, you should see great efficiency and a 20+ year lifetime out of your heat pump system.