Geothermal heat pumps, also known as ground-source heat pumps, can be one of the most cost-effective and efficient ways to heat and cool your home or building. However, occasionally geothermal installations go wrong, often as the result of an inexperienced designer or installer. Below are some of the common pitfalls and ways to avoid these issues:
1. Oversizing equipment.
Oversizing equipment is common not only with geothermal heat pumps, but also in more conventional equipment like air conditioners.
Many contractors still use rules of thumb for sizing equipment, and then add a large "safety factor" to ensure that the equipment is big enough for the job.
Not only do heat pumps use more energy than necessary when oversized, but oversizing leads to short-cycling of on and off, which wears equipment down much faster, just like starting and stopping a car.
Also, because oversized equipment has shorter run-times, you don't get the same amount of filtration and air circulation throughout the home, which can lead to bigger temperature variations in the house.
A good contractor will provide you with a detailed ACCA Manual J heating and cooling load calculation for the home based on your insulation, windows, and other details.
2. Improper distribution system.
Many homeowners spend a premium for a geothermal heat pump, only to integrate it with a lousy duct system or poorly designed radiant system. This is like putting bald tires on a brand new car.
The best geothermal heat pump in the world cannot make an improperly designed air duct system more quiet or comfortable. Likewise, when a radiant system is not optimized to keep floor temperatures low, much of the efficiency of ground source heat pumps is lost.
Residential duct systems should be designed in accordance with the ACCA Manual D.
3. Wrong pump for the job.
The circulator pump is what moves water through the ground and into the heat pump itself. The pump must have enough power to overcome all of the friction from the piping and fittings, as well as be able to overcome the pressure drop of the heat pump.
If the pump is sized to small, there will not be adequate water flow through the heat pump, potentially causing malfunctions or hurt the unit efficiency. On the other hand, an oversized circulator will use far more electricity than necessary. A good contractor will select a pump based on a head loss calculation and the specific heat pump.
4. Ground loop sized incorrectly.
The key to a good geothermal system is the ground loop itself. An undersized ground loop is almost impossible to fix and will lead to an inefficient system at best, and a frozen ("slushy") or overheated ground loop at worst.
At the same time, a grossly oversized ground loop will be prohibitively expensive for little benefit. Never use "rule of thumb" for sizing your ground loop; a good contractor will take into account the heating and cooling load of the building, soil conductivity and temperature, flow rates, and other factors all into account.
5. Poor coordination.
Depending on the size of the geothermal project, any number of different players may be involved, such as the owner, a general contractor, drillers, HVAC contractors, sheetmetal installers, engineers or architects, etc. The more parties that are involved in the project, the more important good communication becomes.
For instance, the HVAC contractor needs to be aware of any changes that the architects make in the floor coverings to make sure the radiant floor works as designed.
Ideally one contractor is responsible for all aspects of the geothermal system, including the equipment, ducting, and drilling or trenching. That way, if something goes wrong there is no "finger pointing" and the contractor can work to correct the problem.
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