Ground-Source Heat Pumps

The most common configuration of a ground-source heat pump has tubing laid in a horizontal trench--often as a series of coils.

Despite being highly efficient at turning kilowatt-hours of electricity into heat or chilled air, ground-source heat pumps have a significant drawback and usually aren't the best heating system choice.

For the past month, I've examined various home energy improvements for which one can earn a 30% federal tax credit. The last of these opportunities I'll cover is ground-source heat pumps. A ground-source heat pump (GSHP) is also referred to as a "geothermal" heat pump, though I prefer the former terminology, to avoid confusion with true geothermal energy systems that rely on elevated temperatures deep underground from the Earth's mantle. GSHPs are eligible for the 30% tax credit with no cap on the dollar amount that can be received. But this system may not be the best choice for home heating, and I'd like to offer some warnings about the potential for exaggerated claims and outright scams.

GSHPs are pretty amazing systems. Like all heat pumps, they extract heat from one place (in this case, underground) and deliver that heat--at a higher temperature--to your house. That may sound impossible, but it's how your kitchen refrigerator works. In the summer, heat pumps can be reversed to remove heat from your house, providing air conditioning.

While electric resistance heat is 100% efficient at converting electricity into heat, a heat pump provides at least two to three units of energy for every one unit of energy consumed. This is often measured as the annual "coefficient of performance" or COP; a COP of 3.5 means an effective efficiency of 350%. (Note that if you consider the "source energy" that a power company uses to generate that electricity, the true efficiency numbers are significantly lower.)

In cold climates, a GSHP has higher efficiency than an air-source heat pump (which extracts heat from the outside air), because temperatures underground are considerably warmer in the winter than the outside air. Installing a GSHP involves digging a trench or series of wells into which tubing is embedded. Either refrigerant, water, or antifreeze solution is circulated through this tubing. Another version pulls groundwater from a well as the heat source (or heat sink), and then returns the water to the well.

To earn the 30% federal tax credit, a GSHP must be Energy Star listed. The Energy Star requirements vary by type and are currently as follows: a closed-loop GSHP has to have an energy efficiency ratio (EER) of at least 14.1 or a COP of at least 3.3; an open-loop GSHP must have an EER of at least 16.2 or a COP of at least 3.6; and a direct-exchange (DX) GSHP must have an EER of at least 15 or a COP of at least 3.5.

The reason I'm not a huge proponent of GSHPs is that they're really expensive. Most of the expense is due to the cost of digging trenches and laying tubing. In addition, field experience shows that these systems often aren't meeting performance expectations. Sometimes the annual COP turns out to be only slightly higher than 2.5.

Meanwhile, performance of air-source heat pumps--in particular, the ductless mini-split heat pumps that have been popularized by such companies as Mitsubishi, Sanyo, and Daikin--have been improving dramatically in recent years, especially at low temperatures.

In the past, air-source heat pumps only made sense in warmer climates, because the efficiency would drop dramatically at lower temperatures; at about 40°F, these systems would usually switch over to electric-resistance heating. New mini-split heat pumps, by comparison, especially "inverter" models, can function pretty well down to about 0°F. And they cost a whole lot less than GSHPs.

It is not unusual to hear about GSHPs in Vermont costing as much as $35,000 for typical homes. For the same investment, one could spend $30,000 reducing heating loads (insulating, air sealing, replacing windows, etc.) and install a state-of-the-art mini-split heat pump.

The 30% tax credit that's available for GSHPs allows homeowners to recover some of that very high cost. Air-source heat pumps also earn the tax credit, but only up to $1,500 and only through the end of 2010 (while the GSHP tax credit isn't scheduled to expire until the end of 2016).

Here's my concern: Because GSHP dealers can show homeowners how much money they can get back from Uncle Sam (over $10,000 for that $35,000 system), they can make a pretty compelling argument. A few unscrupulous companies might also be tempted to inflate the price of the system--allowing you to earn an even higher tax credit--and then offer you a special rebate or pyramid-marketing inducement of some sort so that both you and the company benefit--at the expense of the rest of us taxpayers.

I don't have a problem with subsidizing energy improvements through tax credits and other incentives, but if my tax dollars are being spent on these improvements I'd like to be sure that the money is spent wisely. I don't believe a GSHP costing tens of thousands of dollars is nearly as cost-effective as combining energy conservation and a far less expensive mini-split air-source heat pump.

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