I’m an hvac system designer in Vancouver, Wash., just across the Columbia River from Portland, Ore. We install a wide variety of heating and air-conditioning systems, but in recent years our best-selling products have been super-efficient small-to-medium-sized ductless heat pumps (DHP), most of which are retrofitted into existing homes.
That trend has been helped along by now-expired federal and state tax credits and an ongoing rebate program sponsored by the Northwest Energy Efficiency Alliance, or NEEA. The NW Ductless Heat Pump Project, as it’s called, helps eligible homeowners cover the cost of converting from existing electric resistance heat — which the regional utilities call “zonal heat” — to more energy-efficient heat pumps. Even before the program took effect, though, we’d already been putting in quite a few ductless systems; in fact, we’ve installed more than 600 DHPs in the past few years. They go in quickly and easily, and have proven to be extremely reliable. We almost never have any quality issues with the equipment, and customer satisfaction has been very high.
Heat-Pump Basics
As most people know by now, heat pumps are refrigeration-cycle devices that use electricity to move and concentrate heat energy. During cold weather, the system collects diffuse heat energy from the outdoors and releases it indoors; and during the cooling season, it does the opposite. Although electricity is required to run a compressor, the energy needed to deliver a given quantity of heat with a heat pump is typically a fraction of what it would take to provide the same amount of heat with resistance baseboard heaters or wall-hung units.
Air-source vs. ground-source systems. The NW Ductless Heat Pump Project rebates apply only to air-source heat pumps, which draw heat energy from the ambient outdoor air. In the past, our company has installed some ground-source systems (also known as geothermal systems), which extract heat from soil or groundwater by circulating fluid through a closed loop of buried or submerged tubing. Ground-source systems are at their best under extreme environmental conditions, so they can be a good choice in regions with long, blazing-hot summers or winters that feature long periods of subzero cold.
But in our area, temperatures are quite moderate throughout the year. The design temperature for heating-load calculations, for example, varies from 18°F to 22°F, depending on elevation, while cooling-load design temperatures are in the 85°F to 88°F range. Air-source heat pumps operate very efficiently here, and offer much more bang for the buck than ground-source systems, which must be individually engineered and often cost tens of thousands of dollars to install. As a result, our company now offers air-source systems exclusively.
Going ductless. The output from any heat-pump system — whether ground- or air-source — can be distributed by way of a conventional duct system served by a central air handler. But the ductless systems I’ll be talking about in this article consist of an outdoor unit — made up of a coil, a fan, and an inverter-driven variable-speed compressor — connected by refrigerant lines to one or more indoor fan-coil units, each of which is equipped with a filter, a coil, and a multispeed blower to distribute the conditioned air (see Figure 1).
These kinds of systems are often called mini-splits, to distinguish them from through-the-wall “motel units” that combine the compressor and fan coils in a single package, as in a conventional room air conditioner. But to avoid confusing consumers, we follow NEEA’s lead and refer to them as ductless heat pumps.
Measuring efficiency. One standard measure of efficiency for heat pumps while in heating mode is their coefficient of performance, or COP. A heat pump that is operating with a COP of 2, for example, is delivering twice as much heat as a resistance heater that’s drawing the same wattage (Figure 2).
Many consumers and contractors will be more familiar with heating seasonal performance factor (HSPF) and seasonal energy-efficiency ratio (SEER) ratings, which in effect average the instantaneous COP figure over an entire heating or cooling system. Some of the ductless systems we install have SEER ratings of more than 20, and HSPF ratings of 12 or more — far exceeding the minimum federal SEER and HSPF standards of 13 and 7.7. Very few ducted heat-pump systems can come anywhere near this level of efficiency.
