Ductless heat pumps perform well in cold weather

February 28, 2010 · Posted in Uncategorized · 10 Comments 

Considering that heat pumps started out as air-conditioners, it’s no surprise that they have a sketchy reputation in colder climates. That reputation is changing now that “inverter driven” technology is appearing in the North American market. An inverter drive system – common in Asia and Europe – is essentially a variable speed compressor powered by a direct current motor. Because they are infinitely variable speed, they operate almost continuously instead of cycling on and off. So far, inverter drive is only commercially available in smaller ductless heat pumps, also called mini-splits.

The indoor unit of a ductless heat pump hangs on the wall. This is the only real drawback. Most people wouldn't find this very attractive. However, most people quickly become accustomed after living with them.

Compared to their central system cousins, ductless heat pumps are smaller, quieter, more efficient and more comfortable. About the only criticism that remains of ductless heat pumps are lingering doubts that they will work well in cold weather. I’m hoping to dispel that last doubt – at least to a point.

Typical heating systems, whether heat pumps or gas furnaces, operate at full blast for a short time and then shut off. A heating system must be sized for the highest heating or cooling demand of the year even though that “design temperature” is only reached about 5 percent of the time. The result is like taking your car from 0 to 60 mph, then slamming on the brakes, turning off the engine, waiting for a few minutes, and then repeating the cycle.

Not only is that hard on the equipment, but the house never really reaches a stable temperature. Forced air heating systems have large indoor temperature swings. The variable speed nature of the inverter drive system leads to an indoor temperature that stays remarkably constant. This allows interior surfaces to warm up to a stable temperature, too. It is the temperature of these interior surfaces that – more than anything else – determines occupant comfort. This is called “mean radiant temperature.” MRT of 64°F is generally considered comfortable. Gas-fired forced air systems can have variable speed blowers and modulating burners. The problem with furnaces is that you can’t find one small enough for a modern, efficient home.

Of course, applying only the right amount of energy needed to keep the house at the proper temperature is a big reason that ductless heat pumps are more efficient that their central system cousins. But there are several other reasons.

They don’t suffer heat loss in leaky, poorly insulated ducts. Forced air systems lose 20 to 30 percent of the heat between the air handler and the registers. Unless properly installed (and few are), all forced air systems create pressure imbalances within buildings.

Sizing is critical for heat pumps, because turning on and off frequently causes excessive wear on the heart of the heat pump: its compressor. This specialized electric motor drives the vapor-compression cycle that makes heat pumps work. (That’s a whole different topic that I hope to cover soon in an Oikos Library article.)

Because air-source heat pumps extract heat from the outside air, they become less efficient as the outdoor temperature drops. They continue to operate, but the temperature of the air delivered to the building drops, too. Before long, delivery temperature dips to a level that most people will find uncomfortable. To prevent complaints, most heating contractors “lock out” the heat pump when the outdoor temperature reaches 35 or 40° F, even though the heat pump will continue to operate at with an efficiency above 100 percent. Heat is now supplied by the electric resistance elements (strip heat) in the air handler. That’s much less efficient and much more expensive.

The outdoor unit of a ductless heat pump is small and very quiet.

Even when outdoor temperature is above 40°F, central heat pumps deliver air to the building at about 105°F and they also move a lot of air within the house. For comparison, a gas furnace will deliver air at about 130°F with lower air volume. This has always been a criticism of heat pumps, because 105°F is not much higher than body temperature at 98°F. This feels cool to most people. Moving a large volume of air at a fairly low temperature is a recipe for comfort problems.

So, the question of whether a heat pump “works” at low outdoor temperatures is really a question of occupant comfort. The answer for heat pumps with traditional compressors, single-speed operation and forced air delivery is clearly NO.

Ductless heat pumps answer every one of these shortcomings.

Sizing is much less problematic with variable speed heat pumps than typical single speed models. Larger units perform better at colder temperatures, but there isn’t the same concern about oversizing. A larger inverter-driven ductless heat pump – meaning more heating capacity – will be able to deliver when the outdoor temperature drops, without suffering short-cycling during warmer weather.

Some continue to doubt that ductless heat pumps will perform at such low temperatures. A couple of years ago, Bonneville Power Administration1 sponsored research that measured the efficiency of ductless heat pumps in cold climates. Researchers reported that ductless heat pumps delivered 40 percent of their rated capacity at 5°F, with efficiency ratings from 150 to 250 percent. There is also a YouTube video showing a ductless heat pump operating in Manitoba, Canada with outdoor temperatures between 0°F and -14°F.

Across all these criteria, an inverter-driven, ductless heat pump surpasses the typical central heat pump system. Continuous operation allows very low air speed. Lower air velocity improves occupant comfort. Stable room temperatures maintain a higher mean radiant temperature. Larger units can be installed without compromising light load performance. Eliminating duct losses means better efficiency all the time, but especially when outdoor temperatures are low. You can see a list of companies in the Oikos Product Directory under Ductless Mini-split Heat Pumps.

Ductless heat pumps are an excellent choice for new homes that are small and super efficient. You’ll see them in many homes classified as net-zero energy. They can also be good for older homes that want to convert from electric resistance heating, such as electric baseboards or wall heaters . (Of course, a new heating system should always be the last step in an energy makeover that includes air sealing, more insulation and better windows.)

It seems odd to me is that inverter-drive compressors have been slow to arrive in central air-source heat pumps. I predict that manufacturers of central systems are working on that right now.

– Bruce Sullivan

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