Ductless heat pumps perform well in cold weather

February 28, 2010 · Posted in Uncategorized

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

Tags: Ductless heat pump, efficient, equipment sizing, HVAC

Comments

10 Responses to “Ductless heat pumps perform well in cold weather”

Casey Bradley on March 2nd, 2010 5:44 pm

Heat pumps are great for saving energy, but a little more thought needs to be given when considering the amount of carbon emissions asociated with them. The key when talking to homeowners about swithching from a gas furnace to a heat pump is understanding that the heat pump needs to have at least a COP of 3 for the carbon emitted to be roughly similar to that of a 90% furnace (given average carbon emissions of electric utilities). This isn’t a problem with these inverter drive heat pumps because most seem to have a COP of 3 and beyond, but it is something for us to keep in mind as we push the products of the future.
Dave Brook on March 4th, 2010 7:11 pm

As the article suggests the sizing issue is considerably different for DHPs. In the Northwest, where we are not so concerned about humidity control, oversizing, taking into consideration the “low heating” heat output, is probably recommended. I’m not aware of recommendations in this area and would love to hear them.

The carbon impact of any heat pump is directly related to the mix of coal in the utility’s generation. Thesuggestion of COP of 3 (about 9 HSPF) being a threshhold is good “on average” but perhaps a little too simplified, especially considering the many public utilties in the Northwest.
Jeff Burns on August 14th, 2010 7:53 pm

Good article. I’m actually reading it after my installation of a Mitsubishi upstairs and an LG downstairs, and can tell you it’s generally accurate. Stable indoor temperatures are a extremely nice feature of the mini splits. The article is primarily concerned about cold, but the stable indoor temps and advantages of inverter based mini splits hold just as true for cooling. Particularly in Texas where the temps get above 100 and just stay there (temp drops down to 85 at night and it feels like a cold snap). Major temp swings inside are hard on the AC equipment, and when the temperature really gets up there (supposed to be 108 tomorrow) a properly sized Central AC will never catch up once it gets behind. With the modern mini-splits, you can oversize for some additional margin of error and not pay for it with high humidity inside, not to mention it’s always running at a low capacity to keep from getting behind in the first place.

Regarding different pollutants from a power plant when switching from a furnace to a inverter based heat pump – Something to keep in mind is the energy consumption of the Inverter based mini-splits in normal operation can actually consume the same or less power than just the blower motor of a conventional heat pump/furnace (not including the compressor) since the inside units run at low speed/BTU most of the time with much smaller fans. The only time the units really start working from what I’ve noticed is when they’ve been turned off for a while and they’re having to catch up and cool the room. (My old central AC/Furnace blower motor was sucking down 5-6amp at 115v versus around 1-2A on each leg of my 2 outside 220V 36000 BTU inverter compressors at the end of a 105 degree day when it’s still 100 out running 7 inside units…) For me the energy consumption has dropped significantly ditching the old central furnace/AC. Interestingly, oversized inverter based mini-split units appear more efficient than right sized as well, since they only operate just as many BTU’s as needed to keep cooled/heated and they’re most efficient when running well under capacity, rather than the full on or off central furnace/AC’s. For those of us in warm/humid climates, the humidity of the house remains controlled even if oversized, which doesn’t occur with conventional AC. I’m oversized by at least a ton (probably 2), but my energy consumption so far is well down over the conventional (and dare I say obsolete) central furnace/AC combo that got replaced. Getting rid of a energy hungry blower motor along with the inefficient ductwork was one of my major goals. I’m extremely happy to have gotten rid of all the inefficient and poorly installed ductwork along with a relatively noisy furnace dead center in the house. The internal units are so quiet you have to check to see if there on half the time. I’ve also recovered a ton of storage area in the attic which I didn’t have before, a new hallway closet, and one closet which is now ~25% bigger because it used to have utility bulkheads in it for ductwork. Also for once my house is now balanced temperature wise through every room and both floors, something which is near impossible to do with conventional ducted central air/furnace systems without major expense. All this for about the same energy consumption of the old furnace’s blower motor. Life’s good!
Karl Juul on October 19th, 2010 9:51 am

The article, written in 2010, concludes” 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.”

Mitsubishi introduced such a unit in 2008 to the Canadian market.
It is sold as “Zuba-Central” and utilizes the same technology as Mitsubishi’s mini-splits, but with a conventional air-handler and duct system. I’m hoping it will be available for US sales soon, but Mitsubishi is not commenting on that possibility.
Fire Fly on October 25th, 2010 8:36 am

very interesting read. thanks for sharing
Josh Thomson on February 2nd, 2011 9:36 pm

GREAT POST! Thank you so much for your take on mini split heat pumps. I’ve been selling mini splits for a while now and haven’t yet thought of the advantage of a lower “balance point”. I will continue to use this post as a resource for my home owners to look at in the future when debating between conventional and mini-split heat pumps. Here’s also a video of a mini-split unit that I’ve installed. Let me know what you think.

http://www.thomsonac.com/?p=1403
Mini Splits Guide on February 25th, 2011 2:15 pm

Nice to see an image. Thanks. So, because mini splits are ductless, does it means they can be installed much faster?
Brian Powell on April 19th, 2011 9:30 pm

I have been installing the Mitsubishi ductless splits for 18 years. This product catagory is not as accepted as it should be. They heat and cool like no other system for half the cost to operate. I have a 12K Hyper Heat Mitsubishi Ductless and love it.
Heat Pumps on May 31st, 2011 9:58 am

Just what I was looking for, thank you posting.
Seth on October 7th, 2011 9:05 am

Very helpfull thank you. I am looking to add mini-splits to heat my house and cool my house I am remodeling. It will have gas fired radiant on the first floor and Ductless in the second floor bed rooms since it is easier to retro fit on the second floor. I was concerned with cold weather, but I have a wood stove for those few nights it gets below 10 deg here in Connecticut. I agree, they seem a good option but the masses are filled with doubt.