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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Building hypocrisy in the Malibu hills

February 24, 2010 · Posted in Construction, Opinion, Real Estate · 2 Comments 

Less is more for green building. It must be defined only in terms of using less energy, less water, less material and causing less harm to the ecological processes that support life. I’m appalled that yet another mega-rich celebrity is using “green” to wash away the grime of over consumption.

David Evans, also called The Edge and guitarist for U2 hopes to develop 156 acres overlooking the Pacific coast near Malibu, California. I respect the work that celebrities do for good causes, and I suppose that Mr. Evans has done his share of good deeds. Let’s take nothing away from other accomplishments. Instead, let’s focus on how this particular project reeks of excess.

Mr. Evans’ plans to build five homes on a bluff overlooking the ocean using “every imaginable green building technique”, according a New York Times story. The homes would range in size from 7,000 to 12,000 square feet. Whoa!

I can’t disparage his motives. Let’s assume that Mr Evans’ simply needs some help understanding that “green” isn’t about shiny stuff. Mr. Evans can demonstrate his sincerity by focusing his attention on the outcome not the technology. All five of his houses should meet a few simple goals: net zero energy, net zero water, completely healthy, beneficial to local habitat and certified by an independent third party. I could add more requirements, but I think those five elements should keep him busy enough.

Let’s assume that Mr. Evans is able to meet all four goals and get the house certified. That leaves only a fundamental hypocrisy. He claims he wants to “inspire” others to create a “benchmark of sustainability.” I get it.  A world of eager green acolytes will gaze on this accomplishment and dedicate themselves to building their own multi-million dollar monuments to conspicuous consumption. Let’s see, the land alone cost $9 million, which is more than I would make in… 12 LIFETIMES. And they haven’t even started pushing dirt.

To assert that anyone, but rock stars, investment bankers and mega-millionaires will be able to follow this example is absurd and insulting. It’s bad enough to salve one’s own conscience with green consumption, but it’s contemptuous to say it somehow serves society. I’m sick of rich people claiming that the only reason they build monstrous green houses is to show the rest of us how to do it. If these folks really want to be examples for the huddled masses (who made them rich to begin with), then they should use their money and all their talent to create a truly sustainable home with no more than 500 square feet for each permanent resident. If they need help, I’m happy to offer my thoughts on how to accomplish green development with true elegance.

No matter how many shiny gadgets Mr Evans puts in these small houses, there will be lots of money left over. With that, they could build thousands of truly green houses in Haiti or Africa or New Orleans. David Evans is only the current poster child for this behavior. It happens in every town and to varying degrees. Generally speaking, this kind of greenwashing is unintentional. These folks just don’t get it. Even if Marie Antoinette didn’t really say it, the sentiment applies: “If they don’t have bread, let them eat cake!”

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Can passive solar and hydronic radiant heat live together?

February 7, 2010 · Posted in Construction, Lessons Learned from a Green Building Project · 6 Comments 

We’ve now lived with our radiant floor heat for most of six heating seasons, so I think it’s time to draw some conclusions. The house was built in 2004. Insulation is better than average. The air leakage is impressively low at 1.8 air changes per hour at 50 Pascals. It’s also a true passive solar design with 60 percent of the glazing facing south, high solar heat gain glazing and concrete floors for thermal mass.

Being our first experience with radiant floor heat, we anticipated trotting around barefoot all winter on warm floors. It was immediately apparent that toasty toes were not a given. Because the house is so tight and well insulated, the heating system doesn’t really operate that much of the time. The house requires no heat at all until several days have passed with temperatures in the mid-40s and little sunshine. With the heat system sitting idle, the concrete floor stabilizes at about the same temperature as the room air. That means the floors are considerably cooler than we are. Instead of heat flowing from the floor to us, it’s the other way around. This situation holds true much of the heating season. However, during the coldest, darkest periods, the heat kicks in and our toes rejoice.

Our hydronic system is typical of many radiant floors. Cross-linked polyethylene (PEX) tubes are imbedded in a concrete slab. This slab is only three inches thick and sits atop a typical wood-framed floor. Each of the three zones is controlled by a standard wall mounted thermostat, which measures air temperature. Each zone has a small manifold that feeds three circuits. The flow of water to each circuit can be adjusted up to a maximum of 3/4 gallons per minute (gpm). Over the years, I’ve experimented with different flow rates, and I think I’ve come up with an optimum distribution within each zone.

Zone 1 is the top floor great room with living, dining and kitchen areas. It’s about 600 sq. ft. and contains three large south-facing windows – the source of considerable solar gain. The three-inch-thick concrete floor serves as thermal mass, absorbing heat when the sun shines on it and then slowly releasing the heat throughout the evening. Energy experts often say that this combination of passive solar and radiant floor heat doesn’t work. If the concrete is already warm from the hydronic heat, it will be unable to absorb additional solar heat. That’s certainly correct, but the design of the circuits allows us to have our solar heat and hydronic, too.

Hydronic heat cirucuits are laid out parallel to the solar windows to the left. The short wall will be a kitchen island.

The three circuits of Zone 1 are arranged parallel to the south facing windows dividing the room into three sections. The section adjacent to the south windows is turned off entirely because it receives direct solar gain for the entire heating season. We also keep this section largely free of furniture that would shade the floor. The middle section runs at about 50 percent of maximum flow – between 1/3 and ½ gpm. This section receives solar gain only during mid winter when the sun angles are low and light penetrates deeply into the room. The third section never sees direct gain, so the water flow is set to full. This arrangement allows solar heat to be stored while keeping other sections comfortable.

Hydronic radiant floor doubles as solar thermal mass, if carefully laid out.

Another view of the floor shows the kitchen too far from the solar windows to receive direct sunlight.

Zone 2 is much different. This zone contains the master bedroom, master bath and the home office. The office has a large south-facing window, but sun can’t strike the floor directly because of desks. The flow rate for this room is about 1/3 gpm or 30 percent of maximum. The bedroom has it’s own circuit also set to about 1/3 gpm. The last circuit in this zone comprises the bathroom and a small walk in closet. The flow is set to the full ¾ gpm. Because the thermostat sits in the bedroom, heat concentrates in the bathroom floors where it stays warm, while the bedroom is nice and cool for sleeping.

Zone 3 is the ground floor containing two bedrooms and a bath for the kids. This zone gets no solar gain, so the flow rate is set to full for all circuits.

With thoughtful design of the hydronic system, I think solar and radiant floors can work together. Credit for this design goes to Jim Chauncey at Sunterra Homes.

That leads us to another question. Is hydronic heat more efficient? Let’s break it down into two parts: distribution and conversion.

Hydronic is a far more efficient way to distribute heat than the most common method, namely forced air. Ductwork is notoriously leaky. Typical duct systems, which are located outside the conditioned envelope, can lose 20 to 30 percent of the heat before reaching the register. Hydronic systems lose virtually nothing. Actual leakage isn’t tolerable as it is with forced air. Plus, the pipes are commonly located inside the building’s conditioned space. What little heat does escape the pipes still flows into the house.

The conversion from fuel to distribution medium is also more efficient. With hard floors, the delivery temperature of the water can be as low as 85°F. It takes less energy to raise the water to this lower operating temperature. Because our hot water is supplied by a heat pump, there is an extra operating advantage to this lower delivery temperature with reduced wear and tear on the mechanical equipment.

Water has a higher specific heat than air so heat transfer within the heating plant (furnace, boiler, etc) is probably a little more efficient than the heat transfer to air. Is this enough to make a difference? I rather doubt it, but I think it deserves mention in the interest of completeness.

The typical advantages of hydronic floor heat obviously still apply. It’s quiet and doesn’t affect furniture placement.

Is high-mass hydronic heat like ours the best choice for a super-efficient house? I will agree with critics to some degree. My current vision of a “perfect” green house is so small and so simple that any type of central heating system is overkill. I would rather spend money on R40 walls, R60 ceilings and the best windows I could afford. The complexity and expense of a hydronic system would be a waste of money and resources.

However, not everyone shares my vision of the perfect house. Many new home designs include a mix of solar-heated space and those with no solar gain. Depending on the design, it may be that hydronic heat is the best approach.

The distribution efficiency might also make it an excellent choice for larger commercial buildings, multifamily buildings and district heating. In these cases, a highly efficient central plant can supply a lot of floor space.

Another advantage to hydronic heat, in general, is adaptability. There are many ways to heat water, including biomass boilers, other biofuels and active solar. With uncertainty about the future, this kind of flexibility may be useful.

So, is hydronic heat appropriate in green buildings? I will avoid making any sweeping proclamations. Instead, I’ll say that “it depends” on the requirements of each project and the care with which the design addresses those requirements.

–Bruce Sullivan

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