HVAC systems can be among the most complicated in a house, and that's usually even more true in an ultra-efficient zero-energy home. If your energy-saving goals are ambitious, choosing the type and size of a heating system usually requires some expert guidance, probably even some form of energy modeling. However, as a homeowner looking to build a new home or replace the heating system in an existing home, it's important to have a general idea of your preferences before you discuss your needs with the home's designer.
If you are shooting for net-zero-energy, your choices are mostly limited to high-efficiency electric heat to supplement the passive-solar design of your home, and that means some form of heat pump. Some definitions of net-zero-energy allow for one to use gas heat and calculate the equivalent amount of solar electricity to make up for it, but I like the elegance and simplicity of an all-electric home.
Types of heat pumps
If you are dedicated to using electric heat pumps, there are several types of system to choose from.
Air-source heat pumps
These generally look like a large air-conditioning unit outside, but they can create heated as well as cooled air. The indoor unit is generally hooked into an air duct system, so they can heat and cool a larger home.
Mini-split heat pumps
These are smaller units that also pull heat out of the air, usually with a single indoor air handler or head. If you have a large home, you will probably need multiple units. Mini-splits do not hook into your existing ductwork. This is what we have at Artemisia.
Air-to-water heat pumps
These are similar to standard air-source heat pumps, but the outdoor unit has a heat exchanger, which heats an antifreeze solution that circulates through the home via an in-floor radiant or wall coil-and-fan system. This is what we used in our Seattle zero-energy house.
See also: Air-to-Water Heat Pumps [Green Building Advisor]
Geothermal heat pumps
Instead of pulling heat out of the air, geothermal heat pumps use the ground as a heat source. They are typically much more expensive to install, but they are the most efficient option and can heat effectively in the coldest outside temperatures.
Our experience with Panasonic mini-split heat pumps
We have built two zero-energy houses over that last decade. The first one had an air-to-water heat pump connected to a radiant heat system in the concrete floors, which consisted of loops of PEX tubing placed under the concrete slab before it was poured.
When we built Artemisia, we tried a different approach. As part of our Artemisia Lab program, Panasonic of North America provided two of their EXTERIOS XE - Single Split System, Low Ambient Wall Mounted Heat Pumps. Dave, who did our energy modeling, chose this particular model for several reasons, notably that it was one of the most energy-efficient available (SEER 28.5) and that they can keep producing heat even when the temperature drops to 0° F. Many less technologically advanced models only work when the outside temperature is 25° F or above, which simply wasn't good enough for our cold Eastern Washington winters.
Will the floor be cold with mini-splits?
One thing we really loved about our Seattle home was the warm radiant-heat concrete floors. We were worried that if we built a house with unheated concrete floors that they would feel cold and uncomfortable on our feet. Now that the house has been operational for a year, I can say that our worries were unfounded. There is enough rigid foam under the slab to keep it well insulated from the ground, and the concrete floor eventually reaches the same temperature as the indoor air (usually about 69° F). True, it doesn't feel as toasty on your toes as the floors in the Seattle house, but the floor temperature is barely even noticeable, even with bare feet.
What about noise?
One of the benefits of an in-floor radiant heat system is its nearly silent operation. We wondered if we would be annoyed by the sound of the Panasonic's fan. As soon as we turned it on for the first time after installation, I realized that there was nothing to worry about there. The fan speed adjusts automatically, so during normal operation, it is quite low. The air baffles can be set to swing silently in four directions to circulate air throughout the room without having to raise the fan speed. Even when the fan is running on high, it is practically silent. The outdoor units also operate at multiple speeds, thanks to their inverter technology, and are practically silent. Kudos to the Panasonic engineers for getting this right!
In contrast, we did have some noise issues with our Unico Unichiller air-to-water heat pump at our Seattle house. The outdoor unit was noisy and only operated at one speed, but the real noise came from where the water lines penetrated the house. The outdoor unit vibrated the SIPs wall of the house, creating a loud hum. We solved the problem by adding a loop of flexible hose to each line before it touched the house. Because of this experience, I convinced our group to have the Panasonic outdoor units mounted on concrete piers on the ground rather than being attached to the exterior wall, as they were designed to be. In hindsight, I think this was probably unnecessary because they vibrate so little, but it was an easy, low cost form of insurance against potential noise issues.
A main benefit of mini-split heat pumps: cooling
One thing I love about our Panasonic mini splits is that they can cool as well as heat. With Seattle's mild summers, we rarely felt like we needed air conditioning, but on those rare hot days it would have been nice to have the option. Here in Winthrop, summers get hot, and guests who rent our house expect to stay cool indoors. The ultra-insulated SIPs walls and concrete floor (which acts as a huge heat sink) help keep things comfortable, but when the temperature reaches 100° F outside, air conditioning really helps. There are solutions for air-to-water heat pumps to cool the indoor air, but they require separate coil-and-fan units and more complicated plumbing that we didn't install in our home.
Problems with Panasonic mini-split heat pumps
Now that we've been living with these Panasonic heat pumps for over a year, we have a decent perspective of how they work. The only real problem with the units appears to have to do with the installation. The living room unit leaked some oil from a refrigerant line that appeared not have been installed tightly enough. This also resulted of some loss of cooling until the line was repaired and the refrigerant recharged. Another time, one unit flashed an error light, and we had to call our handyman to come out. He just turned the pump off and back on at the breaker, and it worked just fine.
I have a small gripe with the way the remotes are designed. They just don't seem very intuitive. Now that I've used them for a while, I know what all (18!) of the buttons do, but for a nightly rental, I worry that it might be confusing for guests. However, we haven't received any complaints, so I guess people are figuring it out just fine.
The other challenge isn't a problem with the unit per se but just something to keep in mind when designing your heating system. Two heat pumps were more than enough to heat our whole house, but the heat only comes out of the two indoor units--one in the living room, and one in the master bedroom. Our back bedroom on the west side of the house tended to be a few degrees cooler in the winter, so we ended up adding a resistance-electric cove heater on a special thermostat, which automatically turns the temperature down after a set number of hours. Problem solved.
Why I would choose mini-splits over a radiant-heat system
Now that I've lived with both systems, I think I would choose the Panasonic mini-splits over the air-to-water heat pump with radiant in-floor heat. Granted, we were able to pre-heat our domestic hot water with the air-to-water unit in our Seattle house, but we made up for that in Artemisia with a separate heat-pump hot water heater. Here's why I really like these little Panasonic units:
- Price: You can get them for under $2,000. They don't require all the plumbing of a radiant floor.
- Air conditioning: I like that they are set up to cool as well as heat.
- Redundancy: If one of the units goes down, the other one could probably keep the house reasonably warm just on its own.
- Quiet: This has to be seen to be appreciated, but they are remarkably quiet and don't vibrate.
- Efficiency: We'll be keeping a close eye on the heat pumps' overall efficiency and energy draws, but we are meeting our zero-energy goal so far, thanks in large part to how little energy these units draw.
- Smart design: Not only do they operate efficiently, but they have features like the built-in ECO-NAVI occupancy sensor, which turns down the heat when nobody is in the room. The compressors and fans can also adjust their speed based on the amount of heating or cooling needed, so once the house reaches the desired temperature, they can operate at a low, silent level to maintain the temperature, so it stays very even.
- Wifi: We're going to be adding a wifi component to be able to control the units remotely, so we can turn down the heat when guests check out and warm up the home before new guests arrive.