Minisplit Heat Pumps and Zero-Net-Energy Homes
What we know, what we wish we knew, and an invitation to readers to ask more questions
For the last several years, just about every project I’ve worked on other than large university buildings has used minisplit heat pumps for heating and cooling. Why?
1 – There is no combustion and no need for a chimney or vent.
2 – In space conditioning applications, heat pumps can provide heating and cooling.
3 – The equipment installation costs and the operating costs compare favorably with other options.
4 – Heat pumps are a natural partner to solar electric systems to achieve zero-net-energy buildings.
In addition, we’re using heat pumps to make domestic hot water (DHW) and even to heat swimming pools. All of these products are air-source heat pumps (ASHP) which extract heat from the air to supply heat to the load.
We’ve been metering the energy consumption at a number of buildings using heat pumps. We’ve seen through our own experience, and through more precise lab-based measurement efforts such as those performed by NREL and others, that the Japanese minisplit heat pumps live up to their performance claims in terms of both output and efficiency.
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Interested in learning more?
Since 2012, I've been working with NESEANorth East Sustainable Energy Association. A regional membership organization promoting sustainable energy solutions. NESEA is committed to advancing three core elements: sustainable solutions, proven results and cutting-edge development in the field. States included in this region stretch from Maine to Maryland. www.nesea.org and HeatSpring to teach an online course as part of NESEA Building Energy Master Series with other experts from the NESEA community. Over 150 professionals have taken my Zero Net Energy Homes design course, and the next course starts on February 3rd.
This course is an opportunity to study with me: to ask me questions for a full ten-week semester. You will walk away with a comprehensive understanding of all of the key components of a zero net energy home — envelope, systems, and renewables — and how they fit together, with key pitfalls to avoid, and numerical calculators for sizing peak heat loss, glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. amounts, annual energy use, and solar electric systems that will empower you to confidently design a zero-net-energy home.
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What I know (or I think I know…)
(Caution: unfunded research follows.)
1 – Compact superinsulated homes in climates with design temperatures of 0°F or more can often be heated with a single zone unit with the wall cassette located in the main space. As long as the doors to other rooms remain open, the temperatures in those rooms will usually be within 2°F of the space where the cassette is located. We’ve put electric radiant panels in bedrooms in these homes, and there’s a large variation in how much people use them; the variation seems to be driven mostly by whether the door is open.
2 – Without specific detailed measurement of in situ Coefficient of Performance of the units (COPEnergy-efficiency measurement of heating, cooling, and refrigeration appliances. COP is the ratio of useful energy output (heating or cooling) to the amount of energy put in, e.g., a heat pump with a COP of 10 puts out 10 times more energy than it uses. A higher COP indicates a more efficient device . COP is equal to the energy efficiency ratio (EER) divided by 3.415. ), the amount of energy used by these units, metered separately, agrees well with the usage predicted by simple energy models.
3 – In temperatures below design temperatures, the units have enough capacity to heat the houses even though we don’t size them with an intentional safety factor. Recently it dropped to -5°F here on Martha's Vineyard, and my Fujitsu ¾-ton ducted unit was heating my basement (because the air handler is in the basement, and I haven’t got the ducts installed yet) and one-story house, a total of over 2,400 gross square feet, to 70°F. The load had to have been more than the published output of the unit at that temperature (14,000 BTUBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /hour). I was impressed. The best single zone systems have little drop-off in capacity down to 5°F.
4 – The cold climate Mitsubishi Hyperheats are rated down to -13°F and we’ve seen them running at temperatures below -20°F. Colleagues are reporting minisplits not specifically rated at these temperatures running happily at them nonetheless.
5 – Treating these units like furnaces and implementing significant temperature setbacks doesn’t appear to save a lot of energy because they will run at full speed when the temperature is set back up, which is a less efficient operating point. And because they aren’t usually oversized for the house load, it can take a long time to get the house back up to temperature. At my house we regularly set down to 66°F overnight, or when we are both going to be gone all day at work, and set back up to 70°F when we're there – unless the outdoor temperature is going to be in the teens or below, in which case we don't set it back.
6 – Properly designed and installed ducted systems are usually really quiet. However, I made the mistake on my new system of installing the air handler with a very short return – about three feet and one elbow to the return grille – and I’m going to have to do some sound absorption. The supply side is really quiet. Most of our ducted systems are hard to hear, most of the time.
7 – The systems with high outputs at low temperatures are also delivering air at higher temperatures than traditional heat pumps, which is more comfortable. I’ve measured supply air at 120°F from my air handler this past month.
8 – If the homeowners let the filters get dirty, output can drop dramatically! These aren’t your father’s 100,000 BTU/h gas furnace. The blowers have significantly lower static pressure capability than what most HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. folks are used to. My air handler allows the external static pressure to be selected at the controller (not by the homeowner) and the maximum is 0.36 inches. So ducts may need to be larger than you think if the distances are significant or there are a lot of fittings.
9 – In cooling, the fact that these systems are variable-speed and have such a wide range of operating points means that oversizing appears to have little consequence (usually in a heating climate, the heat pump size is determined by the heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load., so the cooling ends up oversized). Our clients here on humid Martha’s Vineyard have been very pleased with these units' dehumidification performance.
10 – Minisplits are not yet drop-in replacements for fossil-fuel furnaces and boilers in conventional housing, but we’ve been applying them selectively in cases where the existing system is due for replacement. Surprisingly, from a source energy point of view they reduce carbon emissions. On one project, a single oil boiler heated and provided hot water to two adjoining homes totaling 7,500 square feet. It was replaced by three Daikin Altherma air-to-water heat pumps and two electric boilers (for severe weather – the Althermas hadn’t enough capacity, and more importantly, high enough temperature water at low outdoor temperatures). Annual electrical usage into the new system was just under 20,000 kWh. Oil consumption before the retrofit was 3,187 gallons/year, with associated CO2 emissions of 71,389 lbs/year. Using the EPA eGrid figure of 728 lbs CO2/MWh for the NEWE generation region, CO2 emissions dropped to 14,457 lbs/year, an 80% reduction. Fuel cost was reduced by 70%.
11 – Ducted systems need well insulated ducts and they need to be airtight. My system has a design air flow of 353 cfm. Lose 100 cfm of that into the basement, and have ducts wrapped with bubble wrap, and half the output might not reach the living space.
What we’d like to know more about
1 – A consistent and understandable rating system for both capacity and efficiency. It’s frustrating to go to the AHRI Directory and find the heating rating at 17°F (the lower of the two rating temperatures) and then learn that the unit has a higher capacity at 5°F in the manufacturer’s engineering literature.
2 – A good quick method for taking the HSPF and SEER(SEER) The efficiency of central air conditioners is rated by the Seasonal Energy Efficiency Ratio. The higher the SEER rating of a unit, the more energy efficient it is. The SEER rating is Btu of cooling output during a typical hot season divided by the total electric energy in watt-hours to run the unit. For residential air conditioners, the federal minimum is 13 SEER. For an Energy Star unit, 14 SEER. Manufacturers sell 18-20 SEER units, but they are expensive. ratings, assuming they can be made comparable and useful, and modifying them for different climates. I know how the machines we’re using in New England work, but I can’t tell you much about what to expect in substantially different climates.
3 – A rating that includes how much power the system draws on standby, when the thermostat is satisfied. We’ve measured some surprisingly high wattage, and inquired of the manufacturer, to be told that we’re measuring improperly and that because of very low power factors the actual power draw is much lower. I don’t think we really know.
1 – An efficient, affordable air-source heat pump that provides space heating, space cooling, and domestic hot water.
2 – An air-to-water unit that makes 160°F water at outdoor temperatures of 0°F, to serve as a drop-in replacement for existing fossil-fuel boilers.
What do you want to know more about?
Do you have any questions related to designing a net-zero-energy building? Now's your opportunity to get some of your questions answered by someone who has done it for himself and others. As they say, good judgment comes from experience, and experience comes from bad judgment! Perhaps my experience can help you along with one of your challenges.
Here’s how this “mini-consultation” will work:
- Step 1. In the comment section below, write a specific problem that you’re facing or question that you have about zero-net-energy home design. Feel free to ask multiple questions if you’d like. Please, be as specific as possible.
- Step 2. You must submit these questions by Friday January 10 at 9:00 p.m.
- Step 3. I will select five of the best questions and write in-depth responses to those questions on Thursday January 16th.
The goal is that these responses will have tangible impact on your work, and that it can then help others that are facing similar issues. Think of this like a mini-consultation with me. I’d like to go into more detail that I can in the comment section of an article in a forum.
Marc Rosenbaum is director of engineering at South Mountain Company on the island of Martha's Vineyard in Massachusetts. He writes a blog called Thriving on Low Carbon.
- Marc Rosenbaum
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