The Mystery of Misapplied Mini-Splits

Inverter-driven mini-split heat pumps hold tremendous promise for slashing energy use for space heating and cooling in virtually every American climate zone. Equipment from some major manufacturers operates reliably to outdoor temperatures as low as -15°F, as described In our recent blog post on Cold Climate Air Source Heat Pumps. However, a long-time reader and past contributor to the Zero Energy Project challenged the low-temperature capability of mini-splits based on his experience in Massachusetts winters. He observed that his heat pumps did not heat his home below +20 degrees F. And he cited a discussion thread in Green Building Advisor that included some criticisms of mini-split performance in cold climates.

In addition to those comments, heat pump problems reported from the field have included excessive energy use and failure to maintain the comfort originally expected. These reports raise a critical question: how to achieve the promised performance of mini-splits in the real world. The answer is not a mystery — in a nutshell it is good design and installation .

A Fujitsu document sums up the issue: “According to a national survey well over 50% of HVAC companies do not size central heating and cooling systems the right way.” There are some smart, dedicated HVAC contractors that consistently install successful systems. Unfortunately, there is the other half that seems to cling to outdated practices, install poorly performing systems, and give this excellent technology a bad name. How can you ensure that you have a system that lives up to its potential? Whether you are an HVAC installer, a builder, designer, or homeowner there are some basic principles to follow.

Low Load Homes

It’s certainly possible to install a ductless mini-split in a conventional home with “typical” insulation and air sealing, but unless you improve the energy efficiency of the building envelope, it may not meet expectations. The best path in this situation would be to upgrade the insulation and air sealing before engaging a contractor to design a mini-split system. The result will be a heating system that is smaller, less costly, and more efficient, and home that is more comfortable.

For new construction with high insulation levels, low air leakage, and better windows, far less heating and cooling capacity is needed. But since energy efficient, high performance homes come in a variety of sizes and configurations, they have a wide variety of heating and cooling needs. So a successful mini-split system must be carefully matched to the thermal characteristics of the building. Every heat pump installation must be a custom design and not an application based on past experience with other homes or general rules of thumb.


Accurate sizing is not magic. It requires the use of reliable software that follows the industry standard Manual J. Wrightsoft and Elite are two examples of commonly used software programs, and some equipment manufacturers offer their own applications as well. Like any calculation, heating and cooling loads are subject to the old adage, “Garbage in, garbage out.” Each home’s thermal characteristics must be properly entered for the software to yield an accurate result. It’s critical that building insulation, air leakage, window performance, and solar heat gain be correct. The building’s location must be set to the proper climate zone and elevation.

The Manual J software calculates the amount of heating and cooling needed by the whole building as well as the amount needed in each room. A room-by-room Manual J load calculation is essential for proper HVAC design and occupant comfort.

Oversizing is a common problem. It’s often a response to an HVAC contractor’s fear of comfort complaints and call-backs. While large central systems also suffer from poor efficiency when oversized, they seem to tolerate the habit. This has led to oversizing becoming a common practice in the industry. It can result from inaccurate Manual J inputs or from the well intentioned, but mistaken, application of multiple “safety factors.”

One challenge is the simple fact that equipment comes in a limited number of sizes. If the calculated load falls between two standard sizes, most installers will select the larger size “just to be sure.” It’s common practice, but it can lead to problems for mini-splits. One solution is to closely match equipment capacity (size) to the calculated load. If the closest size is more than 10-15 percent higher than the calculated load, it might be best to slightly undersize the system. Any concern about the heating needs of small, isolated rooms or extreme weather can be addressed with small electric resistance heaters. This minor “insurance” measure will allow the mini-split to be sized more closely to the actual load range and run at peak efficiency while the house is covered for the most extreme conditions.

Equipment Selection

To properly size the heating system, it is important to understand the equipment. While inverter-driven mini-splits have the ability to adjust output, they have limits. Unlike the output of a gas furnace or a central heat pump, the “rated” output of a mini-split is not the whole story. To understand output over the entire operating temperature range, it’s necessary to study the specifications. The industry standard for proper equipment selection is presented in Manual S and should be used to select the most appropriate equipment. Good manufacturers offer design tools to help with equipment selection. HVAC contractors and HVAC designers should be sure to use a manufacturer’s sizing tools every time. To be sure that the model is suitable for a cold climate, check the list of cold climate heat pumps available from Northwest Energy Efficiency Partnership.


Some design problems grow out of a concern for distribution of conditioned air to all rooms. HVAC contractors are accustomed to extending a connection to every room. With a typical forced air system in a conventional home, this means a register ducted to a spot near a window on an outside wall. It’s no surprise that they would approach mini-splits in a similar way. It’s common to hear that an indoor unit is installed in every room using a multi-zone, mini-split system. This is seldom the best approach. Interior rooms without much exterior wall or window area will often have requirements far below the minimum output of the smallest indoor unit.

This horizontal ducted mini-split system is located in dropped soffit running down a hallway.

Mini-splits create an entirely new situation that requires a different approach from conventional systems — especially in zero energy homes. High performance features not only slash the total heating and cooling load, but they reduce drafts and increase comfort. Placing an indoor unit in every room is seldom necessary. The size and location of indoor units must match the comfort requirements of each room. Small rooms might need only an open door or a transom to stay cozy. Another option is to install a small transfer duct to connect a small space to a larger one. Air moves through the duct naturally or is driven by a thermostatically-controlled fan.

Multi-zone systems — in which two or more indoor units connect to one outdoor unit — work well, but offer the temptation to install an indoor unit in every room. This is good for distribution, but may lead to oversizing, because the outdoor unit must be sized to equal the sum of all indoor units. Again, it’s common for a very small room to have a load lower than even the smallest indoor unit. Not only does this lead to oversizing for that room, but it could also lead to an accumulation of excess capacity that may well be higher than the total load of the building. In high performance homes where multiple zones are needed, it may be more effective and possibly less expensive to install two or more single-head systems — referred to as a “one-to-one” approach — instead of one larger, multi-zone system. Good design analysis should consider both approaches and choose the better one.

Ducted Mini Splits

One great solution to provide heating/cooling to a group of rooms with relatively small loads is a horizontal ducted mini-split system. For example, one horizontal ducted system can serve several bedrooms, and a typical indoor unit covering the main living space. Some manufacturers offer a vertical air handler that connects to a branched duct system just like a conventional heat pump. An essential element of good design is making sure that comfort is delivered to all areas while keeping the overall system capacity in line with the total building load. Each duct run should be calculated to deliver the amount of conditioned air to each room as calculated by Manual J. Duct design is done according to Manual D.

Installer Training

The HVAC industry has a wealth of training resources for mini-split installers. The American Air-conditioning Contractors Association publishes the industry standards mentioned above and offers training on how to apply those standards. Most installers have a relationship with one or more equipment manufacturers who can provide training, technical materials, and individual consulting. Installers should take advantage of these offerings.

Smart Shopping

If you are a general contractor or homeowner hoping to invest in a mini-split system, you must find a contractor who is knowledgeable and conscientious. Even “reputable” contractors may not be the best choice unless they have the needed mini-split training and experience. Here are a few recommendations for due diligence.

  1. Get bids from three different installers.
  2. Ask them to describe the process they use to select equipment. It should include:
    • verifying the thermal characteristics of the building (insulation, air leakage, window performance, etc),
    • calculating equipment size with industry approved software
    • calculating duct diameter and length
    • delivering the right amount of comfort to each room
  3. What kind of training and certifications have they received?
  4. How many mini-splits have they installed? Ask for references.
  5. Before accepting a bid, review the full, multi-page Manual J report (not just the cover sheet) and check these for accuracy:
    • Is the climate and location correct?
    • Are the building characteristics (floor area, insulation, air leakage, etc.) correct?
    • Do the room-by-room load values closely match the capacity of the indoor units selected for those rooms?
    • How much “safety factor” has been added?
    • Is the total heating and cooling load within 10-15% of the rated capacity of the equipment specified? (There are quite a few variables that factor into this step, so the installer may have good reasons to exceed this range. The goal is to prevent unnecessary oversizing. If the equipment size falls outside this range, be sure that the installer has good reasons, and that they agree to stand behind the long-term performance of the system.)

Chances are good that any contractor that does all this will not be the lowest bidder. But that’s okay. Paying a small percentage more for proper heat pump installation will be money well spent — because it will provide low-cost comfort for decades while reducing carbon emissions.

Make Hot Water the Modern Energy Efficient Way

Water heating represents a big slice of the home energy use pie. As the housing industry moves away from fossil methane (natural gas), it’s essential to heat water as efficiently and economically as possible. The best water heating choice for getting your home on the path to becoming an energy efficient home is to install a heat pump water heater (HPWH). Here’s how to select, locate, install and operate one.


Modern HPWHs are about 300% efficient, meaning that for every unit of electricity consumed, three units of hot water are produced. The basic idea behind these extremely efficient water heaters has been commercially available for more than 100 years and is the same technology used in today’s household refrigerators.

Fossil methane is commonly used for water heating due to its low cost. While gas water heaters rely on less expensive fuel, a typical gas-fired tank is only 60% efficient and the most efficient tankless model tops out around 98% efficiency. At this stage in the evolution of energy efficient homes both tank-style and tankless gas water heaters should be off the table entirely for three reasons. First, natural gas is a fossil fuel that leads to even greater greenhouse gas emissions than are produced from burning coal. Second, heating water with a super-efficient HPWH has a lower life-cycle cost than heating with fossil methane. Third, building natural gas infrastructure into new housing developments and in new homes is an unnecessary expense. HPWH will serve this function without the added expense. It’s time to move past natural gas.

How HPWHs Work

A heat pump uses a refrigerant gas to capture heat from surrounding air. A compressor increases the pressure and temperature so that the gas condenses and releases its heat. This process cools the air around the water heater while pumping heat into the tank of water. As a result the space around the HPWH will be cooled and the air dehumidified as the water is heated.

Image: USDOE

Equipment Selection

Modern HPWHs have benefited from many years of research and development. All major water heater manufacturers now offer them with warranty periods of 6 to 12 years. Reliability has been quite good. Most products are called “hybrid” water heaters to reflect that all HPWHs also contain electric resistance elements as a supplementary heat source. Consumers can choose to operate them strictly with the heat pump or in combination with the resistance element.

In warm climates, just about any model of HPWH will work fine. These are called Tier 1 models. However, in colder climates, it’s important to select a Tier 3 model designed to operate efficiently at lower temperatures. Here’s a list showing the Tier 3 HPWH models appropriate for cold climates. Efficiency is expressed as the Uniform Energy Factor or UEF with higher numbers being better.

Tank Size

Rapid temperature recovery is important for all water heaters because as hot water is drawn it’s replaced in the tank by colder water. Heat pumps definitely take longer than standard electric water heaters to reheat the water, so HPHW tank size is important. Larger tanks help compensate for slower recovery. Since hot water demand is a function of how many people are served, select a tank capacity based on the number of people in the household. A 50-gallon model is sufficient for 2-3 people. Households of more than 4 will need an 80-gallon unit.


HPWHs need generous space for airflow and can’t be tucked into a small closet like standard electric water heaters. Generally, HPWHs need about 1,000 cu. ft. of space which would be, for example, an 11 ft. x 11 ft. room with an 8 ft. ceiling. Specific manufacturers may have slightly lower requirements, but generally speaking more free air volume around the unit is better. Ceiling height is also a consideration, because HPWHs tend to be taller than other water heaters due to the compressor and fan mounted on top of the tank.


The fact that a HPWH draws heat from its surrounding air leads to several important considerations for where to locate the unit. All HPWHs remove moisture from the air and produce a condensate. This water must be piped to a drain of some kind. A utility sink or floor drain are ideal. If an appropriate trapped drain is located farther away from the HPWH’s location, a condensate pump may be used to reach it

In climates that need a lot of cooling, a HPWH can be located in an indoor space that will benefit from the cool, dry air. In climates where homes need more space heating than cooling, HPWHs should be located in areas that are not heated directly but stay warmer than outside. The space must remain within the HPWHs operating temperature range of about 40°F to 90°F. Noise from the HPWH’s compressor motor and a fan is also a consideration. A location near bedrooms would not be a good choice.Taking all of these factors into account, a good location for colder climates would be a basement or garage that is insulated, but not heated directly, or possibly an indoor utility or laundry room.


HPWHs have some requirements for space, air flow, air temperature and noise management that don’t apply to conventional water heaters. Many or all these limitations can be sidestepped by using ducts to supply air to the unit and/or to exhaust air out of the house. All HPWHs offer optional duct kits that allow standard HVAC ducting to be attached. This opens a range of possibilities.

For example, the HPWH could be located in a small interior mechanical room or closet. A supply duct could draw air from outside in mild climates. In cooler climates, supply air may originate in a tempered space, such as a crawlspace, basement, or garage. Similarly, the cold exhaust could be directed through a duct to the outside — or in warm climates to an area where cooling is desirable. This arrangement allows the tank to be centrally located without reducing comfort or increasing space heating energy use.

Financial Factors

According to the the US Department of Energy, a typical household could save as much as $3750 over the 13-year life of a heat pump water heater compared to a standard electric model. This figure is based on national averages and may vary significantly depending on specific situations. In the process of designing a zero energy home, an energy model will give a much more accurate prediction of savings and how a HPWH meshes with other energy efficiency features. Many utility programs offer incentives for HPWHs – so check with your utility for details. They may require product and installation specifications.


Basic operation of a HPWH is not only easy, but may offer features that are more convenient than most standard water heaters. You can set the operating mode to “Hybrid” and walk away. This setting will ensure the high efficiency heat pump provides hot water whenever possible. When high hot water demand exceeds the capability of the heat pump, the built-in electric element kicks in to meet the need. Many HPWHs offer other helpful options, such as a convenient digital interface for setting water temperature, a mode called “Electric” or “High Demand” that locks out the heat pump and forces the electric element to do all the work, a “Vacation” mode that reduces energy use during long absences and triggers the water heater to resume normal operations on a specific day, and a control interface that allows the household to participate in a utility demand response program that reduces water heating operation during certain time periods in exchange for favorable electricity rates.

Hot water is an essential part of modern life but also a major contributor to residential energy use and carbon emissions. Installing a HPWH is a step in the right direction toward getting an efficient home on the path to zero energy.

Production Builders, It’s Time to Sell Zero Energy Homes

Production builders are uniquely positioned to identify and fulfill the needs of mass market home buyers. Most buyers assume that a new home is more comfortable, healthier, and more durable than an existing home, including the one they are leaving behind. Zero energy homes check all these boxes. Unfortunately, there is one misconception that prevents many production builders from offering zero energy alternatives. It’s the perception that zero energy homes are not affordable. In fact, zero energy homes cost less to own, provide their own buying power and build wealth starting the very first month.

The housing market is starting to change as the number of zero energy homes and zero energy ready homes grows steadily each year. Here are a few points for production builders to consider as they contemplate the pros and cons of joining the shift to zero.

  • Energy efficiency is slam dunk cost effective for the consumer. They can afford a slightly higher mortgage payment because the energy costs are lower.
  • This selling point must become second nature to the sales team and lenders.
  • Many builders are moving incrementally from their current construction practice toward higher efficiency.
  • A good first step is to have all homes certified by a third-party certification program, such as ENERGY STAR or Zero Energy Ready Homes.
  • Experience gained by participating in one of these entry level programs can be used to plot a course toward zero.
  • Energy modeling can be used to cost-optimize a package of performance features to get the most energy efficiency for the least cost.
  • While some builders start by offering zero energy options, it is best for builders to transition all their homes to high performance and then to all zero energy.
  • There are several successful production builders who have made the transition to 100% zero.

The biggest obstacle to production builders getting their developments on the path to zero is finding a way to pass on the higher initial cost to buyers while showing them that they are getting a better home with a lower cost of ownership. This is not a technical construction issue but involves educating customers regarding the high value they are receiving, finding the right financing vehicle, crafting an effective marketing strategy that meshes with the builder’s business model.

Sales Team Training

Successful production builders recognize the critical importance of an effective sales team, whether they are in-house or independent brokers. While it’s easy to see solar panels or a heat pump water heater, most energy saving features are invisible, so selling high performance benefits takes specialized knowledge and skill. Detailed broker training is available online and in person. Every person on the sales team must be enthusiastic about energy efficiency, knowledgeable about its benefits, and effective in communicating these points.


High levels of energy performance, along with the additional benefits of health, durability, and affordability, must be featured prominently in print and online advertising. Thrive Homebuilders in Denver is a good example of how energy features and benefits are presented on an equal footing with the common themes of location, finishes, and aesthetics.

Model homes must be staged to feature not only architecture and decor but must also highlight hidden energy performance features and benefits. One way to accomplish this is carefully executed signage that effectively identifies energy saving features and their benefits. Mandalay Homes in northern Arizona offers a good example of a production builder who has fully incorporated energy efficiency, water efficiency, and health into their business model and marketing.


It’s ironic that many production builders target the price-constrained end of the market, while virtually ignoring a major factor in the affordability of home ownership. While energy is no less significant than other monthly costs, energy efficiency offers the unique ability to pay for itself. The tangible result for the home buyer is a lower cost of ownership each month. If marketed appropriately, it will create a competitive advantage for builders at the lower end of the market. Monthly utility payments can be redirected into higher mortgage payments — without adding to the buyer’s overall housing cost, giving them a better home, and a profitable, long-term investment in the home’s structure and equipment.

The financing conversation needs to be re-framed. Consuming energy (and water) every month is an expense. Reallocating that money to efficiency features is an investment with an immediate return. The average homeowner spends around $1945 per year on energy, according to the latest official figures available from the US Department of Energy. Reallocating those funds to a 30-year mortgage payment would allow the buyer to finance additional loan principal of $39,000. That is more than enough capital to pay for the improvements, increase builder profit, and reduce the total cost of ownership for the buyer.

Applying zero energy features to a home is a sound financial strategy that turns an expense into a profitable investment for the home buyer. When monthly earnings exceed the monthly debt service, it’s called a profit. Zero energy homes are investments that bring their own buying power to the table. The return on investment can often beat returns from stocks.

This cold, hard economic advantage seldom finds its way into the sales and marketing efforts that sell homes. Savvy production builders leap over the first-cost barrier and show buyers how zero energy homes are good investments. Builders who can facilitate this investment will reap the reward of more sales and higher profits.


One way to overcome any first cost barrier is long-term financing, which is exactly what a 30-year mortgage is intended to do. Most mortgage lenders will acknowledge that lower monthly expenses equal higher qualifying income, and that means streamlined underwriting and higher loan values. Research also shows that occupants of energy efficient homes pose a lower default risk.

For conventional mortgages with buyers who can afford large down payments, mortgage qualification is not a big issue. These buyers have the capability to borrow enough principal to reap the rewards. It’s the job of the sales team to show the financial benefits to these higher income buyers.

It’s borrowers on the margin that may bump into income limits and other underwriting obstacles. For them, conventional guidelines often prevent lenders from taking advantage of a buyer’s greater buying power. Two relatively new secondary market programs address this issue. They are Greenchoice from Freddie Mac and HomeStyle by Fannie Mae. Both programs offer higher loan-to-value and debt-to-income ratios. If lenders are uninterested in these opportunities, builders must press the issue by demonstrating and documenting the added value of zero homes to lenders and appraisers. Some areas offer financing innovations such as PACE loans, or green banks, such as the one in Connecticut.

Production builders and their agents can clearly document the added value of these highly energy efficient homes. Then they can request the lender utilize appraisers with green appraisal training. If certified green appraisers are not available, they can fill out the green appraisal addendum and make sure that the appraiser uses it. These methods make financing available to lower income homebuyers and help them bring the higher initial costs of energy efficient homes within their budgets.

Shape the Market

Finally, every successful production builder has developed an approach that works for them. Many target affordability and effectively use the economies of large scale and corporate buying power as instruments of their success. While there may not be fat in the profit margin, there are many ways to cover the costs of greater efficiency within the finite “buying power” amount described above. These ideas for affordable zero energy design and construction can be applied to the zero energy home developments, so both developers and their customers benefit.

Of course, most businesses are reluctant to fiddle with the formula that brought them past success. However, the businesses that continue to enjoy success are those that embrace and to some extent shape the future. Production builders have the capability and responsibility to spearhead the zero energy homes movement.

It would seem that any business person would jump at the chance to sell a slightly higher priced, more profitable, product that cost less to own for buyers. With thoughtful design, careful construction, skillful marketing, and enlightened financing, zero energy homes provide production builders with a superior business model that pleases customers, adds to their bottom line and is good for the planet.