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 Unfortunately, there is the other half that seems to cling to outdated practices, install poorly performing systems, and give this excellent technology a bad name.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.
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.
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.
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.
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.
- Get bids from three different installers.
- 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
- What kind of training and certifications have they received?
- How many mini-splits have they installed? Ask for references.
- 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.