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Best Practices

Getting Ductless Heat Pump Installations Right

Factors that cause a heat pump system to function improperly and how to prevent common callbacks

Thorough testing of refrigerant line sets can prevent many ductless callbacks. Here, newly flared line sets are pressure-tested with nitrogen.

Ductless heat pumps are surging in popularity. This growth has been driven in part by improvements in technology that allow them to perform well in cold climates. Recent increases in fossil fuel prices have also contributed, as have state and utility programs that aim to reduce carbon emissions by electrifying buildings. The recently enacted Inflation Reduction Act will further boost heat pump adoption by increasing federal tax credits and funding new rebates for low- and moderate-income households.

The boom in ductless heat pumps is bringing new installers to the field. Some come from traditional HVAC backgrounds and are more familiar with conventional furnaces, boilers, and air conditioners. Others—electricians, home performance contractors, solar installers, and carpenters—come from related trades; some are new to contracting entirely.

The barriers to becoming a ductless heat pump installer are relatively low in terms of basic skills and required tools. But, despite their seeming simplicity, ductless heat pumps can be unforgiving of poor design and workmanship. A bad ductless installation can become a nightmare for homeowners and contractors alike. And bad outcomes lead to bad publicity, slowing heat pump adoption and climate progress.

A few easily avoided errors are responsible for most problems I’ve seen on ductless systems. If you’re a contractor, paying attention to these critical details can avoid costly and emotionally draining callbacks. If you’re a homeowner, knowing about these pitfalls can help to vet potential contractors and ensure that your new system is trouble-free.

Refrigerant leaks

Heat pumps work by moving heat between the indoors and the outdoors. The fluid that carries the heat is known as a refrigerant. As refrigerant moves through the system, its pressure is raised and lowered, and it changes back and forth between gas and liquid states.

As it enters the warm side…

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  1. rondeaunotrondo | | #1

    Thanks Jon. Where do you source those “tiny roofs” that I’ve seen pictures of attached to the top of units to deflect snow and change the drip line, mostly up north?

    1. Jon_Harrod | | #2

      We make ours in-house in our sheet metal shop and paint them. A similar product is here, with options to fit Fujitsu, Daikin, and Mitsubishi outdoor units:

      1. rondeaunotrondo | | #4

        Thanks Jon.

  2. Expert Member

    Thanks, Jon. Another one of your blogs that goes straight into my tech file for future reference.

    1. Jon_Harrod | | #7

      Thanks, Malcolm!

  3. conwaynh85 | | #5

    Great info Jon. If we only had enough qualified people to actually install them we would be in a much better situation. I rarely see heat pumps sized or installed correctly. Testing is like a myth to most of them. Does anyone know a good heat pump technician in central NH? I haven't found one.... Thanks

  4. adam5532 | | #6

    Is there a way to permanently install refrigerant pressure guages so that correct pressure can be verified at any time and slow leaks can be quantified (rather than an arbitrary "it doesn't seem to be working as well as it should")?

    1. Jon_Harrod | | #9

      This should be possible. One of the complications is that refrigerant pressures can be affected by factors other than leaks, like dirty filters or changing indoor/outdoor temperature conditions. But these units have a lot of information to draw on--many have 5 or more temperature sensors, plus pressure sensors and current sensors, so it should be possible to sense a degradation in performance and send a notifcation to the customer and/or installer. I am not aware of any ductless systems that have this capability.

  5. Deleted | | #8


  6. synergytodd | | #10

    It is my belief that most ductless heat pump installations are cop-outs for not understanding proper sizing and installation techniques for duct systems. Maybe ductless mini-splits work well in small homes in the Northern United States, but in humid, Southern areas especially in larger homes, you need ducted inverted driven heat pumps. 1) Size, design, install, and commission the inverter driven all-electric HVAC system correctly. 2) Bring the HVAC system into the thermal envelope and we do it by encapsulating the attic with open cell foam and encapsulating the crawlspace using closed cell foam. 3) Use jumper ducts or transfer grilles and make sure the air has a return path to the unit to be reconditioned. 4) Install makeup air and ventilation air without creating additional moisture issues. 5) Make sure the condensate is drained correctly away from the building. Occasionally, mini-splits work well in bonus rooms without access for ductwork or in sunrooms and small additions. Inverter driven ducted heat-pumps with proper zoning is the way to go.

    1. Jon_Harrod | | #11

      I don't disagree! When possible, a fully ducted system offers a lot of advantages. I talked about some of these in an earlier post here:

      But I think there is a role for ductless systems. Bonus rooms and small, low-load homes, but also retrofits of all-electric homes and apartments, and replacement of steam and water-based heating systems where installing ductwork would be prohibitive. They can work well in the right applications, but they can also turn out poorly.

      1. Expert Member
        MALCOLM TAYLOR | | #12


        The context I'm seeing most ductless mini-splits put in in my rural community here on Vancouver Island are in existing homes as a replacement for other central heating sources (mainly wood stoves), or to provide heat for open living spaces, rather than rely on baseboard electric resistance heaters. Since the houses were already set up to deal with supplemental heat in closed off rooms, one or two ductless units seem to work really well, and are simple to install (if they would just follow your instructions).

    2. 23992134929 | | #14

      The issue with this is a roof encapsulation retrofit is prohibitively expensive, sometimes dwarfing the cost of a well-designed system of ductless units. (That's if you can even get an insulation contractor to call you back.)

      1. kiwiscott | | #16

        Can someone explain this "roof encapsulation retrofit". I think this is adding exterior insulation, air barrier, rafter insulation and a vapor barrier but is there potentially another R60 to my attic, which contains my existing HVAC, without going outside?

  7. 23992134929 | | #13

    I was hoping to install one of these systems myself and hire a professional for the commissioning/charging. However, it doesn't seem like any of the manufacturers will deal directly with a homeowner, even with a permit. Is there a good way to source these products without being a certified contractor?

    1. user-723121 | | #15

      HVAC is not DIY in my opinion and it seems like only the best of them can install the new heat pumps correctly. When I replaced my furnace and AC in 2006 with high efficiency equipment my HVAC installer, who is a pro, hired out the brazing to someone more well versed in brazing. The system has held it's charge to date needing no refrigerant.

      1. 23992134929 | | #17

        I might agree with you if I knew an HVAC contractor who read GBA, cared about my air barrier and comfort, etc. That's basically a mythical creature in my area.

        There are basically three difficult parts to an HVAC install:
        * Design. No HVAC contractor I've talked to is willing to do a manual J calculation, so that needs to be done yourself or hired out.
        * Attention to detail regarding air sealing, water abatement, and condensation: I don't trust the average HVAC contractor with this in the slightest.
        * making the connections, charging the lines, testing; this is what a DIYer should hire an HVAC professional for.

        1. kiwiscott | | #18

          100% agree with you here. Finding the right contractors is proving to be the hardest part of any retrofit idea for me.

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