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Green Building News

Startup Promises Window-Mounted Heat Pump by 2022

San Francisco-based Gradient has not disclosed the price but says the unit will help cut carbon emissions

A San Francisco company is developing a window-mounted heat pump it says will operate more efficiently than AC units already on the market while also providing space heating. Photo courtesy Gradient.

A San Francisco startup says it will introduce a window-mounted heat pump next year that will provide both heating and cooling at a lower cost than professionally installed systems.

The device is shaped like a pair of saddlebags, with the evaporator and fan on the inside below the level of the window sill and the compressor on the outside of the building. The design keeps the noisy part of the system outdoors and blocks less of the window than conventional window AC units.

Fast Company posted an article about the Gradient late last month, claiming that when used for both heating and cooling the device can reduce the carbon footprint of HVAC by 75%. “As the grid shifts to renewable energy, that footprint could essentially be eliminated completely,” the article says, “so turning on the AC doesn’t have the ironic effect of making future heat waves more likely through global warming.”

The company’s website offers basic information about the unit, including its weight, dimensions, and capacity (9000 Btu/hr in cooling mode, heating output to be announced). Gradient says the unit will run on a 120-volt circuit and will use an inverter so it can adjust its heating and cooling output, much like minisplit systems do now. It’s designed for rooms up to 450 sq. ft. Final specs will be available after adjustments are made in Beta testing and third-party lab verification, the website says.

Replying to a list of written questions, Gradient CEO Vince Romanin said in an email that the unit would be commercially available before next summer.

“We aren’t announcing the price yet,” Romanin said. “Gradient will have the efficiency and power of professionally installed products with similar features at a fraction of the price of those units.”

Gradient said it expected the unit would lower carbon emissions by 75% when compared to “today’s systems,” although it didn’t provide details on how that calculation was made.

Photo of Gradient heat pump from the outside
The low profile Gradient heat pump keeps the noisy part of the mechanism outdoors. Photo courtesy Gradient.

“This is based on using more environmentally friendly refrigerants, being more efficient, and displacing fossil fuel heating with electric heat pump heating,” the statement said. “Our end goal is a fully decarbonized heating and cooling system.”

The company says the device will use R32 refrigerant, a replacement for R410A that has a lower global warming potential. “Our goal is to move to even more climate friendly refrigerants in the near future,” Romanin said. “Natural refrigerants create more efficient and climate-friendly products; we believe they are the future of the industry. We are working with regulatory and industry standards to get full approval in the U.S.”

Romanin didn’t say how the Gradient’s  efficiency ratings would compare with heat pumps and window AC units currently on the market. But he said the device will be evaluated and get a CEER rating.

Fast Company reported that Gradient grew out of Otherlab, a research company that studies energy use and helps design new products. Romanin told Fast Company that lowering carbon emissions is a key goal. “We realized that air conditioning is an important public health need,” he said. “And we realized that we’re in kind of a vicious cycle where today’s systems are really high in carbon emissions and growing in use. It doesn’t have to be this way. Technology exists to make heating and cooling systems that don’t have high carbon emissions.”

In an apparent reference to ductless minisplits, Romanin said more efficient air conditioners already exist but are too expensive for many people to afford.

Others share U-shaped design

The U-shaped design of the Gradient is unusual but not unique. Fast Company’s article attracted the attention of several GBA readers, including Giles Winden, who posted a link to the article in the Q&A Forum. That prompted Sean Cotter to point out that the Gradient heat pump shares its design with an AC unit made by Midea.

Media’s website lists a U-shaped window AC unit that also separates the fan from the noisy compressor. Its shape allows the window to be opened while the unit is installed, but unlike the Gradient the Midea air conditioner sits on the window sill, not below it. The unit has a rated capacity of 8000, 10,000 or 12,000 Btu, but it does not provide space heating. The 10,000 Btu unit lists for $399.

Soleus Air also makes a window AC unit that hangs from the window sill. Its 8000 Btu saddle model sells for $570 on Amazon.

Several posters offered comments on the Media and Soleus models and said they had signed up with Gradient in hopes of becoming beta testers for its new heat pump.

AC efficiency is a big deal

Air conditioners that use less electricity and run on refrigerants with a lower global warming potential would help mitigate a growing global energy problem. In a 2018 report, the International Energy Agency said increased AC use would be one of the top drivers of global electricity demand over the next three decades as the number of AC units surges. Energy use from air conditioners is expected to triple by 2050.

“Growing electricity demand for air conditioning is one of the most critical blind spots in today’s energy debate,” IEA Executive Director Dr. Fatih Birol said.

In response to growing demand for air conditioning, the Indian government along with the international organization Mission Innovation and the Rocky Mountain Institute launched the “Global Cooling Prize” to encourage the development of more energy efficient designs.

Earlier this year, two winners were announced: Gree Electric Appliances, with its partner Tsinghua University; and Daikin and partner Nikken Sekkei Ltd. According to the announcement, these companies have developed prototypes with five times less climate impact than standard AC units now on the market. The units should be on the market by 2025 and have the potential to mitigate global warming by 0.5°C.

UPDATED: This article was updated on Aug. 23 to include new information from the company and revisions to its website.

Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.


  1. Deleted | | #1


  2. GBA Editor
    Scott Gibson | | #2

    Thanks, Charlie. The link has been repaired.

  3. 1910duplex | | #3

    I wish I could try one of these, but my radiators block where it would go.

    1. Charlie Sullivan | | #4

      If this in fact can heat well, your radiator wouldn't be needed anymore, so you could simply remove it to make room for this. But we'd need to have more spec for it and see how it does in your climate to know.

      1. sprockkets | | #27

        If they have a radiator system then most likely this unit will not help them out for heating, even if inverter based. It can help for some months though.

  4. CarsonB | | #5

    I have the midea ac unit. It’s great but really hard to air seal well. There are many awkward gaps at the top and bottom of the unit. This will likely have the same issue. These are great for rentals, otherwise I would have to question how much easier it really is then a diy minisplit like mr cool which will likely look and perform better.

    1. Charlie Sullivan | | #6

      I fear that DIY miniplits are leaking a lot of refrigerants. This will probably be more climate friendly as well as being accessible to people who are intimidated by a DIY minisplit install.

      1. CarsonB | | #7

        how do they leak? The linesets are pre-charged on the DIY kits, you just plug them in. Granted they are certainly harder then a window unit to mount and drill holes.

        1. Trevor Lambert | | #11

          The DIY types are precharged (as are plenty of non-DIY ones), but surely you still have to connect the tubing, don't you? I can't picture how you could have a pre-assembled mini split, with the line set already connected. That's where the majority of leaks occur.

          1. CarsonB | | #15

            Trevor, this is the first I’ve heard of leaks from just connecting them. Would make a good article? A lot of diy installs out there based on the number of reviews.

      2. Expert Member
        Malcolm Taylor | | #8


        Your discussion with Charlie got me thinking. The market for these U-shaped units seems limited to those (older) buildings with appropriate type and size of windows. But there is something alluring about being able to purchase a mini-split with the two parts already integrated and not have to run lines between them. I wonder if some version of this will become available that could be mounted through the wall without any addition work required beyond plugging it in? Perhaps with a gasketted sleeve that is installed during the framing?

        1. CarsonB | | #9

          Malcolm, you mean like a single large unit that you just put into the wall and through the framing to the exterior, similar to installing a window? that would be interesting. That seems the least DIY friendly though.

          1. Expert Member
            Malcolm Taylor | | #12


            I haven't thought this through at all, but the idea of a plug and play system for which all y0u needed was power on the interior, a small universal sleeve that fitted every model, and two back to back components you just snap together (and could remove for repair or replacement) sounds appealing to me.

      3. Bryan Coplin | | #10

        >I fear that DIY minisplits are leaking a lot of refrigerants.

        If DIY minisplit installation proves to be at all similar to DIY automotive AC repair, I imagine you're right. It would be interesting to know if DIY installs get serviced by professionals, or if they are simply charged by end users (like many DIYers do to their cars on an annual basis). R410Aa is less available to DIYers than R134a, but it isn't particularly arduous to find second hand.

        1. CarsonB | | #14

          Bryan, diy units aren’t charged by users. They come with fixed, precharged linesets.

          1. Bryan Coplin | | #17

            Precharged linesets are still connected by non professionals, and connections are where most refrigerant leaks happen. (I see Trevor mentioned this above).

            Sorry if I was unclear; I'm most curious what happens after installation. Do homeowners take over maintenance of these units?

            When I was an auto tech and played DIY AC repair clean up in the summertime, it was quite clear from their complaints that most people working on their vehicle's AC system didn't know what they doing, and hadn't invested in tools to do the job properly. Many of these vehicles had refrigerant vented to the atmosphere in the course of DIY service, either intentionally or unintentionally.

          2. CarsonB | | #18

            "I'm most curious what happens after installation. Do homeowners take over maintenance of these units?"

            probably. Though based on the logs taped to the systems of past homes I've bought or rented, I would be curious what percentage of people get regular professional maintenance on their hvac system at all.

          3. Expert Member
            Dana Dorsett | | #20

            Bryan has it right- the quick connect pre-charged line sets still has to remain clean and be torqued properly to keep the leak risk low. (...and as always the more idiot-proof they make them the more creative the idiots become. :-) )

            The linesets are only charged with only a very small amount of refrigerant- just enough to maintain a slight positive pressure (much much lower than the operating pressures) during shipment to avoid contamination with air/water. If the seals for the pre-charge leak and the linesets take on air it may install just fine and may not leak refrigerant AFTER installation, but the contamination can dramatically shorten the lifespan of the unit. A savvy DIYer might be able to tell if it's seal has been broken by releasing a tiny amount of the pre-charge before making the final testing just to verify that it's positively pressured, but I'm sure that a large fraction of the DIY installers never fully absorb the instructions and treat it like a plumbing problem.

            There are quality-control issues with linesets too- probably more so with the DIY units than with flared connections. (An obviously dinged flare end can be cut and re-flared.) I recently saw a video posted by an of an unhappy Mr. Cool customer installing a multi-zone unit chasing down a leak which was found in the brazed connection between the line and the quick-connect refrigerant connector on one of the linesets. It may or may not have been leak free at the lower lineset shipping pressure, but clearly leaky when the charge inside the compressor unit was being released. I'm not sure if or how that was ever resolved, but even if it was eventually fixed it likely dumped a substantial amount of R410A by the time it was up and running properly.

          4. CarsonB | | #26

            Thanks for the comments. Sounds like it may be worthwhile to have a professional look it over.

  5. Eric Habegger | | #13

    After seeing the design of this unit I'm very interested in it. I need a more carbon neutral method of heating my home than my my present method using a propane fireplace. I can imagine Midea must be kicking themselves on their inferior design. There is no real incentive to buy a Midea over a window wall shaker as far as aesthetics are concerned. Some of us care about that stuff. Gradient's design is so similar to it but much more elegant and not balanced so precariously. If they are at all as promised I will replace any window that is required to a single hung window just so I could install one of these Gradients. The best designs are always the ones where you do a face plant and say "why didn't someone think of this earlier?"

    1. CarsonB | | #16

      Agreed. The midea is better than a wall shaker ( I can see around the sides), but yeah making the indoor unit go down instead of up seems pretty obvious.

      1. Joshua Van Tol | | #19

        Going up is simpler though, as you don't have to accommodate varying wall thicknesses.

        1. Expert Member
          Dana Dorsett | | #21

          That's right, AND since the sash is a handful of inches back from the lip of the sill the inside half doesn't project into the room as much, which makes a difference if furniture (or a radiator) happens to be placed close to the window.

          In my tiny loft-office at home that was one of several reasons for going with a Midea rather than a Soleus, which would have been much simpler to install, but would have made an already narrow room a few inches narrower due to the necessary placements location of a work table & desk. (The ability to modulate to very low levels along with significantly higher efficiency were also major factors on the decision tree, rendering the R32 vs. R410A mere icing on the cupcake.)

        2. Ross M. | | #22

          The Soleus is also hopeless in a dormer or other situation where there is a roof below the window.

          I put the Midea in a dormer window and it required a small modification to the AC bracket but otherwise works well.

  6. user-6958335 | | #23

    Does this type of window-mounted heat pump violate fire codes for egress windows?

    1. Expert Member
      Malcolm Taylor | | #24


      I don't see how it would, unless it reduced the size of the opening to less than the minimum required.

  7. Fortunat Mueller | | #25

    One possible benefit of these factory charged and sealed units is that they can possibly be built to use refrigerants with lower GW potential and higher low temp performance, even before the local tradespeople make the conversion. Of course, they'd need to be pretty reliable since you won't have many folks who can service the refrigerant side in the field, if for example they use CO2 (R744) refrigerant.

    Here's another through the wall unitary HP product that i think is interesting: Haven't seen it in person yet, just in a spec sheet. Not as simple for DIY as the window units, but also probably higher performance and doesn't introduce the air sealing issues other commenters have raised with having a window open 24/7/365.

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