Gradient May Be a Revolution in Heating and Cooling

This little heat pump could be the answer to a lot of prayers.

Gradient heat pump on outside of building


Professor Cameron Tonkinwise once called air conditioners weeds, view-destroying, and inefficient. He noted: "The window air conditioner allows architects to be lazy. We don't have to think about making a building work, because you can just buy a box." They are often known as "window shakers."

This new Gradient heat pump unit changes all that. It doesn't shake. It doesn't block the view. And it could be the start of a revolution in heating and cooling.

Window air conditioners are by definition heat pumps, moving heat from indoors to outdoors. Conditioning air doesn't just mean cooling either: The terminology we use is archaic and confusing. So we will call the Gradient unit a heat pump because it can move heat in either direction to heat or cool as required.

Another more recent term is "mini-split" describing a heat pump system where the compressor/condenser end is outside, and the evaporator/air-handling end is inside, with refrigerant running in pipes between the two units. This requires skilled trades for installation and a lot of refrigerants filling all those pipes and coils.

Inside a Gradient


The Gradient needs a new term, perhaps "micro-split" in that the compressor is in the outside part, and the air handling is on the inside.

The creative leap they have taken is that the evaporator is on the outside too—it is connected to a heat exchanger, transferring heat to a secondary coolant loop that isn't under pressure. The heat exchanger seems to be the big deal and was patented by CEO Vince Romanin, Saul Griffith, and others. Gradient was spun out of Griffith's Otherlab and we know Griffith loves heat pumps.

Gradient inside


All the noisy stuff is outside; inside, the only moving parts are the quiet fans. Anyone can connect them, so if you have single or double-hung windows, you can install the outside unit—they have a very clever frame design so you won't drop it on anyone—and then the inside. No skilled trades are required to deal with refrigerants connecting the two.

The unit's cooling capacity is 9000 BTU/hr (2637 watts). They have not released the heating or noise information yet, it's still being tested. Gradient says that's "great for rooms up to 450 square feet" but my first thoughts were that this would be perfect for tiny to small houses and houses designed to the Passivhaus standard, where even the smallest mini-split is often overkill. My second thought was, why are these designed for double-hung windows, the leakiest design that is never used in Passivhaus, or for that matter, any building designed with energy efficiency in mind?

Gradient CEO Vince Romanin responded in an email, telling Treehugger:

"We can design different brackets for many different types of windows, but we have to start somewhere, and if someone has a window AC (typically the lowest efficiency and worst user experiences on the market), we can be pretty sure they have a sash window (either single or double hung). So while we will expand to different window types eventually, we chose to solve the biggest problem first."
Exterior installation


In a subsequent interview, Romanin tells Treehugger that in the U.S. "our system is expected to fit into about 80% of windows that currently use a window AC, or 80% of sash-type windows." 8 million window units are sold each year in the U.S., and they determined this would be the quickest way to bring the unit to market–a design addressing the biggest problem, and delivering a new form of split system that doesn't make you wait until you can find an installer, which these days are a very long time. Romanin said "it's trivial to change to other systems" but this is the place to start.

Conceptually it is not hard to imagine it as two separate units where you drill a hole in the wall and run hoses between, much like hooking up a washing machine, since they are not pressurized or full of refrigerant. Then you have a heating and cooling revolution on your hands.

Global Warming Potential (GWP) of refrigerants.

IPCC via Air Conditioning News

The refrigerants are another interesting point. The unit is charged with R-32 or Difluoromethane, which is a hydrofluorocarbon that has a global warming potential (GWP) of 675 times that of carbon dioxide, but a quarter the GWP of the refrigerants it replaced. However, the Gradient was designed to run on R-290, which is propane and has a GWP of just 3. Propane is flammable, so the quantity is regulated for indoor use; the international standard is 2.2 pounds, or a kilogram, a tenth of a barbecue tank. In the U.S., the limit is 4 ounces (114 grams) because the big chemical companies that make refrigerants are (surprise!) fighting the change in the standard that would permit R-290. But Romanin tells Treehugger they will change to it as soon as they can.

When I wrote about Griffith's proposal for rewiring America and electrifying everything, I complained that you have to reduce demand first, or you were going to need big hardware.

"This means bigger heat pumps made with more metal and more refrigerants that are powerful greenhouse gases. One of the benefits of efficiency is you can use smaller heat pumps that can use refrigerants like propane, which are limited in size for fire safety."
Gradient interior straight on


The Gradient is exactly the kind of unit I was thinking about: a little heat pump that can heat and cool super-insulated homes—with no sash windows—filled with climate-friendly refrigerants while sipping clean electricity. In an #efficiencyfirst world, this is what we have been waiting for.