Stone Mountain Technologies
Michael Garrabrant, President and CEO of Stone Mountain Technologies, with Anesi ammonia-absorption heat pump at AHR Expo 2024

Production Under Way for Ammonia-Absorption Home Heat Pumps in North America

Tennessee manufacturer Stone Mountain Technologies has produced 30 of its Anesi units and shipped four so far.

Stone Mountain Technologies (SMTI), a U.S. manufacturer of ammonia (R717)-absorption heat pumps, has started production of its Anesi-branded units at its plant in Piney Flats, Tennessee, with 30 manufactured, four shipped and “orders for more,” said Michael Garrabrant, President and CEO.

Garrabrant provided this update at the AHR Expo 2024 trade show, held in Chicago January 22–24.

The Anesi heat pumps are designed to provide space heating and domestic hot-water production for residential homes and commercial applications. In North America, where vapor compression propane refrigerant heat pumps are not yet available for residences, the Anesi unit is one of the few natural refrigerant-based heat pumps for private homes that deliver both space heating and domestic hot water. Heat pumps for residential and commercial applications that use CO2 (R744) are typically limited to domestic hot water production.

In development for 10 years with field tests starting in 2015, the Anesi system received venture capital finance backing in October 2022. Stone Mountain also exhibited the system at the AHR Expo in 2023 and 2022.

Unlike many heat pumps, the Anesi units are well suited for colder climates, delivering heat down to −40°F (−40°C), noted Garrabrant; for example, pre-production models have been installed in four homes in Edmonton, Canada, for more than a year. The units have a heating capacity of up to 80,000BTU/hr (6.7TR/23.6kW) at rated conditions and offer a COP of 1.43 at standard ANSI rating conditions of 47°F (8°C) ambient and 95°F (35°C) return.

The heat pumps are also designed for hot water generation for such commercial sites as restaurants, hotels, schools and prisons. Three of the first four shipped units are for commercial hot water heating in Atlanta, Georgia.

Stone Mountain is selling its heat pumps via manufacturer representatives in many parts of the U.S. and in Canada. “We’re trying to get the units out to the reps,” Garrabrant said. The reps, in turn, recommend contractors who are trained in installing and maintaining the Anesi units. So far, around 700 contractors have received this training, and Stone Mountain intends to “get a couple of thousand trained,” he said.

“This is a transition year for us,” added Garrabrant. “We’ve got to educate people.”

The Anesi units, located outdoors to avoid safety concerns, generate heat via an ammonia-water absorption cycle, which is powered by heat from natural gas or propane; the ammonia charge is only 11lbs (5kg). The system uses 30–50% less natural gas than a gas-fired boiler system, said Garrabrant, adding that the use of gas enables the system to function at very low temperatures and low cost.

In addition to generating heat from absorption of ammonia gas into water, the system releases heat via the condensation of ammonia at the condenser. Heat from both the absorber and condenser is carried by an glycol-water mix. With the additional source of heat from the condenser, the AFUE (Annual Fuel Utilization Efficiency) of the system goes up to 140% (and never below 92% in very cold temperatures), he said. The AFUE of most high efficiency gas furnaces or boilers averages between 90 and 95%.

The hot glycol-water solution is pumped into a residence, where it heats up water in an 80gal (300l) hot water tank for domestic use. For space heating, it employs an air handler or a hydronic system. In an existing home, the infrastructure and utilities (gas and electricity) should support the Anesi system without significant modification, Stone Mountain says on its website.

For space heating, the temperature of air in the air handler is from 110–125°F (43–52°C) while for hydronic heating the glycol/water solution ranges from 100–150°F (38–66°C) depending on the type of system. Domestic hot water ranges from 120°F (49°C) in the U.S. to 140°F (60°C) in Canada.

Cutting emissions and costs

Garrabrant noted that his heat pump, by using 30–50% less natural gas than a gas-fired boiler system, reduces annual carbon emissions and costs by the same amount while taking very little (400–500W) from the U.S. electrical grid; widespread implementation of electric heat pumps, he noted, would add a considerable burden to the existing grid system. Moreover, the U.S. grid still relies on fossil fuels, with about 60% of electricity in the U.S. generated from natural gas or coal in 2022.

The first cost of an Anesi system that uses an air handler for space heating and produces domestic hot water averages from $13,000 to $15,000, Garrabrant says. (Hydronic space heating costs may require additional steps.) That can be reduced by $2,000 via the U.S. Inflation Reduction Act (IRA) and further from utility incentives. Even before incentives, the Anesi heat pump has a payback period of three-to-six years, depending on the local climate and gas prices, for the premium over a gas system, said Garrabrant.

While the Anesi system could hypothetically reverse to offer space cooling during summer months, this functionality is not currently offered. However, Stone Mountain is working on a heating-cooling model.

As an ammonia absorption heat pump, the Anesi system avoids the use of a compressor. However, Garrabrant noted that a highly efficient vapor-compression monobloc residential heat pump using ammonia is feasible, except that an appropriately sized ammonia compressor is currently not available.

Italian manufacturer Robur, another producer of ammonia-absorption systems, also showcased its commercial heat pump and chiller at the AHR Expo 2024 as well as at last year’s AHR Expo.

“This is a transition year for us. We’ve got to educate people.”

Michael Garrabrant, President and CEO, Stone Mountain Technologies

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