Tennessee-based manufacturer Stone Mountain Technologies (SMTI) will start production of its ammonia (R717)-based absorption air-to-water heat pump for the North American domestic market in July, according to the company’s president, Michael Garrabrant.
SMTI’s Anesi heat pump can provide space heating and hot water, offering a more efficient, cost-effective and sustainable alternative to commonly used gas furnaces and water heaters, explained Garrabrant in an interview at AHR Expo, which took place in Atlanta, Georgia, February 6–8.
While many natural refrigerant-based heat pump systems are entering the North American residential market, they typically use CO2 (R744) and are limited to hot water production.
Notably, rather than using electricity to drive a compressor, the Anesi heat pump is thermally activated using low-charge ammonia-water absorption technology, which is triggered by natural gas.
But with an annual fuel utilization efficiency (AFUE) of up to 140%, the manufacturer says the Anesi heat pump can cut heating bills by up to 50% compared to a gas furnace with an AFUE of 80%.
While SMTI’s system has higher upfront costs – up to 50% higher – cheaper bills means that the Anesi heat pump has a payback period of three-to-six years, depending on the local climate and gas prices, he said.
“In Atlanta or Dallas an electric heat pump might make more sense, but in Connecticut or Chicago a [gas-triggered] heat pump is the better option,” he added. “We’ll save you money, we’ll keep you comfortable, and you get to help the environment with better efficiency.”
Garrabrant also noted that rebates are available via the U.S. Inflation Reduction Act (IRA), as well as from some utility providers.
Initially, SMTI will focus its efforts on certain regions around North America, including eastern Canada, western Canada and the Northeast and Midwest of the U.S.
Having been in development for 10 years and first debuted back in 2019, the enhanced production of the Anesi heat pump follows an injection of venture capital finance in October 2022.
Absorption technology
According to Garrabrant, ammonia absorption technologies were popular in North America in the 1960s and 1970s, but SMTI has adapted it to be “significantly more efficient.”
“There is currently nothing like this for the residential market,” he said.
The Anesi heat pump uses a small amount of electricity – around 2% of its total output – which minimizes its impact on the electric grid, he explained. This is important because the drive to electrify numerous aspects of society will place significant pressure on utilities to meet capacity, he said.
The air-to-water heat pump unit, which contains around 10lbs (4.5kg) of ammonia, is installed outside the house to mitigate safety concerns surrounding refrigerant leaks and gas combustion. It produces hot water that is sent to Anesi’s custom air handling unit (AHU), which replaces the gas furnace and connects directly to the air ducts to circulate hot air around the house. If hot water is needed, the AHU can also be connected to an 80gal (300l) indirect storage tank to create a combi system.
The unit has a heating capacity of up to 80,000BTU/hr (6.7TR/23.6kW) at rated conditions.
Efficient in coldest climates
The Anesi heat pump is capable of operating in ambient temperatures as low as -40°F (-40°C), although temperatures this low will have an impact on system efficiency, Garrabrant explained.
“Even in the coldest climate, efficiency never drops below 95%, so it’ll always be better than a gas furnace,” he said. “It is the ideal solution for cold climates.”
At 0°F (-18°C) the system’s AFUE is around 115%, with efficiency only dropping below 100% at temperatures below -20°F (-29°C), he added.
For hot water production during the summer when space heating is not needed, the system runs at an efficiency of around 120%.
While the system could hypothetically reverse to offer space cooling during summer months, this functionality is not currently offered. However, Anesi is working on a heating-cooling model and is currently testing prototypes.
