BANGOR, Maine — The doors are locked, the parking lot is empty, but a sign for Hallowell International still frames the warehouse-style building on Hildreth Street in outer Bangor.

The phone rings and rings but no one answers. An answering service seems to indicate that the business is in operation, but no one returns calls.

Less than two months ago, Duane Hallowell said he was fervently trying to find an investor for his company, which produces commercial and residential heat pumps that run on electricity.

Some customers have indicated that the company recently went out of business, but officials with the city of Bangor, which has assisted Hallowell International with financing, said its lease is still valid.

Hallowell has not responded to numerous calls or e-mails over the last few weeks. His attorney, Benjamin Marcus of Portland, also has not returned multiple calls.

What is going on with Hallowell International?

No one seems to know.

Meanwhile, numerous customers have contacted the Bangor Daily News to express frustration with a company that once received national praise for its innovation.

A former employee of Hallowell International, however, said the heat pumps were “doomed to fail.”

“The product engineering would never be successful in 90 percent of the climates it was sold in,” Randy Margraf wrote in a recent e-mail. “This was a tightly kept secret at Hallowell International.”

Customers have confirmed problems with the heating units.

Kelley Fitzpatrick of Houlton purchased a unit in 2008 and said it never worked properly. She has bring trying to reach Hallowell International for months with no luck.

Ronald Carlow of Hermon has had similar problems and said he even contacted the Maine Attorney General’s Office to no avail. Martha Demeritt, a spokeswoman for the AG’s Office, said the consumer protection division has received only one call about Hallowell International, but she encouraged anyone with complaints to call the complaint line at 1-800-436-2131.

In the last year, eight complaints have been filed with the Better Business Bureau against Hallowell International. On a scale of A+ to F, The BBB ranks the business with an F.

About two months ago, Duane Hallowell, who founded the company in 2006, said he has reduced his staff from 40 employees to three to stabilize operations and prepare it for a sale or merger. He declined to identify any possible business partners but was confident that there was interest.

Customers, however, said they have not been served during this transition.

One of Hallowell’s biggest customers is a military housing project in New Jersey that has purchased 1,370 Hallowell units over the past three years. Thirty percent have failed, according to Matthew Haydinger of Architectural Renovation and Construction, a private military housing contractor for Fort Dix-McGuire Air Force base in New Jersey.

“Despite holding a warranty for the units, the additional labor, and headaches resulting from removing, reinstalling, packing, shipping, stocking and warehousing the hundreds of failed units became overwhelming and costly,” Haydinger said.

His company is exploring legal action.

In several online forums, Hallowell International has been criticized as being unresponsive to service calls. Some customers are threatening to sue.

In early February, Hallowell said he understands concerns but stressed that customers need to contact their local Hallowell International dealer for assistance per language of the warranty. The company has designated installers in every state in which its pumps have been sold.

The pumps, including the Acadia heating and cooling system that is marketed specifically to colder climates, are among the latest heating alternatives for a nation heavily dependent on fossil fuels.

Heat pumps are a popular source of heat in southern parts of the country but are relatively new to Maine. At Hallowell International , the pumps are designed to harvest heat from the outside air even when it’s cold and deliver hot air inside to warm homes. In the summer, the process reverses, sucking heat and humidity from the inside air and blowing it outside, like an air conditioner.

The initial investment is large — between $10,000 and $15,000 — but the payback is strong because fuel costs are eliminated.

Hallowell leases property on Hildreth Street from the city of Bangor and also is on the hook to the city for an equipment loan. All told, the city provided conditional finances in the amount of $200,000 back in 2006.

The lease is still in effect, according to Economic Development Director Rodney McKay, and payments have been current at least until the last month or two.

When Hallowell first was granted city financing in 2006, its president and CEO said the company could create up to 900 jobs. In 2008, the company employed as many as 40 people and sold 1,600 units. Now, the company is not selling any new units.

In addition to the local financing, Hallowell got a boost in 2009 from Maine’s congressional delegation, which lobbied on the company’s behalf to secure a Department of nergy-recognized energy-efficiency rating. That paved the way for a host of incentives, but Hallowell said earlier this year that those incentives were counteracted by the company’s failure to qualify for American Reinvestment and Recovery Act funds.