Some air conditioning and heating companies that sell new units promise customers that their utility bills will drop -- and offer a partial refund if they don't. But before buying one of these energy-savings packages, make sure there's a math wizard in the household.

That's what John Harkey of Mansfield is learning.

He has spent more than a year fighting Hobson Air of Weatherford over his electricity and gas bills. He says his bills didn't decrease the promised 45 percent. Hobson, which also operates under the names Air Genius and Comfort Experts, says Harkey's bills in total did drop close to what the company promised.

Hobson offered Harkey $400. Harkey says his math shows that Hobson owes him $2,500.

Well, it's not actually his math. His daughter is a math teacher. She helps him.

Armed with his calculations, Harkey took his dispute to small-claims court this week, but something strange happened there. He won, but then he didn't.

After Harkey initially sued, Hobson didn't file an answer. That caused a default judgment in Harkey's favor. All Harkey had to do was go to court this week, show his daughter's math calculations to the justice of the peace and win. He did that.

But after the hearing, I called company President Brett Hobson to ask him what happened. It's not like Hobson Air to ignore lawsuits. Hobson's lawyer vigorously defends the company, in operation for 40 years, in these matters.

"This is the first I've heard of this," Hobson told me. "Quite frankly, talking to you, it's all I can do to maintain my composure and not be upset that it got to this point."

He promised to find out what happened. A day later, Hobson lawyer Trey Harris called and explained.

The process server, a Parker County constable, did not properly serve the lawsuit papers to the appropriate company official, he said. Under law, Harris said, court papers must be served to a registered agent or a top company official. In this case, the papers were served to a company accountant. That means the service wasn't legal, Harris said.

Hobson Air will get a new trial. (I called the Parker County constable's office, but my calls were not returned.)

The dispute began after Harkey bought new heating and air conditioning units from Hobson. He paid for the "Double the Difference Utility Savings" package, which promises a 45 percent savings. Hobson advertises in its flier: "Your utility bills will be reduced by 45 percent as promised [or] we will double the difference not realized for one year."

Harkey's bill to Hobson was $15,000, which included other energy-savings measures installed by Hobson, such as attic insulation, outlet seals, reconfigured duct work and even lower-energy bulbs.

Eventually, Harkey sent two years' worth of utility bills to Hobson, which sent him back its own, quite different, calculations.

"My figures show that I received no savings," Harkey said.

Hobson first responded in a Better Business Bureau complaint: "Mr. Harkey saved on his electrical savings; however he did not on his gas. Therefore, the money saved on electric is deducted from money owed for not saving on gas."

Then Hobson Air offered its own calculations: "634 kWh @ $.13/kWh = $82.42 x 2 = $164.84. Did not save on gas -36.2 CF x $7.89 = $285.62 x 2 = $571.24 totals $406.40 due back to Mr. Harkey."

In an interview, Hobson said the company takes a customer's lowest bills -- an electricity bill from winter, when the air conditioner doesn't run, and a gas bill from summer, when heat isn't used.

That's considered the base load to the house, or what's needed to run electricity and gas appliances that aren't related to cooling or heating.

Harkey, Hobson says, is trying to deduct the savings from the base load, too. But Hobson said the company doesn't include that part for savings.

Harkey says he got a runaround from the company when he tried to claim the money. "I had to keep sending and sending copies of gas and electricity bills. They said, 'Oh, we're missing this, this and this.' I had to sit down and redo everything. That's why it frustrates me. I put a lot of time in this."

Hobson has an F rating from the Better Business Bureau, which reports on its website that the company met with BBB officials a year ago to discuss complaints. Harkey first complained to the BBB, but when the problem couldn't be solved, he went to small-claims court.

Now he has to go back again.

Sunday: Fort Worth considers a new vegetation control ordinance for abandoned houses.

Dave Lieber, 817-390-7043

Twitter @DaveLieber