A nationwide trend is getting to be a big problem in the Triangle: thieves making off with air conditioning units.

It's not a new issue; Clayton and Carrboro have had problems before, but it seems to be on the rise.

Durham police say at least 19 have disappeared in the last month, and they're asking for help tracking down suspects.

The big draw for people is the copper tubing inside that's gotten to be worth enough that thieves can rip it off, and sell it for scrap to make a quick buck.

Larry Tilley, president of One Hour Heating and Air in Durham, says he's certainly noticed a spike in air conditioning thefts, especially in the last year.

And it's not just from abandoned houses -- but often, occupied homes.

"They got to turn the air conditioning on and nothing happens. They quite often might call a service company like us to come out there and repair it, only to find out the unit's gone," said Tilley.

Because of that, a lot of people are taking steps to protect their air conditioning units.

Some go up on the roof, where people can't reach them.

Tilley keeps his company's units behind a fenced gate that he locks every time someone leaves the parking lot.

And some people literally keep them in cages.

"A few of the units are inside a higher wood fence that you would enclose a dumpster or something like that inside," said Tilley.

Tilley says it's not worth the effort for his company to try and sell used copper they end up with when replacing units, but some people obviously go to great lengths to get it.

Depending on the type of copper, people can get close to $3 a pound, and police say it's not an easy crime to track.