CONCORD, N.H. – For years, New Hampshire’s juvenile delinquents have sweltered on hot summer nights, locked in rooms without air conditioning, behind security doors and heavily screened windows in decades-old buildings.

“When the kids go into the rooms at night, we shut the doors. It’s very, very warm,” said Tricia Lucas, chief of administration at the Division for Juvenile Justice Services. “Kids come off the mattresses to sleep directly on the floor so they can have the coolness of the floor.”

Next summer should be different: The state reformatory, the Youth Development Center in Manchester, will be in a new, 144-bed facility with air conditioning.

The long wait for the $33 million complex illustrates the sort of Yankee frugality that has spared state governments in northern New England the debt problems plaguing some other states.

Nationwide, median state debt per capita is $703. New Hampshire’s is a low $457, Maine’s is $634 and Vermont’s is $716, according to Moody’s Investors Service, a credit rating firm.

Vermont actually has the highest credit rating accorded to the three states by Standard & Poor’s, another rating firm. Vermont’s AA+ rating is one notch below the top AAA rating and one notch above New Hampshire’s AA score. Maine is one step below New Hampshire at AA- after a slight downgrade this year.

Geoff Buswick, the Standard & Poor’s analyst who keeps tabs on the three states, says the ratings are based on the steadiness of a state’s revenues, how well a state predicts its income over time, its spending patterns, the amount of its debt, the health of its economy and what kind of work force it has.

Analysts also watch closely to see that states pay their “living expenses” with current income. When states dip into savings, analysts look to see that the money is put back as quickly as possible.

Intangibles – like being Yankees – also are considered.

“We talk about that Yankee frugality as something seen as a credit positive,” Buswick said.

In addition to being conservative about borrowing money, New Hampshire lawmakers typically give maintenance – such as new roofs – a higher priority than new buildings like the new reformatory.

“There are certainly things you just have to do,” said Rep. Gene Chandler, chairman of the House committee that handles New Hampshire’s capital budget.

When something like the new reformatory does get considered, it helps if everything possible has been done to hold down the cost. In the case of the reformatory, some of the money will be federal and some of the rest will be scraped together from savings from other reformatory projects.

Lucas says using the federal money – which the state stood to lose if it didn’t spend it – and the savings cut the state’s borrowing almost in half.

The good credit ratings that flow from such tightfisted practices translate into low interest rates and cheaper costs for public works projects in all three northern New England states.

Vermont secured its strong credit rating by building its reserves and paying down its debt for more than a decade under then-Gov. Howard Dean, now national Democratic chairman.

New Hampshire and Maine also aggressively pay off their debt, but both states lose points with rating firms by relying too heavily on savings and one-time income to pay operating costs.

In New Hampshire’s case, the state took in less money than it spent for four consecutive years, 2000-2003. The state also compensated partly by spending its reserve funds for health care and school aid. As a result, two firms cut New Hampshire’s credit rating in 2003.

Last year, the state began turning things around, and the current state budget is built almost entirely on income from existing taxes.

Every two years, New Hampshire’s treasurer advises lawmakers on how much they can borrow for projects like the reformatory without alarming people like Buswick. The state’s goal is keeping debt payments well below 8 percent of revenues. Last year, it was 5.8 percent.

New Hampshire’s unusual tax structure – the state has neither a general income nor a general sales tax – does not produce big surpluses even in good economic times, which forces lawmakers to weigh spending and borrowing choices carefully.

But careful planning can pay dividends besides a good credit rating.

The new reformatory, which is on budget and on schedule to open in January, will include a renovated recreation area with a swimming pool, gymnasium and chapel, Lucas said.

“We would never, ever have been able to build the equivalent space new,” she said.