Today's home buyers - millennial first-timers and everyone else, it seems - are looking for houses in move-in condition.

Move-in translates to houses with no expensive problems. Even though credit remains rather tight these days, home buyers are opting to take on larger mortgages rather than take on the work required by shells or fixer-uppers they can purchase more cheaply.

Among older resale houses, one issue that keeps coming up is old wiring, especially the first-generation electrical system known as "knob and tube."

Such a system consists of single-insulated copper conductors in walls or cavities in ceilings. The conductors, protected by porcelain insulating tubes, penetrate joists and studs through holes drilled through them.

Along the length of the stud or joist, the wiring is supported by porcelain knob insulators nailed into the wood.

"Knob and tube was intended for lighting only," said Harris Gross, president of Engineers for Home Inspection in Cherry Hill.

But when major electrical appliances such as refrigerators and washing machines first appeared in the 1920s, he noted, "knob and tube was asked to handle major draws of power that it wasn't designed to do."

Perhaps surprisingly, many knob-and-tube systems remain in good shape, though their use by electricians in household applications ended after 50 years - around 1930.

Yet real estate agents in the region are reporting delays in settlements or, worse, sales lost altogether, because mortgage lenders and homeowners' insurance companies are wary of knob-and-tube wiring.

"We deal with knob-and-tube issues from time to time, usually as a result of discovery during home inspections," said Paul S. Walsh, managing partner of Elfant Wissahickon Realtors in Chestnut Hill.

Some homeowners' insurance companies want the systems to be removed before they will write policies, Walsh said, while other insurers will allow knob and tube.

Val Nunnenkamp, of Berkshire Hathaway Home Services Fox & Roach Realtors in Marlton, said two of the houses he has sold so far this year have had to be rewired because the buyers could not get homeowners' insurance.

Rewiring costs for the houses ran between $10,000 and $12,000 each, Nunnenkamp said. One job delayed closing on the house because it had to be completed first.

For lenders, the issue isn't clear cut, said Jerome Scarpello, of Leo Mortgage in Ambler.

"If knob and tube is 'common to the [geographic] area' and is sufficient for the needs of the home, most lenders will be OK with," he said.

But if a mortgage has a high loan-to-value ratio - meaning that the lender is assuming more risk for loss - and the knob-and-tube wiring is insufficient or is deemed unsafe, Scarpello said, "I have seen lenders have an issue with it."

A major problem with knob and tube that S. Clark Kendus, of D. Patrick Walsh Real Estate in Swarthmore, has seen is when "it is hidden beyond sight."

"I have seen such wiring connected to 'tails' of modern wiring connected to the outlet or switch box," Kendus said, "and when you look in the box, you just see evidence of the modern wire."

That has been done to give the impression that the home had been rewired, he said.

"A home inspector would almost surely recommend that, when knob and tube is uncovered, [you] have a qualified electrician evaluate the electrical system," Kendus said.

When he asked an electrician he frequently uses about knob and tube, Kendus said, "his response was that he knew of no licensed electrician who would recommend anything other than replacing it."

Older generations of wiring typically are removed during major renovations, in which walls are opened and the system is upgraded.

Remodeler Jay Cipriani is working on a house built in the 1920s. "When we run across knob and tube, we can't tie into it," said Cipriani, president of Cipriani Builders in Woodbury. "[Building] code says that we need to change it out all the way down the end of the line."

Said Gross: All knob-and-tube wiring "has simply reached the end of its useful life, due to its decay, safety concerns, and inferior capability with respect to modern wiring."