This old (dangerous?) house (© Yellow Dog Productions/Getty Images)

More than 30 million homes — roughly one-third of the homes in the United States — are at least 50 years old. That’s a lot of beautiful old houses — with lots of potential problems.

What should you look out for, whether you’re the owner of a Victorian painted lady or a Beaux Arts brownstone? We’ve done your virtual inspection for you and identified seven key villains, along with their solutions, so read on. (Bing Cube: Check out photos of Victorian homes)

1. The villain: Old electrical systems

Why it matters: A study by the Consumer Product Safety Commission in the 1980s found that fires occurred disproportionately more often in homes that are more than 40 years old.

Over time, wiring in these homes can corrode, says Underwriters Laboratories (PDF file). Wires using a rubber compound that was widely used before 1950 are known to become brittle with age and when “they are subjected to bending, abrasion or harsh usage,” UL says.

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How to know if you have a problem: The only way to really know what kind of problems you have is to hire a trusted electrician to inspect the house, the experts say.

Here are some other common warning signs of electrical problems in older homes, according to the Electrical Safety Foundation International:

  • Circuit breakers that frequently trip or fuses that often need replacement
  • Dim and/or flickering lights
  • Unusual sounds and smells from your electrical system
  • Hot switch plates
  • Electrical shocks

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What to do: Two professionally installed updates to your old home can help reduce the fire danger significantly, the foundation says:

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The cost: Electrical inspections will vary by location. In the Boston area, for instance, Holt Electrical Co. charges $100 to inspect a home’s electrical system (and waives the fee if work results from the inspection).

AFCIs cost $25 to $50; GFCIs cost just $5 to $10.

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2. The villain: Old or leaking oil tanks

Why it matters: Many old homes relied — and still rely — on oil fuel. That fuel is either stored in tanks in the basement or buried in the yard. Typically, the old steel tanks have a life expectancy of just 10 to 15 years, writes Daniel Friedman, an inspector, consultant and author of the exhaustive home-inspection site InspectAPedia.com, which contains oodles of information about dealing with old oil-storage tanks.

The mess from a leak can range from trivial “to tens of thousands of dollars if oil has leaked from a buried oil tank into surrounding soils or, worse, into nearby waterways or wells,” Friedman says.

How to know if you have a problem:
If your tank is inside your house:

  • Look for obvious clues, like an oil spot under the tank, Friedman says.
  • Know your tank: How old is it? What type is it?
  • “We recommend an ultrasound test that measures thickness of the tank steel along its critical bottom section, the area more likely to be corroded and leaky,” he says.
  • Water is terrible for tanks. Look for signs such as rust and debris in the oil filter or at the oil burner when it’s serviced, he says. (Read more about testing here.)

If you think the tank is outside your house:

  • First, track down the tank. Look for fill or vent pipes in the yard or sticking out of the house.
  • Only a pretty bad leak will be obvious to the eye. Hire a pro. A specialty company can test surrounding soil and do a low-pressure test of the tank to check for leaks. If you suspect any water, have the oil company pump it out.

What to do: If there are no leaks, either an above-ground or buried tank can be abandoned in place, Friedman says. That could be as simple as making sure it’s empty and leaving it — or emptying it and filling it (in the case of a buried tank) with something like sand (more expensive).

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The cost: An ultrasound test for an above-ground tank is roughly $125, Friedman says. Adding limited-term insurance is roughly $100.

Removal or replacement costs vary widely by location, tank size, etc., but can be significant: Removing an old above-ground oil tank and installing a new one can run $2,000 to $4,000, according to InspectAPedia.com. (This is a job for the pros, Friedman says.)

Luckily, states sometimes offer financial aid to replace aged tanks.

3. The villain: Radon

Why it matters: Radon is a cancer-causing natural radioactive gas that emerges from the ground, according to the Environmental Protection Agency’s information site. You can’t see, smell or taste it, but it kills about 20,000 people annually in the U.S., the EPA says.

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How to know if you have a problem: Radon naturally dissipates into the atmosphere; every home that sits on or in the ground has it, but some homes trap it too well.

Homes with a radon level of 4 pCi/L (that’s picocuries per liter) need to be fixed, the government says. 

How do you know your home’s level? Test. In fact, the U.S. Surgeon General recommends that every homeowner test his or her home for radon.

The test kits are cheap and easily available. Here are a few things that can increase the odds of radon in a home, says Marianne Saulsbury, co-owner of Pittsburgh-based Saulsbury Environmental Consultants:

  •  A lot of the ground floor is underground. (This is often true of homes in the Northeast, she says.)
  • “Any crawl spaces with dirt or gravel floors or porous surfaces; those are potential entry points.”
  • If the lowest level of the home has double-paned (i.e. well-sealed) windows, or no way for radon to escape the home.

What to do: Get a reading slightly over 4 pCi/l? You might want to do a long-term test through a pro before deciding whether to fix the home, Saulsbury says. If the measurement is a lot over that mark, the home needs to be fixed quickly.

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One part of a solution usually involves sealing cracks in the foundation. Also typical is a “radon reduction system” — a pipe-and-fan venting system from the basement floor out through a wall. (Check out the EPA’s Consumer’s Guide to Radon Reduction.)

The cost: A home-test kit often costs only about $20. A pro will charge between $125 and $150 for a test, Saulsbury says. Mitigation costs can vary widely; depending on the severity of the radon problem, a few pipes might be needed. Expect the cost of a fix to be $800 to $3,000, according to the EPA.