Q Six years ago, we purchased a two-story, gas-heated, centrally air-conditioned home built in 1970. The home had two separate furnaces (one in an unfinished basement and the other in the attic) and two separate central-air units for the two floors. From the inspection, we knew that we would have to replace the central air. Four years ago, we replaced the central air and added heat pumps at the same time. We now heat using gas and heat pumps. The system is set to heat with heat pumps at moderate temperatures and kick over to gas when temperatures drop below 40.

We recently had a heat-system check. We knew that our basement furnace was 10 years old and that the attic furnace was older. The technician advised me that our attic unit is 20 years old. He was concerned about the age of attic furnace and the hazards of carbon-monoxide leaks.

Would it make sense to disable the second-floor gas furnace and heat the second floor exclusively with the heat pump? We would continue to heat the first floor with the combination gas/heat-pump system.

A Many homes that have more than one heat pump normally have a heat pump and furnace for supplemental heat on the lower levels and a heat pump with only a blower system for the upper floors. A two-story home can be heated with just one furnace, but a lot will depend on the size and design of your home and the amount of insulation in the attic and walls.

A 1970s-era home might or might not have wall insulation, but one furnace should still be able to heat the home if the attic is insulated. A simple walk-through energy audit might determine if the home is insulated properly. A more thorough energy audit, which includes a variety of testing devices, would determine how much insulation the home has and if additional insulation and other repairs would be cost-effective.

A properly insulated and air-sealed home will use a lot less energy than a poorly insulated one. Replacing the aging air conditioners with heat pumps was a good decision, and the heat pump for the second floor should be adequate without the heat from a gas backup furnace.

If you suspect the furnace has a cracked heat exchanger, do not simply turn the gas off to the furnace. Have the gas pipe to the furnace removed so that someone unfamiliar with the hazardous defect can't operate the furnace. Carbon-monoxide poisoning is a real danger because it is a colorless, odorless and tasteless gas that can kill.

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Correct code violationsQ I need a new gas furnace and will be getting a new gas line installed. But my house has had some unpermitted work over the past 70 years under previous owners. The work appears to have been done well, but I realize that it's now my responsibility.

I'm worried that the inspection required for the mechanical permit will bring up issues about unrelated electrical, construction and plumbing work that was done without a permit. Would it be wise to begin the city's "get legal" program now, knowing that anything found that is not to code will instantly become my responsibility to correct?

A Yes, as the owner, you are legally required to correct work done by previous owners who failed to pull the proper permits.

"Get legal" now by verifying the status of work previously done on the home. Besides the obvious legal need, you will also be assured that the work actually was done properly and that, for example, you don't have a wiring issue that could eventually lead to a house fire.

You can check the permit history for your home with the local development services center. You might discover that everything is up to code, which would put you at ease.

But if you do discover permits that weren't pulled, talk to the agency about your options and your need to install the new gas line. The city should work with you to get projects legal and up to code, and even offer programs for homeowners with complex situations.

If work needs to be corrected, you can obtain permits and correct it yourself if you have the ability. For any electrical work, though, you'll need to hire a licensed electrician.

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