Enlarge

Brown County. Here is a photo of an efficient side vent furnace. Source: WHS - State Historic Preservation Office.

The best way to ensure that the furnace in your historic house reaches its maximum lifespan is to follow a regular maintenance schedule. However, whether your furnace is heated with hot water, steam or forced air, it will stop working someday and you will need to replace it.

You might also want to replace your furnace for energy efficiency reasons. Many older furnaces are only 60% to 80% efficient based on today's standards. Modern hot water and steam boilers, as well as forced-air furnaces, can be up to 98% efficient.

Maintain Your Furnace

The lifespan of most contemporary furnaces installed today with regular maintenance is about 20 years. Here are two tips to help make your older furnace last as long as possible:

1. Have professional inspections. Always have a professional inspect, clean and service your furnace at the beginning and end of the heating season. This is critical for the longevity of the unit as well as your health. A dirty filter will make the blower fans work harder and reduce the life of your furnace.
2. Replace the filter regularly. It's best to replace your standard furnace filter once a month. If you have a heavy-duty pleated (accordion-style) filter, replace it about once every two months. However, if you have a woodworking shop in the basement, you should check your filter more regularly and replace it when it is full of dust.

Address Special Furnace Maintenance Needs

If your historic house has one of these two types of heating devices, you will have to address some special maintenance needs:

• Forced-air furnace. Forced-air furnaces have a heat exchanger. As a furnace ages, the heat exchanger can develop cracks or rust holes. The cracks and holes can allow dangerous — even deadly — levels of carbon monoxide to leak into your home. Only a professional heating contractor can properly inspect a furnace for these leaks. Occasionally a heat exchanger can be replaced, but usually a forced-air furnace with a leaking heat exchanger has reached the end of its life.
• Hot water and steam boilers. Hot water and steam boilers are prone to cracks and rust. If you do not properly maintain your boiler, it could leak and even build up enough steam or water pressure to explode. You should have your boiler cleaned, oiled and checked for water pressure annually by a heating contractor who specializes in boilers.

Follow Best Practices to Replace Your Furnace

If you must replace the old furnace in your historic house, you should follow preservation best practices.

For example, if you are replacing a boiler with a forced-air furnace, you might have a radiator that is a character-defining feature of your house. Depending on the design and location of this radiator, you should consider keeping it even though it will no longer be used. Here are two types of radiators you should consider leaving in place:

• A radiator with an intricate design pattern that is located in a prominent location, such as the main parlor.
• A multi-use radiator, such as a pie-warming radiator located in your kitchen.

If you have basic radiators in other areas of your house, you can remove them and dispose of them properly.   

Follow Best Practices to Install Ductwork for a New Forced-Air System

If you must install ductwork for a new forced-air system, you have two best-practices options:

1. Conventional Heat and Air Conditioning with Conventional Ductwork. The easiest and least costly way to approach a retrofit is a conventional heating and air conditioning system with conventional ductwork. Conventional ductwork consists of square and round metal return air and supply air ducts made of galvanized sheet metal. Your approach to installing the ductwork will depend on the type of house you have:
• One-story house. You can install all the new ductwork in your basement or crawlspace up to vents placed on the first floor flooring. Do not install salvaged or antique floor vents, because they will create a false sense of history in your house. Instead, use modern vents that blend into the surrounding areas. For example, you could install flush floor vents into your hardwood floors and finish them to match.
• Two-story house. It is nearly impossible to effectively run conventional ducts from the basement to the second floor without disturbing historic elements like trim, plaster and flooring. If your two-story house has over 2,000 square feet of living space, you should consider installing two complete systems: One in the basement for the basement and first floor, and one in the attic for the attic and second floor. The attic unit is called a downdraft system. You can run the conventional return air and supply ductwork into the second-floor ceilings through closets and into walls from above. This approach disturbs historic elements the least and allows you to have separate thermostats or zones for each floor.
2. High-Pressure, High-Velocity Heating and Air Conditioning Ductwork. High-pressure, high-velocity central air conditioning systems are outstanding for historic homes with boiler heat, but they can also be set up to provide heat. They adapt to geothermal systems as well. However, these systems are also the most expensive system to install. The interior air-handling units for these systems push the heated and cooled air through small ducts that are the size of a vacuum hose. The high-pressure, high-velocity air handler pushes air into the room in such a manner that the air is virtually the same temperature from floor to ceiling. These systems also have a very high efficiency rating.
   
   The ductwork for these systems is so small in diameter that the ducts can be run through 2 x 4-inch wall cavities and closet and floor joists without harming your original plaster and trim. The vents are round and look like a porthole in a speaker.