In an existing home, it's important to consider the impacts of replacing your current heating system with a different type of heating system. The easiest route is simply to replace your furnace or boiler with an updated model of the same type, but this may keep you from your most efficient heating and cooling options. Here's what you need to consider in the following situations:
Electric resistance heating, also called electric baseboard heating, is the cheapest to install and the most expensive to operate, as it is the least efficient source of heat. Unfortunately, upgrading to ducted systems, as discussed below, will involve a significant expense for ducting. Another option is to switch to a hot water baseboard system. Baseboard heaters can probably replace electric resistance baseboards in the same location, but will still require extensive plumbing.
Steam systems are generally less efficient than hot water heating systems, but the efficiency gain of upgrading to hot water is probably not worth the expense; it would be better instead to upgrade or replace your boiler. However, it is sometimes possible to convert existing steam distribution pipes to hot water heating, which reduces the cost of the system to the cost of the boiler, the baseboard heaters, and the installation labor. This is generally only feasible in newer two-pipe steam systems (that is, systems in which two pipes go to each radiator, one for the steam and one for the condensate return). For some homeowners, the aesthetic and space-saving benefits of eliminating the large steam radiators may be worth the expense.
If you currently own a forced-air heating system, you may wish to add central air conditioning (if you don't have it already) or switch to a heat pump system. Adding central air conditioning is fairly simple, but be sure your contractor matches the system to your existing ductwork. Switching to a heat pump system could be problematic, since heat pumps generally require larger ducts. However, many heating systems are oversized, particularly if your home is well insulated. It may be feasible to upgrade the energy efficiency of your home (if it hasn't been upgraded already) and convert to a smaller-capacity heat pump that is matched to your existing ductwork. To evaluate this option, consult a heating and cooling professional.
Heat pumps are among the most efficient sources of heating and cooling, but in areas with high electricity costs, they may still be more expensive than other options. Switching from a heat pump system to a furnace system (possibly with a central air conditioner) is feasible but probably not cost effective. However, if you wish to do so, the ducts should have no problem accommodating the new system, assuming it is sized correctly.
Many homes that use steam or hot water heating, radiant heating, or electric resistance heating (or have no heating at all) do not have ducts. Homeowners may want to add ducts to their homes for a number of reasons, but the most common reason is to provide central air conditioning. If that's your only reason for wanting to add ducts, consider instead adding a ductless mini-split air conditioner or ductless mini-split heat pump.
If you are considering switching from your current system to a ducted system simply to upgrade your heating system's efficiency, it probably is not worth it, except perhaps for electric resistance heating. Instead, you should look into upgrades to your existing system (see the heating section for more information). For steam systems and electric resistance heating, consider switching to hot water baseboard heating, as discussed below.
Adding ducts to an existing home can be a difficult proposition. In a single-story ranch home with an unfinished basement or crawl space, the ducts could be located underneath the main living space, and nearly any ranch home could accommodate ducts in the attic space. In homes with high ceilings, it may be possible to hide the ducts in a dropped ceiling. In all cases, DOE's Building America program recommends that the ducts be located within the home's conditioned space, even if that means extending to conditioned space into the crawlspace or the attic. See the section on ducts for more information.
In multi-level homes or single-story homes with finished basements and attics, the logistics of adding standard ducts are nearly insurmountable, unless you plan to largely gut the home as part of a remodel. Fortunately, there is an alternative: several companies now offer "mini-duct" air distribution systems that force air through plastic feeder ducts that are only 2 inches in diameter. These ducts can be easily threaded through cavities in walls, floors, and ceilings.
A downside with mini-ducts is that they general require more outlets: roughly 5 outlets per ton of cooling, or one outlet for every 2400 BTUs of heating. This will likely result in higher costs to install the system. The Air Conditioning and Refrigeration Institute (ARI) certifies high-velocity air distribution systems for use with numerous heat pumps, air conditioners, and heat sources. For more information, see the "Product Info" category in the "Learn More" section at right.
If you add ducts to an existing home, the question is whether to continue using your existing heating system or to install a new forced-air heating system. For homes using electric resistance heating, adding either a furnace or a heat pump is clearly preferable. With steam, hot water, or radiant heat, the decision is less obvious: it really depends on how efficient your current system is, how efficient the new system would be in the heating mode, and the cost difference between installing just a central air conditioner versus installing both a heating and cooling system. A heating and cooling professional should be able to help you evaluate these options.
Note that changing to a forced-air system has its pluses and minuses. On the plus side, not only does it allow for central air conditioning, but the furnace could be much more efficient than your current heating system. On the negative side, the fan for the forced air system could increase your use of electricity significantly. This may be an important consideration in areas with high electricity prices.
U.S. Department of Energy - Energy Efficiency and Renewable Energy