Gas furnaces and boilers can be fueled by either natural gas or propane with simple modifications accounting for the different characteristics of the fuels. Many furnaces and boilers operate on either natural gas or propane. Propane is usually more expensive as a fuel, but is available throughout the United States. Natural gas supplies depend on having a natural gas distribution system in your area, and areas at the end of the pipeline (such as the Northeast) tend to pay higher prices for natural gas.
A number of retrofits are possible for gas-fired furnaces and boilers, but before pursuing any retrofits you should consider the potential added benefits you could receive by simply replacing the furnace. The following retrofits are possible:
The most common retrofit is the addition of a vent (or flue) damper. A vent damper prevents chimney losses by closing off a boiler's vent when the boiler isn't firing. Steam boilers benefit from vent dampers more than hot-water boilers, and bigger boilers benefit more than smaller ones. Vent dampers, however, may not be cost effective with properly sized, newer furnace models.
Older furnaces and boilers with a continuous pilot light can be retrofitted with intermittent ignition devices. These devices are difficult to install and should only be installed by professionals. They cost about $250 and typically have a payback period of less than 10 years.
Although these intermittent ignition devices can save you some in fuel costs, they are not always cost effective when installed on aging equipment. If it's possible to turn off your furnace's pilot in the spring and to turn it on again in the fall, you can usually save the same amount of money as you would using one of these devices.
Many boilers and furnaces in today's homes are oversized, particularly if you've upgraded the energy efficiency of your home. It is sometimes possible to reduce the heating capacity of your gas boiler or furnace to make it operate more efficiently by reducing the size of the gas burner orifice, and possibly also the baffles. This is a difficult process that should only be performed by a qualified technician, and in some cases, it could violate local building codes and void manufacturer's warranties. If allowed, though, the modifications should cost less than $100 and can save up to 15% of your fuel costs.
Steam boilers should only be derated if the steam system is also modified to remove excess radiators, which is a tricky procedure.
An aquastat controls the temperature of the hot water in a boiler, typically keeping it around 180°F. But when heating needs a lighter in the spring and fall, that's actually higher than needed, and wastes energy. A modulating aquastat, also called an outdoor reset, senses outdoor temperatures and adjusts the hot water temperature accordingly. The units can save up to 10% of fuel costs, and cost several hundred dollars.
A cheaper alternative is to manually adjust the aquastat yourself, turning it down to around 120°F during the milder heating season. Consult your owner's manual or a service technician to locate the aquastat.
A time delay relay is a way to squeeze the most heat out of your system without running the boiler. When the thermostat clicks on, the relay causes the boiler to circulate hot water through the system without turning on the boiler. After a set time, the boiler will fire up. A time delay relay costs about $100 and can cut your fuel costs by up to 10%.
U.S. Department of Energy - Energy Efficiency and Renewable Energy