What is a forced air furnace anyway? It’s just a big box with a fan in it, right? Right. It’s just a fan in a box with a flame-fired (direct-fired) heat exchanger. One way to get more mileage out of a furnace is to convert it to a hot water air handler using an efficient tankless water heater and some proper planning. When a furnace is converted, the burner, heat exchanger and gas valve are removed and a hot water coil, similar to a car radiator, is installed inside the box. The fan and some of the controls are retained, but otherwise the system is now without any combustion gases or flame whatsoever. This means the system is more safe and reliable; furnace flues and combustion air holes can all be eliminated, making the building more energy efficient simply by eliminating some huge sources of air infiltration. It also eliminates gas-fired heat exchangers that can create noxious gases.
After the conversion, when the thermostat calls for heat, a pump comes on and pumps water from the tankless water heater through the coil and back to the water heater. This circulation brings on the burners in the water heater, taking the temperature of the water up to 140-150 degrees. This is where the modulation I mentioned earlier really benefits the system. The burners will only produce the BTU’s needed to keep the water hot and no more. Then the fan on the “hot water air handler” comes on and produces warm air, just not as dry as that from a standard furnace. This gentler source of heat will likely never need maintenance, with the exception of fan servicing and filter replacement. The pump is a cartridge pump that is easily repaired if it fails. Heat is being produced at the efficiency of the tankless system and has little effect on the amount of hot water that is available for domestic use. This type of system solves many problems and means nobody will ever have to haul a furnace or water tank out of your basement again. Ever.
Tankless systems can even be used in hydronic heating situations, as replacement for old, inefficient boiler systems. Again, with full modulation, the efficiency of a radiant floor system or baseboard hot water will be greatly increased over a standard boiler. In many cases, space heat and domestic hot water can be integrated into one unit, if the unit is rated for space heat. If solar or geothermal is used as a pre-heat source, the savings can be tremendous. It is just a matter of proper design and load calculation.
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