Like other gas-fueled appliances, a gas boiler can be adapted to burn natural gas or propane gas. Most gas boilers don't actually boil water; they just heat it. The hot water is then pumped to radiators, under-floor tubing, or to a heat exchanger. In a duct system, all of these heating systems are referred to as "hydronic" (water-based) heat.
Wall-mounted radiators and hot water baseboards use their boiler-supplied hot water to warm rooms by radiation and convection. Hot water circulating in under-floor tubing turns the entire floor surface into a giant radiator. A gas boiler can also supply a forced-air heating system (referred to as "hydro-air") with hot water for its heat exchangers.
If you have an older boiler, it will have a metal flue that is connected to a chimney. This type of boiler relies on natural "atmospheric" combustion. In contrast, a gas-fired, direct-vent boiler is designed to extract more heat from a given quantity of fuel. In fact, so much heat is extracted that the flue gases are cool enough to vent outside through a plastic pipe. No chimney is required. Direct-vent gas boilers will have AFUE ratings in the 85%-90% range —much better performance than you could expect from an older boiler that exhausts up a chimney.
Some direct-vent boilers not only vent directly to the outside through a plastic pipe; they also get their combustion air from outside the house. This "sealed-combustion" boiler is often recommended in a house that's been upgraded to be more airtight.
Regardless of the fuel used, a boiler can supply hot water for heating alone or for heating and washing. A boiler that does both jobs is often referred to as a "combi-boiler." With a combi system, no separate hot water heater is necessary. In fact, when the boiler is running to provide water for whole-house heating, hot water for washing is heated for free.
"Hydronic" is a term used to describe any HVAC system that uses water or fluid. There are several different types of hydronic heating systems that utilize boilers. Hot water can be pumped to baseboard and other types of radiators located throughout the living space. It can also be circulated through plastic tubing that has been installed beneath finished floor surfaces. In this type of "hydronic radiant floor" heating system, the flooring becomes a giant radiator to provide home heat.
A third type of hydronic heating system, called "hydro-air," circulates hot water from the boiler to a heating coil in a duct system, where a fan circulates the heat to the rooms in your house.