Boilers use electricity, gas, fuel oil, or biomass to produce heat. This heat is transferred to water that is circulated for warming a building, producing hot potable water, and even melting snow from sidewalks and driveways. They are classified by whether they produce steam or not, the type of fuel they use, and their input rate. In the U.S. the input rate is given in btu/hr while most other parts of the world use kilowatts (kw). Here, we will discuss water heating boilers.
Gas fired units are the most common type but they are not all the same. Some units take conditioned air from the home and use it to burn the gas. This air that is required to burn the gas is called combustion air. This air must be replaced with unconditioned air from outside the home which in turn must be heated. This is called an atmospheric unit and is the least efficient. The next type brings air in from the outside to use in the combustion process. These units are called sealed combustion units. They can be up to 10% more efficient than atmospheric units. The next type is called a condensing boiler. These units extract more heat due to the design of their heat exchangers. Their efficiency is >90%. The exhaust from these units is cool enough to be vented with pvc pipe. Some of these units can be coupled with an indirect fired water heater to provide for the familys' hot water needs. Some of these units combine the features of the condensing units with modulating capacity. Modulating capacity is where the unit senses the load on the system and adjusts the amount of gas it is burning. This eliminates a lot of cycling of the unit on and off.
A combi is a unit that produces hot water for central heating as well as the domestic hot water needs of the home. The unit typically has two thermostats (programmers) which turn the unit on and off. When a tap is opened for hot water the unit starts a circulator pump and positions the diverter valve to supply dhw. This causes a pressure switch to close and the water temperature is checked. If the temperature is below the setpoint, a flue fan is energized. After a time delay, the gas valve is opened and a spark is generated to light the gas. A flame sensor verifies that the unit is firing and the gas valve stays open until the set temperature is reached. The process is much the same in the heating mode except the other thermostat is used and the diverter valve is positioned for heating. The most advanced units can be fitted with weather compensation controls. This is a system that measures the outdoor temperature and the heating water supply temperature. The two are compared and the heat output of the boiler is adjusted to provide longer more efficient run cycles. The most advanced systems produce electricity while they provide for the home's heating needs. These units are called combined heat and power systems or CHP for short. They can be a very cost efficient way to heat a home or business.
The biggest advantage of using a boiler to heat the home is the fact that it provides a more consistent room temperature than a warm air furnace. They can easily be connected to a renewable energy source such as a solar water heater to provide energy savings. There are some disadvantages to using these units to heat your home. They can be very expensive to install in a new home. The price of copper has risen sharply in the last several years. Therefore the cost of the radiators and piping system has increased sharply. This can be overcome with infloor radiant heating systems. These systems use cross-linked polyethylene (PEX) tubing instead of the standard copper pipes and radiators. If you use hot water for heat, and you want to have central air, you have to install a separate system. Therefore, it is usually more practical to use another type of heating system.