A water heater stores, heats and supplies hot water to multiple fixtures, such as a laundry machine, shower and sink, while a boiler is responsible for the heating of the entire house. Simply put, a water heater supplies hot water, and a boiler heats the house and its water supply.
Compared to water heaters, boilers, or hydronic heat systems, require complex pumps and controls. This is because boilers not only heat but also boil water, changing it into steam. While a water heater is effective for only small spaces, a boiler can heat an entire complex, pool or tub.
Because boilers operate in a closed-loop system, they are highly efficient compared to water heaters. A closed-loop system means that after the extraction of heat from steam, the steam changes into water and goes back to the boiler through the pipes where boiling takes place once again.
Another difference between these two is that water boilers can run on natural gas, heating oil, electricity or propane, while water heaters can run on solar, electricity or gas. Water boilers can also run on alternative fuels, such as wood pellets.
The type of maintenance needed for each is different. With water heaters, changing air filters every month is necessary. Medium efficient boilers only require annual service while the highly efficient ones can operate for years with minimal maintenance.