Boilers and water heaters both heat water to either heat your space or keep your water warm for showers, etc. At the Factory Furnace Outlet, we sell combination boilers, no-pump boilers, water heaters, tankless water heaters, and other modern versions of boilers and water heaters.
As a rule of thumb, for boilers, you will need 50 BTUs per square in a cold climate, 35 BTUs per square foot in a moderate climate, and 20 BTUs per square foot in a hot climate. Boilers also come with AFUE ratings, and the higher the rating, the more efficient the boiler will be.
Disclaimer: Before making a purchase, you should always check first with your qualified installer to figure out what unit is best for your home or space.