Most types of boilers, also known as water furnaces, can be installed any place they fit, with the exception of power-vent boilers. Since power-vent boilers draw air from inside the home, they need to be placed in a relatively open area.
While it might be tempting to place a boiler in a crawlspace or other confined area, doing so can have some drawbacks. Boilers generally require regular maintenance, and placing one where it's hard to access can lead to difficult and potentially expensive service. Some boilers place all control components on one side, but others have important components in other places, and these control components must be accessible for servicing.