There are different types of boilers so when hiring for boiler repair in San Francisco CA, make sure the boiler repair company can prove lots of experience with exactly your kind of boiler. The same goes for a company you choose to buy a boiler from and to install a boiler.

Boilers come in different forms including hydronic boilers, fire tube boilers, water tube boilers, packaged boilers are mos common.

Hydronic boilers: Most commonly used for central heating systems of buildings and which uses hot water rather than forced hot air. Circulates hot water through the home to hydronic baseboard heaters or radiators. Is sometimes also configured to perform as a home's hot water heater at the same time. 

Fire tube boilers: Water is heated and turned into steam by coming into contact with the fire tube in which air or other gases have been heated by a flame.  Fire tube boilers are used for steam pressures up to 40lb/in2 and rates of 26,000 lb/hour

Water tube boilers: Boiler feed water is heated in tubes that come directly into contact with the flame and fed into the boiler drum and converted to steam inside the drum.  Water tube boilers are used when steam pressure requirements and demand are higher than fire tube boilers with rates as high as 132 tons/hour of steam

Packaged boilers: Boilers that come manufactured as a complete package and needing only a water supply and fuel to be operational. Package boilers are usually fire tube boilers.  Packaged boilers are rated based on the number of passes the hot gases make through the boiler and there can be one to three sets of fire tubes. A three pass unit with two sets of fire tubes is the most common type. 

Waste Heat Boilers: Hot waste gas (heated air) as a byproduct of another machine (like exhaust from diesel engines or gas turbines) is fed into the boiler past the water tubes, heating it to steam. If the waste gas air is not hot enough itself, an auxiliary fuel burner is used to increase the temperature of the waste gas being used. 

Resources About Boilers, Hot Water Heaters in San Francisco CA  
The following is a list of trade associations and publications relevant to boilers & water heaters.

Regulatory Bodies & Trade Associations for Hot Water Heaters & Boilers in San Francisco CA
CSLB Contractors State License Board 

DCA California Department of Consumer Affairs

ABMA (American Boiler Manufacturers Association)

CIBO (Council of Industrial Boiler Owners)

AABA (American Association of Boiler Assessment)