Rocket plumbing, chicago’s best emergency plumber and water filtration service company. To find out more about the options available please. Buying guides to help you make the right choice. Our research took place in 2016. Service may not be available in all areas. To provide hot water, combi boilers are on constant standby and connected to the mains water supply when a tap is turned on, the boiler is triggered into action and it heats the.