How might such a situation work? A server-boiler would be installed much like an existing boiler, with connections to the water and electricity supplies, and additionally a network interface. Based on the home's heating requirements, the server would be timed to start up and accept distributed computing tasks. The heat from the servers is transferred to the water, and the computation continues until sufficient hot water has been generated.
From the users point of view the boiler acts entirely like a normal boiler, but with the added feel-good for them that their boiler is helping the cure diseases. The server management would be entirely remote, potentially by the distributed computing project.
Is it economical? At today's prices a server-boiler would be more expensive than a standard boiler. But as computer pricing becomes cheaper it might become cost-effective, particularly if subsidised by commercial distributed computing projects.