[VLM]

The term to google for is cogeneration. Sometimes all people need is the correct search term.

The problem with cogeneration is its usually incredibly heavy and there aren't many uses for very low temperature process heat in most real-world applications.

Another problem with cogeneration is you might get a small percentage boost by connecting a backup generator to the HVAC system but the capex can be VERY high if done safely and reliably, and system complexity seems to scale at an exponential rate. It seems a no-brainer to dump the radiator heat from a backup generator into an office building thereby burn less natgas to heat the building; however you factor in that you have to frost-proof it all and its going to be hundreds of gallons of anti-freeze in those pipes which is expensive and all pipes leak eventually with has ecological issues, and you can't have exhaust leaks into the building and over half the time you need cooling not heating anyway and the HVAC cannot be made smaller because you still need to heat even when the gen is off and the HVAC system will be less reliable because it'll be more complicated and the backup gen will be less reliable because its more complicated, perhaps making the backup gen less reliable than wall power. So shrug shoulders and dump the gen heat using an air cooled radiator, even in the winter.