The answer to 'boiler smells bad' is that a foul odor coming from your boiler is typically caused by a build-up of sludge, scale, or other debris in the system. This can lead to the growth of bacteria and mold, which produce unpleasant smells. Common symptoms include a rotten egg or musty odor, and the smell may be more noticeable when the boiler is running.
In many cases, the solution is to have a professional HVAC technician perform a thorough cleaning and maintenance of the boiler. They can flush the system, clean the heat exchanger, and address any underlying issues. While some basic boiler maintenance tasks like air filter changes can be done DIY, dealing with a foul odor usually requires the expertise of a licensed HVAC technician.