The answer to 'boiler smells bad' is that a foul odor coming from your boiler is usually a sign of a problem that requires professional attention. Common causes include a buildup of sludge, corrosion, or a gas leak. The smell may be sulfuric, rotten eggs, or like burning rubber.
While some minor issues can be addressed through DIY maintenance, a persistent or strong boiler odor typically indicates a more serious problem that requires diagnosis and repair by a licensed HVAC technician. Attempting to fix the issue yourself could be dangerous and may even make the problem worse. It's best to call in a professional to properly identify and resolve the root cause of the smell.