Heaters are a convenience of modern life, but a malfunctioning heater can be dangerous. Odours coming from heaters can serve as clues to a number of different circumstances and sometimes potential dangers. Similarly, sometimes danger comes with no odour at all.
Some Possible Causes
One complaint is a water heater smelling like "rotten eggs." This may indicate a problem with the sacrificial anode of a water heater. This demands repair, but the odour may not be dangerous of itself. Any natural gas smell from a heater should be taken as dangerous due to the flammability and explosive potential of natural gas. Evacuate the home immediately and call your local gas provider for assistance.
An extremely unpleasant cause of foul odour from a heater or radiator may come from a dead animal. Some rodents, particularly mice, may crawl behind the unit and die, especially if you have distributed slow-acting poisons around the home. If someone in your home suffers from respiratory illness, then the odour could hurt them. You should remove the animal to avoid the spread of their parasites, and ultimately, their stench.
Lack of Odor
Remember that one of the greatest dangers coming from a malfunctioning heater will have no smell-- namely carbon monoxide. Unless you have electric heat, you may be in danger of carbon monoxide poisoning. Place CM detectors throughout your house and to replace their batteries regularly. This is one danger that won't give your nose a warning.