The answer to 'heat exchanger smells bad' is that a foul odor from the heat exchanger is often caused by a crack or leak that is allowing combustion gases to escape. This can be a serious safety issue and should be addressed promptly. Common symptoms include a rotten egg or sulfur-like smell, soot buildup, and signs of corrosion on the heat exchanger.
The best solution is to have a licensed HVAC technician inspect and diagnose the problem. They can check for cracks, leaks, or other damage and make the necessary repairs or replace the heat exchanger if needed. Attempting to DIY this repair is not recommended, as it requires specialized knowledge and tools to properly assess and fix the issue.