Tenant complains of intermittent smell from water heater

[Carlos Lez]

tenant says “The water heater smells like rotten eggs, sometimes like a foul smell and sounds like almost a gas leak near the water heater at night to clarify.”. I am a qualified engineer with construction skills and have maintained and installed water heaters. However I am not licensed. I went there to check out the water heater which is only 5 years old and is working correctly. There was no evidence of any leakage of combustion gases or hydrocarbons. There was no smell and gas detectors did not detect any leakage. 

I reported back to them but they now say “is it possible to have receipts of the tests done for the water heater because it did have a foul smell. I want to ask who did you have come in to check the water heater. I just want to insure you did have someone come in who knows what they are looking for and is experienced in checking out a water heater not just people in charge of renting the place.”

what would be an appropriate response to this?