What does your realtor say?
When I sold my house the Home Inspector did the classic "should be checked out by a licensed hvac company".
Put your tools away and hire an independant company to come and diagnose it. You are opening up a can of worms if you say its ok and something happens 6 months from now.
Doesn't seem like anything is wrong with the heaters and its not the first time a homeinspector has found something wrong that isn't wrong. (In my case, the HI said there was something wrong with the water pressure in the master bedroom shower, should be checked by a licensed plumber.....duh, the massaging shower head valve was shut off!!!!!! I turned the valve on, hired a plumber, and he was out of there in 10min with a diagnostic $)
It's not the Brand with the fewest repairs-It's all in the install!!! Attention to detail and using the best materials!