Maintain your portable tankless water heater!
Winter is coming so make sure you store your portable heater inside when not in use. Insects and different outside elements and variables that gets in the heat exchanger can cause a buildup and would block the flame from escaping upright and as a result it would try to get out in other directions and that's why flame may come out the front and side of the heater.
Here is an example when you leave your portable heaters outside: