Homeowners can come across different issues depending on the type of tankless water heater they have - for gas ones, the burner may get clogged and cause hot spots to burn the heat exchanger. For electric ones, the heating element may need to be replaced if it is acting up.
Mark checked on the electrical wiring relating to the furnaces to insure that the furnaces are functioning efficiently with respect to one another.
Minor adjustments were made and both furnaces are functioning properly.
Kale suggested that the wiring should be examined by...