Length of line is too long. When the tank was at same height, the 5 foot up and 5 foot down canceled each other out.
As said by rep. Run a return lie. You'll need to add a bypass plug to the pump. Another way is to install a tiger loop at/near the burner.
As said by rep. Run a return lie. You'll need to add a bypass plug to the pump. Another way is to install a tiger loop at/near the burner.