The 108 error code appears due to low pressure in the heating system. The pressure drop can be caused by heating radiator leaks and poor pipe connections.
Also, there may be leaks in the heat exchanger’s piping system. In some cases, leaks can be soldered. If not, the heat exchanger will need to be replaced. The tank itself may be leaking too. To check it for leaks, press on the valve at the top of the tank. If it causes water to gush out, the tank must be replaced.