An E7 error code indicates that there is an issue with the solenoid valve on the inside of your 45 model tankless water heater.
An E8 error code typically pertains to the 45HI, as the 45H is an outdoor only model and does not have ventilation attached to it. The E8 error code indicates that there is a blockage in your vent pipe. First, remove the top venting from your 45HI, and try to run the heater briefly to be sure this clears said error code. If the heater does not repeat the E8 error code without any ventilation attached, you would need to locate and clear any blockages inside of your ventilation to resolve this matter.