In reply to the video posted by John from softcellelectrical – this should only be performed by a licensed electrician! This is quick overview to augment what he describes in his video using a Gecko flow through heater. I have a different flow through heating unit and purchased the replacement online from Balboadirect.com web site. I show the difference in the sensor on mine. John’s video is very detailed and helpful showing flow through heater / element from Gecko.