There is a reason this controller is cheaper than the others

Hi Michael. 
We understand the frustration experienced with this setup and would like to clarify the behaviour. The interaction between the Geyserwise MAX controller and the XTEND element is influenced by the position of the thermostat probe. With XTEND elements, the thermostat sits lower in the geyser than in a standard configuration, meaning it measures cooler water below the main heating zone. As hot water rises, the actual usable hot water is at a higher temperature than what the probe detects, which can lead to longer heating cycles or inconsistent control.This is also noted by the manufacturer of XTEND elements, who advises reducing the thermostat setting due to the lower probe position and the effect of thermal stratification. In line with SANS 10254 principles, temperature control systems rely on sensing representative water temperature. Where the probe position does not reflect this, the system may not regulate as intended. This is therefore a limitation of the configuration rather than a fault with the equipment or installation. For further reference, please see the manufacturer’s installation guidance: https://xtendelements.co.za/installation/. You will have similar behaviour with any other electronic controller or mechanical thermostat.