There is no adjustable setting available on these devices are far as I can see. The pipes are extremely hot when touched. Very strange that no heat at all going right of the T connector. Completely cold.
https://www.amazon.co.uk/Tesla-External-Immersion-Heater-Willis/dp/B00XLBAAEG not sure what red button is prob thermostat.
Instructions are below but for example 10 minutes only give 3L of hot water. :
OPERATING INSTRUCTIONS
A Willis Immersion heats approximately 4.5 litres of water to a high temperature every 5 minutes, so if one can decide before switching on how much water is required the pre-heating time can be calculated as follows:
4.5 litres for hand-washing, baby’s bath, shaving, breakfast dishes etc. = Pre-heat for 5 minutes
9 litres required for washing-up dinner dishes, medium sized clothes wash = Pre-heat for 10 minutes
9–14 litres required for a shower = Pre-heat for 10/15 minutes
54 litres required for a bath = Pre-heat for 1 hour