As a standard immersion heater is 3KW, so it would take 2 hours to heat 100 Litres of water. After an hour and a half, it will only have reached about 53 degrees.....
Podiatrists cite Plantar Fasciitis as being the most common cause of heel pain. Typical symptoms of this condition include sensations of pain, burning or heat in one or both heels.
The pain in the middle of the heel on the bottom of your foot is most often due to a problem called plantar fasciitis, which is caused by inflammation of the tissues that attach at the bottom of the heel. It will hurt when you walk as the heel touches the ground, so every step may be painful.