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.
Inflammation of the plantar fascia can also cause this pain -- normally at the bottom of the heel. Known as plantar fascitis this can cause your symptoms. Both of these occur due to overuse of the foot, often with an abnormal foot arch. Often, shoe inserts or orthotics are needed.