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.
It is caused by long-term strain on the plantar fascia and muscles of the foot, especially in obese people, runners or joggers. As in plantar fasciitis, shoes that are worn out, poorly fitting or poorly constructed can aggravate the problem. Heel spurs may not be the cause of heel pain even when seen on an X-ray.