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.
Heel pain can have many causes. If your heel hurts, see your primary care doctor or orthopaedic foot and ankle specialist right away to determine why and get treatment. Tell him or her exactly where you have pain and how long you've had it.