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.
Pain Beneath the Heel. If it hurts under your heel, you may have one or more conditions that inflame the tissues on the bottom of your foot: Stone bruise. When you step on a hard object such as a rock or stone, you can bruise the fat pad on the underside of your heel.