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.....
Another common cause of heel and calf pain is Achilles Tendinitis. Symptoms are frequently described as an aching pain in the back of the lower leg and just above the heel. This sensation is typically worse after exercise and upon first waking up in the morning.
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.