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.
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.