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.
Again, plantar fasciitis is usually brought on by a biomechanical imbalance of the foot, which causes inflammation of the soft tissues of the feet. If you are inactive most of the time, but suddenly find yourself on your feet for days at a time, you may see an increase in heel pain.