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 is one of the most common and irritatingly painful foot problems. It can strike at nearly any age, regardless of health status or activity level. There are a variety of possible causes of heel pain, but the most common cause are heel spur syndrome, or plantar fasciitis. To determine the cause of your heel pain, it's best have an evaluation by a podiatrist (foot specialist) or other health care provider.