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.
It is caused by long-term strain on the plantar fascia and muscles of the foot, especially in obese people, runners or joggers. As in plantar fasciitis, shoes that are worn out, poorly fitting or poorly constructed can aggravate the problem. Heel spurs may not be the cause of heel pain even when seen on an X-ray.