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.
Plantar fasciitis. Plantar fasciitis causes pain in the heel and bottom of the foot. It is due to inflammation of the plantar fascia. The plantar fascia is a muscle-like connective tissue at the bottom of the foot. Plantar fasciitis is associated with overuse, such as with runners, or in cases of obesity.