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.
Underpronation is also a factor of the most common cause of heel pain, which is plantar fasciitis. Along the bottom of the foot is a tough ligament called the plantar fascia. This ligament attaches to the heel bone and runs the length of the foot towards the toes.