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.
Conditions that may cause heel pain include: 1  When the tendon that connects the back of your leg to your heel becomes swollen and painful near the bottom of the foot. 2  Swelling of the fluid-filled sac (bursa) at the back of the heel bone under the Achilles tendon (bursitis)  Bone spurs in the heel.