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. The most common cause of heel pain is plantar fasciitis, inflammation where this long ligament attaches to the heel bone. The pain may be sharpest when you first wake up and put pressure on the foot. Arthritis, excessive exercise and poorly fitting shoes also can cause heel pain, as can tendonitis. Less common causes include a bone spur on the bottom of the heel, a bone infection, tumour or fracture.