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.
The most common causes of heel pain are plantar fasciitis (bottom of the heel) and Achilles tendinitis (back of the heel). Causes of heel pain also include: Achilles tendinitis; Achilles tendon rupture; Bone tumor; Bursitis (joint inflammation) Haglund's deformity; Heel spur; Osteomyelitis (a bone infection) Paget's disease of bone; Peripheral neuropathy