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 is probably the most common cause of heel pain. Symptoms come on gradually and are often worse first thing in the morning. The plantar fascia is the tissue under the foot which forms the arch.