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: Most common cause for heel and foot pain is plantar fasciitis which is an inflammed fascia or ligament that supports your arch. It originates in the heel and ends in the toes. When you pronate or your arch flattens than there is tension in this band of tissue causing pain and inflammation.