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 Spurs. Another common cause of heel pain is heel spurs. A heel spur is a pointed bone fragment that forms on the heel bone and can often develop in conjunction with plantar fasciitis. Similar to plantar fasciitis, heel spur pain is most noticeable when walking after periods of rest.