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 pain in the middle of the heel on the bottom of your foot is most often due to a problem called plantar fasciitis, which is caused by inflammation of the tissues that attach at the bottom of the heel. It will hurt when you walk as the heel touches the ground, so every step may be painful.