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 pain is a very common foot problem. The sufferer usually feels pain either under the heel (plantar fasciitis) or just behind it (Achilles tendinitis), where the Achilles tendon connects to the heel bone. Even though heel pain can be severe and sometimes disabling, it is rarely a health threat.