Editor's Blog1 min ago
I have a large well lagged. Hot water cylinder, is it cheaper to leave the immersion heater switched on constant,or twice daily? I don’t use a lot of hot water,but have a bath/shower daily
I went to Buenchico's link. It just said that the common belief was not true. It did not, imo, "debunk it". The fact is that, no matter how well insulated the tank is, it will still lose heat. Some heat will actually go along the copper pipes that will carry hot water when it is needed. The heat loss is proportional to the fourth power of the absolute temperature of the hot body, so the hotter the tank, the hotter the ooutside of the insulation, the faster heat is lost. It's obvious that, when the tank has cooled down to the same temperature as the house, no further heat will be lost. If the myth were true, you would leave the immersion heater on if you went on holiday for two weeks.
Bet; I agree with your conclusion, but I think that you don't take account of temperature difference between tank and its surroundings when you say "The heat loss is proportional to the fourth power of the absolute temperature of the hot body." Apart from that I agree with your analysis and conclusion. (I'm nit-picking).
I heat my water with an immersion heater and I've heard all those comments over the years about leaving it on all day and have taken no notice whatsoever. The dial on the electric meter spins round so much faster when the immersion is on, so I only put it on when I want hot water usually for about 20 minutes a day or longer if I want a bath. Works for me !