I spent yesterday replacing the immersion heater in our hot water tank. It was not an easy job, but I found this site and others very useful to get ideas about what to do.
The old immersion heater was fitted about 35 years ago when the house was rewired (a few years before we bought it). About a month ago it started making singing noises like a kettle and the heat output dropped. The other day it stopped working altogether.
When I tried to take it out, the thing was totally seized. The tank is quite sturdy, being an old 1950s cylinder with a brazed-in boss at the top. I drained it about 9 inches below the top so most of the water was still in it. The box spanner I bought from B&Q looked quite sturdy, however the bar supplied with it just bent when I used it with a bit of extra leverage.
The old unit looked like it had been fitted with Bosswhite or a similar cement that had gone rock hard. We have soft water, so limescale is not a problem. I used a hacksaw blade to cut through the fibre washer between the heater and the boss - a laborious process. I put in some penetrating oil and left it for a while.
Meanwhile I welded the box spanner to a 2 foot length of 2" angle iron so I could apply a lot more force and also apply some impact with a club hammer. Without this improvised tool I would never have got the old unit out.
It still wouldn't budge even with quite a lot of hammering. Finally I heated the boss with a flame for several minutes. Then I tried knocking it back and forth. Finally I tried turning it again applying quite a lot of torque. You do have to watch how the cylinder is coping.
At last I got a bit of movement and slowly I unscrewed it. The threads and been completely covered with Bosswhite or similar.
To refit the new one, I put PTFE tape round the threads and applied a good bead of blue silicone sealing compound as used in automotive applications to both sides of the gasket. At least If I have to take it out again it should move!
One final point. The mating surface on the boss had been slightly damaged by using the hacksaw blade. I had to carefully file this down to get a smooth surface. Make sure the filing is done evenly round the boss, so the mating surface remains at 90 degrees to the thread.
I hope this is useful to others. I've done a lot of plumbing of all sorts over the years including central heating, bathrooms etc., but this was one of the hardest jobs. In summary, you are going to need quite a big wrench, with a two foot lever. After seeing why the threads were stuck, I don't think the penetrating oil made much difference. Hammering it alone, didn't seem to work. The crucial thing seem to be the heat as the differential expansion probably cracked the cement bond.
Hugh