I recently bought a flat that doesn't have a gas supply, so hot water is provided by a Megaflo dd145 immersion heater. The hot water is controlled by an economy 7 timer with a separate boost switch. There is only one cable from the timer which goes into the bottom heater on the immersion tank (see photos). The flat was built about 8 1/2 years ago and it is the original immersion tank provided with the flat.
Last week the tenants called to say they had no hot water so I went out to have a look. After switching off the power to the immersion tank, I had a look at the thermostat to see if the reset switch needed to be reset. Instead I found that the thermostat had burnt out by one of the prongs that attaches the thermostat to the immersion tank. From paperwork we received when we bought the flat, I can see this thermostat was fitted just over a year ago.
As there was no cabling into the top heater/thermostat, I replaced the bottom thermostat with the top thermostat as I assumed this was not being used. The plumber I use doesn't do megaflo/unvented systems so isn't able to help me on this. I have done some research on the internet but would like to clarify some things.
1. My understanding is that it doesn't matter that the top heater/thermostat is not wired. As it is not wired, I also assume it is not being used and so it doesn't matter that there is no longer a thermostat there. The only disadvantage is that if the boost switch is used then the whole tank is heated up rather than just the top half of the tank. I also assume the top thermostat doesn't need to be replaced. Is this all correct?
2. When I changed the bottom thermostat there was some green colouring inside where the thermostat attaches to the immersion tank (see photo). I assume this is from a previous water leak. The area was bone dry when I changed the thermostat last week. I also went back 5 days later to check it and the area was again bone dry. Is this something I should be concerned about?
3. The immersion heater was serviced last year. Is it sensible to have it service annually? What would be done on a service?
Thanks for your help.
John