Megaflo dd145 questions

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

del.

1/ yes

2/ I don't think it's an issue

3/ yes someone with g3 certification can confirm all the safety devices/components are working satisfactory and they can recharge the internal air bubble in the cylinder

Thanks for the reply.

Is it necessary to have the immersion tank descaled? It's in London which is a hard water area. Is there any way of knowing if an immersion tank needs descaling?