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Immersion heater replacement in sealed system - tips?

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TimC:
My all-electric house has an unvented hot water system based around a Heatrae Sadia (Santon) PP170D dual-immersion cylinder. I've noticed over the past few months that the amount of hot water available has significantly reduced, and I suspect that the lower element has failed (if it was the top element, I assume that the amount of hot water would be fine, but it might never get to its design temperature in the Economy 7 heating period).

Of course, restrictions permitting, there will be three of us in the house over Xmas rather than just me, so the demand will increase somewhat - I've been coping fine with the reduced supply on my own.

I can source the element, and the associated thermostat (I'll replace both), but are there any things I should know before I attempt the replacement?

aidan.f:
Find and read the manual? I think these devices have an  air space to allow for thermal  expansion and there are (strictly speaking) rules about who is qualified to work on them.

Diver300:
Obviously, isolate power before doing anything to the elements.

Firstly check the thermostats. They are in the dry, and can be taken out by removing the wires and pulling them out. They seem to be more expensive and complicated than the ones that I have seen. They seem to be like this:- https://www.dhsspares.co.uk/product/heatrae-95612719-control-assembly-stat-wire-ind.-1735847

You can check the thermostats in a pan of water with a thermometer and a multimeter. There should be no resistance between the terminals of the thermostat when cold, and they should go open circuit when hot. If a thermostat is open circuit, it may be that it got too hot and the thermal trip inside it tripped. I can be reset, but the fact that it tripped may mean the the thermostat isn't working very well and it may be easiest to change it.

While you have the connections to the elements available, measure the resistance. Each should be around 17 - 22 Ohms for a 3 kW, 230 V system. There should be no conductivity to ground from either end of an element.

If the electrical resistances are correct, it would usually mean that the element is OK. I would not change an element with the correct resistance unless I had some other reason to think that it was the wrong type or there was some other problem.

If you do change the element the instructions are here:- https://mediacdn.heatraesadia.com/-/media/themes/heatrae/literature/installation-manuals/megaflo-eco-installation-manual.pdf?v=1&d=20171113T161519Z on page 26

If you do change the element, I suggest that the first thing to do, before turning the water off, is to get an element spanner and loosen nut that holds element in place. When loosened, it will start to leak, but until you get it more than about 1 turn unscrewed, it will be a very slow leak. Just get the element to move a bit with the water pressure still in the tank. You only need a fraction of a turn. The reason for leaving the water pressure on is to make the tank as stiff as possible. The weight of water also stops the whole tank moving if you have to apply a lot of force. Once it's loose, turn off and drain the water.

TimC:
Thanks Diver. Some good stuff there. I think I have everything necessary to follow your guidance. Fortunately the previous owners were very good at hanging on to the manuals and tools, so I have the installation paperwork and the element spanner. The stats I have are slightly different to the one you link to (and a bit cheaper!), but not significantly. I'll have a go at checking it in a pan! I can confirm it hasn't tripped its reset.

Aidan, the expansion vessel is a separate component. I had the delight of repressurising that yesterday (it's very easy once you have access). Owing to its location, it was quite a wet affair! Draining the cylinder if I have to replace the element is also likely to be a bit of a wet T-shirt event, and is probably the most daunting part of the whole process!

Edit: definitely the thermostat. New one sourced for £28. Hopefully I'll stay dry while I arrange proper water heating again!!

Feanor:
The thermostats on a Megaflow unit have 2 separate things:

The twiddly adjustable thermostat, and a second Safety Cut-out wired in series.
The Safety cut-out will sometimes just decide to activate, and can be re-set.

It's a non-obvious small red rectangular recessed push-button right next the twiddly adjuster.
On mine, the word 'Safety' is marked on the body of the unit, with a small arrow pointing to it.

Use a small tool to give it a push in to re-set it.

Worth a check before replacing it.

Oh, never mind. I see that point was covered already.

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