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I am not familiar with MegaFlow unvented cylinders; however, my Oso unvented cylinder has the sensor fitted as follows:
1. Access via the electrical panel.
2. Probe inserted against tank by pushing it up to the desired height.
3. Probe lies against the cylinder and held firm by foam and tape.
4. Evohome temperature set below tank stat, and top limit safety stat working in series. If Evohome fails then tank stat will manage the temperature with the top limit as the safety fallback.
Ovo suggested this as their preferred solution for my tank model. Have you contacted Megaflow Support?
I disconnected and isolated the immersion heater probe and inserted the temperature sensor down the hole instead. The hot water relay is then wired in series with the existing dual stat set to its maximum temperature as overheat protection only. The chap that came to service the boiler agreed that none of the above required certification as the wiring consisted of replacing an existing unit and the cylinder stat remained untouched and in service.
Honeywell also recommend that the micro-switch output from the motorised valve is wired through the dual stat in case the contacts weld closed or the valve sticks (both of which are fairly common and entirely defeat the point of a dual thermostat anyway).
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