I wrapped the boiler and electrically heated it so I could estimate the heat loss. This way I could then compare unclad to maximum cladding.

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At the designed operating point of 40psi and 140°C bulk water temperature the heat loss unclad is ~35W and with maximum cladding it is around 12W.

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The cladding is a lightweight fleece, then a aluminium coated bubblewrap.

This has made me realise that I could make a low voltage miniature immersion heater - I will post something on this.