"Pete C" wrote
| Also is the secondary cylinder connected to the main cylinder
| top and bottom? If the thermostat is reading the temperature
| of the water from the bottom of the cylinder you might be better
| off connecting it via a pipe stat on the top pipe returning to
| the main cylinder.

However the top connecting pipe will get hot very quickly, before the heater
has made much difference to the bulk of the water in the cylinder.

| Also check the second cylinder is not scaled up.

I suspect scale in the lower pipe between the two.

Owain