I had a message from a recent purchaser of an Essex tank to say that the thermometer had broken and could his washing it in hot water be to blame.

The short answer is yes. The thermometers in these tanks are designed to work over a short range. If the temperature is too high, the alcohol expands but, being a liquid cannot be compressed. Therefore, the glass breaks at its weakest point. Some limited-range thermometers have an expansion chamber at the top of the scale but I cannot see one in either the Rondinax 35U or the Essex. Therefore, the tank should not be washed in water higher than the maximum temperature shown on the scale.

The instruction manuals say nothing about this problem. I suppose when they were written the ability to get hot water straight from the tap was very uncommon.