Horstmann quartz - Auto boost setting

[pmx_super]

I've got an immersion timer that looks a lot like this:

http://www.tlc-direct.co.uk/Products/HOE7Q.html

except instead of a 0-1hr manual boost, it has 0-2hrs and 'Auto'. What do you think the auto setting does?

I'm guessing it leaves it up to the thermostat in the 'boost' immersion heater to control temp - and is therefore effectively always on. It's not really accurate to describe such a setting on the timer as 'Auto' though ('On' seems more appropriate) so maybe I'm wrong. Anyone know for certain?

 

[dal5band]

I think this is something to do with controlling two immersion heaters -
A "night rate" heater at the bottom of the tank and a "day rate" heater at the top of the tank. IF you only have one imersion then you are probably right. If you had two heaters, in auto it would switch to the other heater at night, but the day heater would be on all day. - That's my guess !!