Problem or I dont understand how it should work

ai ing

So recently moved in to a new place and trying to figure out how things should work. Hoping someone who understands this stuff can maybe give me a few pointers

• Gas boiler: set to ~67C
• Boiler timer: Hot water + heating (rads) timing separate
• Rads a mix of thermostatic and regular valve ones
• Wall thermostat in hall: set to 21C
• Thermostat on upstairs hot water cylinder
• Actuator valve beside hot water cylinder (to heat rads, from boiler timer when heating on)

So here is the weird one:
When I set the thermostat on upstairs hot water cylinder to a lower temp (0-~50) the gas boiler turns on even when the boiler timer is set to off. It never turns off but only heats the water in the cylinder
When I set to a high temp (60+) the boiler stays off and just runs off the timer only as I would expect

I dont really understand why this thermostat on the cylinder would work in this way. I had assumed it was to set a max temperature for the hot water in the tank and when that temp was reached it would turn off the boiler (even when on the timer)

Any pointers or is something nor working right?

K.Flyer

Sounds like a minor wiring error, which should be easy enough to rectify.
Also the stat is positioned too high on the cylinder, should be further down and moved away from the coil pipework a little.
The auto vent valve doesn't look very healthy either, should consider changing it as well.