My rads are each fitted with a Honeywell knob that goes from position "Snowflake" to position 6. There is a loose grey button at position 4.
What is the point of the grey button? There is a very slight resistance as one turns the knob across the button's position. I don't see any point for it!
Why not simply mark the circumference with a 4, i.e. "snowflake" - 1 - 2 - 3 - 4 - 5 - 6?
MM
Commonly called a TRV (Thermostatic Radiator Valve)
Normally you have to depress the button to allow it to be turned up past that point, I suppose to dissuade people form just whacking it all the way up without thinking.
Same reason there is a little tactile bump on the 5 key of many phones.
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