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Unique to the Optitherm is its complete lack of a cold start transient 'spike'.
Most thermostatic taps display a momentary spike in temperature at first start up: during periods of rest, or non-use, the internal thermostatic element cools to the ambient temperature, which means that the valve's 'hot port' may be partially open. On opening the flow, an initial slug of system temperature hot water enters the mixing chamber. The thermostat element immediately expands, moving the internal slide valve to alter the mix of hot and cold water and this achieve control. Unfortunately, this very short temperature 'spike' can be a real disincentive to handwashing as the user perceives the tempeature to continue rising and pose an increased risk of burning. The natural reaction is to draw the hands away from the flow to avoid the rising temperature, by which time the thermostat has started to work to maintain the flow at a safe and comfortable temperature.