There’s not much reason for that to happen unless the building’s water heater has failed or is undersized. I have had that happen once in the decades I’ve lived at this house and it was a simple matter of turning the handle up a bit hotter.
Otherwise, the effect thing you described (the mixer lowering the pressure to maintain a constant output temperature, assuming the hot water supply is constant temperature) works just fine. I have never had the water suddenly become unexpectedly hot or cold after the initial warmup phase.
There’s plenty of reasons for that to happen, but it depends a lot on how your heat source works.
Most regular furnaces will heat up the water quite high, and just leave it at that temperature, and as soon as the water temperature is about to drop, it starts heating again, therefore the temperature rarely changes, unless you have used up all the hot water (teenagers running long and hot showers for rather long, or just many people showering in short succession).
But many newer smart furnaces, and especially heatpumps will change the amount of hot water available depending on demand, and even dynamic electricity costs.
I have a modern heatpump which both controls the temperature and amount of hot water depending on usage patterns, as well at optimizing for heating up the water when the electricity is cheap. Just today electricity is 5x as expensive at 20:45 as it was at 15:00
This means that the hot water temperature and volume can change quite significantly, depending on the time of day and the electricity cost.
Right now I can see that my hot water is only 45.7°C/114.1°F, which fits well because no one in the house is usually showering at this time, and the hot water is only used for washing up and washing hands.
In the morning the hot water tank will likely be around 55-60°C/131-140°F, because that’s when people are usually showering, and the electricity is usually cheaper during the night hours.
Having a thermostatic mixer that can be set to a fixed temperature, regardless of the input hot water temperature is quite nice in this case.
Where I live it’s against building code for hot water to be kept below 60C, due to the potential for growth of water-borne pathogens. A hot water tank that allows water to fall to 45C would not be allowed. In practice, the companies who supply water heaters set them much higher than 60C to avoid the risk of water cooling below that temperature during extended power outages.
There’s not much reason for that to happen unless the building’s water heater has failed or is undersized. I have had that happen once in the decades I’ve lived at this house and it was a simple matter of turning the handle up a bit hotter.
Otherwise, the effect thing you described (the mixer lowering the pressure to maintain a constant output temperature, assuming the hot water supply is constant temperature) works just fine. I have never had the water suddenly become unexpectedly hot or cold after the initial warmup phase.
There’s plenty of reasons for that to happen, but it depends a lot on how your heat source works.
Most regular furnaces will heat up the water quite high, and just leave it at that temperature, and as soon as the water temperature is about to drop, it starts heating again, therefore the temperature rarely changes, unless you have used up all the hot water (teenagers running long and hot showers for rather long, or just many people showering in short succession).
But many newer smart furnaces, and especially heatpumps will change the amount of hot water available depending on demand, and even dynamic electricity costs.
I have a modern heatpump which both controls the temperature and amount of hot water depending on usage patterns, as well at optimizing for heating up the water when the electricity is cheap. Just today electricity is 5x as expensive at 20:45 as it was at 15:00
This means that the hot water temperature and volume can change quite significantly, depending on the time of day and the electricity cost.
Right now I can see that my hot water is only 45.7°C/114.1°F, which fits well because no one in the house is usually showering at this time, and the hot water is only used for washing up and washing hands.
In the morning the hot water tank will likely be around 55-60°C/131-140°F, because that’s when people are usually showering, and the electricity is usually cheaper during the night hours.
Having a thermostatic mixer that can be set to a fixed temperature, regardless of the input hot water temperature is quite nice in this case.
Where I live it’s against building code for hot water to be kept below 60C, due to the potential for growth of water-borne pathogens. A hot water tank that allows water to fall to 45C would not be allowed. In practice, the companies who supply water heaters set them much higher than 60C to avoid the risk of water cooling below that temperature during extended power outages.