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When you shower are you aware whether the water was heated an hour ago or a day ago?

Yea...hot water heaters aren't perfect insulators.

Actually they are pretty good. I did a test where I switched off a completely heated 150L standard home water heater for 36h and after that it only needed to heat for 0,6hWh to become back to the starting temperature.

Also you should always store hot water at a very hot temperature (Nasty bacteria thrive in warm water so you mustn't store it) but you should always mix it (with fresh cold water) to a lower temperature for bathing and washing.

So whether it's a tank of 80°C water or a tank of 65°C water make no difference to the 38°C water coming out of the thermostatic mixer in your shower.

You don't need to keep the water at a high temperature all the time.

I don't have the regulations to hand but it's enough to bring it to a high temperature once every few days for some period of time (to kill the bacteria that cause legionnaires disease).

0.6kWh sorry for the typo.

If the temperature is beginning to dip below it's acceptable threshold and it's waiting for a certain price before it kicks back on, yes.

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