The heat buffer is a great place to dump excess heat from air conditioning or ventilation
, to use later as hot water or heating. If more than one heat source is used (not counting furnace, stove, fireplace, etc.), then more than one set of coils at the bottom of the tank may be needed. The most efficient way to place them is one under the other, but with a second funnel between them, going to a second, smaller chimney. The cooler of the two sources (heat pump) should use the outer chimney, while the hotter (solar) should use the inner chimney to send its heat to a higher level.
Electric heating elements can not charge the buffer using cheaper night tariffs, but can also be a dump load for wind, hydro, PV, etc.
It's good practice to put 4-8 temperature sensors inside the buffer at different levels to know how much energy is stored. Only one sensor is not enough, since heat is stored in different temperature layers, so it's possible to have a thin layer of 80°C at the very top while most of the rest will be around 50°C, or to have 80°C in the top 70% of the tank. In the latter case the buffer has much more stored energy.