"Michael J. Mahon" wrote:
> the slab stayed warm several hours after the heat was turned off

That very same problem is already present with simple old-style metal
radiators. Turning them on they get hot in minutes, turning them off
changes nothing at first. But a small wall radiotor in a large room has
a large power to mass/area ratio, so it cools down fairly rapidly.
Several tons of pavement material distributed over the whole floor area
take a long time to cool down.

So what you have to do is turn off long before the desired temperature
is reached, wait an hour or so, and look. That's exactly what the
internal feedback by heating resistor does. Simple and easy in analogue
electro-mechanical hardware, tricky to emulate digitally. Of course the
downside to all this is, that it takes a log time for a room in a colder
equlibrium to reach a new higher desired temperature. As a human coming
home you turn up the thermostat a few degrees higher until the heating
"gets going" appreciably. Having got a running digital emulation that
bit should be amenable to be added in, but that's an extra second step
for later.

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