[cmcnab]

I had a house for a time that had just what you describe: a concrete block thermal mass set in the foundation. There were ducts running through it as part of the forced-air system. During the days, especially sunny days, the block soaked up the excess heat and then radiated it back out during the night. It worked quite well; indoor temperatures stayed remarkably consistent. A company called Adirondack Alternate Energy were the architects. https://aaepassivesolar.com/low-energy.html