Tom Newton wrote:
PJ -

What's a setback thermostat?

So, it's been 2 days. Any change? It may take a week (or even more)
to warm up that slab and the several feet of earth underneath it.
The exterior insulation sounds like a really good idea. If the
return water was warm and you still had cold rooms, then you would
have a disaster, ie. not enough heat exchanger surface area.
I suspect you will be seeing some improvement already.

Jon