I had a Smart thermostat for a while, but removed it because it was costing me extra energy.
My boiler is more efficient if it runs at low. But with the thermostat allowing the house too cool off, the boiler had to run at high to warm up the house in time. (I don't remember the exact number it's something like 98% vs 80% efficient.)
This ended up using more energy, than leaving it to run at the lowest possible output temperature the entire day.
When I googled this I found that Energy Star does not consider smart thermostats to be energy saving.
I suspect the same will be true of A/C - letting the heat pump run at very low all day will save more energy than high speed to quickly cool the house.
My boiler is more efficient if it runs at low. But with the thermostat allowing the house too cool off, the boiler had to run at high to warm up the house in time. (I don't remember the exact number it's something like 98% vs 80% efficient.)
This ended up using more energy, than leaving it to run at the lowest possible output temperature the entire day.
When I googled this I found that Energy Star does not consider smart thermostats to be energy saving.
I suspect the same will be true of A/C - letting the heat pump run at very low all day will save more energy than high speed to quickly cool the house.