I have an older generation of the Infinity Controller. I have furnaces and not a heat pump, but I will give you an idea of what may be happening based on what I have read. It may not apply to your controller, so you may have to do more research. For some reason Carrier is not good at explaining the operations of their controllers.

I think there are two available lockouts. One for the heat pump and one for the hydronic heating. Each lockout is set independently. If you set the heat pump lock out at 4 degrees, then the heat pump will not come on below this temperature. If you were to set the backup lockout at say 10 degrees, then the hydronic heating would never turn on above that temperature. In between 4 and 10 degrees, I think the algorithm is to first start with the heat pump, and then turn on the back up if it is taking a long time to satisfy the thermostat.

Here is a link to the installation manual. You can go into the service menu and review the lockout settings. Be careful with any changes. Talk to your installer if you are not sure of what changes to make.

Smart recovery means that the recovery of a set back will start 90 minutes early. Therefore if you were to set back the temperature at 11pm to 65 degrees, and you set the temperature to be 70 degrees at 7am, the heat pump would start warming the house slowly at 5:30am. I personally don't care for smart recovery and have it turned off.

Here is a link that might be useful: Installation instructions

Thanks mike_home, that was helpful.

I ran a couple of experiments this week.

I set the hydronic upper limit to 5degF and the HP lower limit to 0degF. So, backup heat should not run above 5degF (essentially no backup heat use). Outside temps dipped below 10degF at night. The house maintained temp with no backup heat which was impressive for a HP, however energy use was 158kwh for the 24 hours. This temp was lower than we have been seeing but normal energy use has been below 60kwh.

It is apparent that if the upper limit on the backup is not set, the thermostat will decide how to use the backup.

I now have set the upper limit of the backup to 15degF and the lower limit of the HP to 0degF. Will see if this achieves what I wanted which is to mainly depend on the HP and to use oil sparingly.

I will now experiment with setbacks at night to see if they help or cost $.

Would be nice if Carrier would publish how these thermostats work.

For conventional heat pumps, I think it is easy to select the lock out temperature. The temperature the heat pump can no longer provide enough heat the house is usually above the point where it is more economical to use the back up heat source. But the Greenspeed inverter technology allows it to supply heat even at low temperatures where it may not be economical.

Keep us updated on your experimentation. Perhaps you can write a white paper on how the controller works!