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I have a heat pump and an oil furnace for backup.

How does the HVAC system know when to use the heat pump and when to use the oil furnace?

Specifics: I have a Honeywell TH8320U1008 thermostat which handles 2-heat systems, and when I pull it off the wall and look at the wires one column of wires is labeled "Heat Pump" and the other is "Conventional". The manual makes no mention of setting a temperature where it switches from one to the other. Is there any way to control this?

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A heat pump thermostat prefers to operate the heat pump under "usual" conditions as that is expected to be less expensive. It will invoke the back up (or "emergency heat") when any of several conditions occur:

  • The current indoor temperature is more than 4 °F/2 °C cooler than the temperature setpoint. This is the blockhead approach taken by primitive thermostats and intelligent thermostats when "anticipator mode" is disabled.
  • The outdoor temperature is cold enough that the heatpump can not operate inexpensively.
  • The "emergency heat" switch is enabled on the thermostat.

Some (advanced) thermostats might have an adjustable delta temperature value, but it does not look like the TH8320 does. According to the installer manual (see page 17), there is a deadband adjustment, option 0310. Besides directly controlling the deadband, it might also be used as the temperature delta or half of it. Try adjusting the deadband, save it, and then adjust the setpoint temperature above the current reading (which will drift up as your hot hand and body will warm the thermostat) and see if the delta is affected.

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  • Yes the information is in that Technician manual. It wasn't in the user manual. Thank you. – mankoff Sep 17 '14 at 21:46
  • I think the correct answer is the "Heat Pump Compressor Lockout" sets when it switches to backup. This is called "Balance Point Temperature" elsewhere in the manual. This is 0350. Also 0360 "Heat Pump Auxiliary Lockout". See also page 33 and 34 of that document. – mankoff Sep 19 '14 at 12:16

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