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For DIY context: I recently got a heat pump installed; I know many things are not optimal (e.g. ductwork) and I am scoping what I want to fix/improve myself.

All offers I got for variable speed heat pumps required a proprietary thermostat (e.g., Mitsubishi). It seems this is generally required for variable speed systems, as described here for example:

[...] while thermostats that accommodate variable-capacity systems must be communicating.

Question 1: I do not understand why this is must be the case. Can't the control electronic of the heat pump / air handler set compressor speed and airhandler blower speed itself, based on criteria such flow rate etc.?

Question 2: My system (Carrier Performance) was sold to me as variable speed system. Indeed, the datasheet of the heat pump as well as the description of the air handler both state they are variable speed:

• Variable speed compressor

and:

The 40MBAB multi-poise air handler has a variable-speed ECM motor that synchronizes with the Carrier single-zone ductless heat pump models 38MARB , 38MBRC sizes

The manual explicitly lists "conventional thermostat support" and I got an EcoBee Lite thermostat installed. Why does this work, despite seemingly being a variable speed system?

Question 3: Does my Carrier Performance with the EcoBee Lite perform sub-optimal? In other words, does it not reach its full performance because it is not communicating? If yes, what is the best way to improve this? Using the thermostat that came with the air handler?

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  • 1
    why would one need variable speed ac ? for what ?
    – Traveler
    Jul 1, 2023 at 1:38
  • 1. yes it can be designed to adjust its operational parameters in response to anything you wish ... the question is off topic here because it asks for an opinion ..... 2. because it is designed to work that way
    – jsotola
    Jul 1, 2023 at 2:00
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    Vast energy savings by running the compressor only fast enough to suit the cooling load, instead of always running to suit the peak cooling load and shutting off a lot when the load is not peak. It's hardly a new concept at this point in time.
    – Ecnerwal
    Jul 1, 2023 at 2:07
  • Variable speed AC unit costs about 2k to $4k more than regular AC. The achieved savings on electricity are about $200 per year. So it will take you 10 to 20 years to recover the cost.
    – Traveler
    Jul 1, 2023 at 3:36
  • @Ruskes - It also keeps the temperature more consistent. Instead of switching the system on and off causing continual small swings above above and below the setpoint, the system settles into a speed which holds the temperature closer to setpoint and makes small speed adjustments as needed. I find it much more comfortable with varible speed than any constant speed system I've had.
    – Mark
    Jul 1, 2023 at 14:50

3 Answers 3

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I've had the same question, and I found this article by the Northeast Energy Alliance that discusses options for communicating thermostats for variable speed inverter type heat pumps. The takeaway is that while most thermostats can control heat pumps, to actually get the rewards of the inverter design they recommend using the OEM proprietary thermostat for the unit. This is because there is no official communication standard/protocol for inverter heat pumps and their thermostats. Thus each brand is unique.

you can download the pdf here, but they require you typing in name and email address. https://neea.org/resources/variable-speed-heat-pump-smart-thermostat-findings

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The controller will operate most efficiently by matching the capacity of the unit to the thermal load of the space being heated or cooled. For this purpose, it is useful to know, where is the set point, what is the current temperature, and how fast is it changing.

A conventional thermostat can only communicate "temperature below set point, temperature above set point". In that case, the controller cannot determine how much heating (or cooling) capacity you need.

The AC controller might support "2 stage" heat and cooling signals from the thermostat, if so it will run at lower capacity when only first stage in called for.

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I can only really speak to Question 1 (which illustrates why "one question per question" is a good idea for this site.)

I have inverter-drive variable speed heat pumps (mini-splits.) For those, the actual thermostat is built into the indoor head - the remote control (or "smart" enabled remote control replacement - which I don't have or want) merely talks to the head to set its mode and temperature - temperature is not measured or controlled anywhere but at the head. This can be an adjustment for folks used to a thermostat remote from the heating/cooling unit, but it was not a big adjustment for me to get used to.

So to change the head settings, whatever is doing that needs to communicate with the head. All the actual varying speeds with load is done without external input, between the head and the outside unit.

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