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  1. #1
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    Confused Inverter heat pump modulation control techniques -- efficiency/performance impact?

    First off, I am new here, so please let me know if there are any corrections I need to make to this post. Second, this question is about control approaches, not manufacturers, so please don't give me any specific make/model/manufacturer recommendations, as I have researched from AAON to Whirlpool to come to the conclusion that is this question.


    Now, to the meat of things. I am certain that most of the folks here are familiar with the fact that most higher-end residential split system heat-pumps (and all mini-splits) use an inverter (variable frequency) drive to accomplish compressor modulation. However, there seem to be two different approaches taken to control said inverters, and I am not sure which is superior.

    Bosch and Nortek (Frigidaire/Whirlpool) outdoor units take the simple way out, using conventional control inputs from a 24VAC thermostat, either single or two stage, and parameters sensed at the outdoor unit (I presume, although please let me know if I am wrong ) to determine what speed to run the compressor at. The other manufacturers, however, appear to rely on the thermostat to tell the outdoor unit how fast to run via a proprietary communications link, although some seem to support the "self-modulated" operation mode that the Bosch and Nortek units use as a fallback in case the proper thermostat cannot be used. Furthermore, mini-splits apparently take this further, having the remote communicate a set-point and room temperature to the main control system (presumably in the outdoor unit) which then uses that information to compute the appropriate compressor speed.

    Am I correct about the way communicating inverter split systems and mini-splits operate with regard to compressor controls? (If not, please correct me!) Further more, is either approach significantly more efficient/performant than the other, and if so, which is the superior approach?

  2. #2
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    Thread Starter
    ...did I send a question sailing over the heads of this entire forum? Would this be more appropriate in the AOP Commercial forum given the depth of the question?

  3. #3
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    Bosch uses a single stage stat and sensors and pressure transducers to decide modulation. The 19 SEER Nortek does use a 2 stage stat and it is a 5 stage machine.

  4. #4
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    Thanks for the confirmation on how the self-modulated systems work, at least. However, should I expect one of these Bosch or Nortek units to modulate as well as a setup based on a communicating control?

  5. #5
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    The communicating units are better, they have more finite control over the compressor and fan speeds, Allowing for better dehumidification and comfort control. These can also be adjusted at the t-stat to allow more finite control based on the part of the world the equipment is installed in. Say your in a more humid area, you can tell the unit to cool less aggressively so you get longer runtimes (more dehumidification), and to stage up faster in case of higher sudden heat loads to recover in a reasonable amount of time. Where as in a dry humid area you might tell the compressor to cool more quickly and stage up slower in response rapid heat gains, because higher staging wont be needed to recover the temperature in a reasonable amount of time.

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  7. #6
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    The difference is whether the entire system is controlled by the thermostat...
    Or the thermostat is one level of control... while the rest of the system attempts to tag along.

    As noted in the previous post...
    The integrated systems (which cost more) DO provide better results.

    Most of the better quality mini-splits (both single head and multi-head) work the same way as the integrated inverter systems.

    Beyond this... we cannot go into details in the open sections of the forum.
    If you are in the heating and AC business... get your PRO status and we can explain things in the PRO forums.
    GA-HVAC-Tech

    Your comfort, Your way, Everyday!

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  9. #7
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    Thank you, this seems to confirm my suspicions. It looks like my options are limited to:
    1. Putting up with vendor lock-in on a communicating split system or multi-split (something I would much rather not do)
    2. Putting up with the about 1-2 SEER efficiency hit, as well as the performance penalties, of a modulating system with staged control and self-modulation vs. a communicating system (the efficiency hit probably isn't that huge of a deal in a high-performance envelope, at least)
    3. Finding a way to do open-architecture command modulation (it appears that the AAON CB series may be able to do it in theory, but it'd take an awful lot of controls integration work to handle things like defrost and low ambient, and the docs needed for that aren't exactly readily available -- I'd also be stuck with unloader modulation and its associated limitations, but those aren't too bad in the grand scheme of things)

  10. #8
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    While there are efficiency gains with modulating equipment, the major benefit is comfort control more so than anything else. Assuming your not zoning that is. There is currently no open architecture modulating equipment, at least not supported by the factory. Your right to say it's possible but would it be worth it? Whats wrong with picking a specific vender and using their communicating equipment? What are you trying to do?

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  12. #9
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    I work on a lot of Carrier 25VNA and 24VNA9 (Heat pump and Straight cooling inverter). (Infinity systems). They can be run using conventional thermostat wiring but you loose a lot of functions that you have when using the Infinity Touch/Control. With conventional wiring, and TSTAT, you donít use all 5 stages on the Inverter units that would be available when the Infinity Touch is used as the control. I would never recommend buying such a complex unit when you can access all of the functions that the infinity touch gives you to dave a few hundred dollars. The price to upgrade later would be much more due to all the rewiring and purchase of the Infinity Control separately. (Also lose of manufacturers rebates)

    Charging the system with the Infinity touch calculates the total charge that needs to be weighted in according to line set length and suction line diameter, and Evaporator coil size/model. The control will Calculates your target Subcooling to check after adding the total calculate charge and give you a count down while the system is stabilizing before making any adjustment to the charge. The unit will also not allow you to charge he system if the indoor and/or outdoor temps are not within the proper/acceptable range preventing those who refuse to read the installation manual from doing more damage then they already have done. The system doesnít stage up and down like it would with the infinity touch in control. You lose the logging function if malfunction occurs with time/date/# of events and the dehumidification feature doesnít work as well.
    Iím not sure if you can control the 3į overcooling option when using dehum mode that is adjustable on the infinity touch but not with a multistage thermidistat.

    I enjoy working on the more complex electronic controls and communication function between all the equipment and components. You also lose the WiFiís ability to contact the installing contractor when a malfunction occurs and when software updates are released in hay are downloaded automatically to improve upon the systems performance and remove any bugs that might been in the previous software version. This is great because we can schedule a service call ,rather then an emergency call, before getting a frantic call from the customer that their system isnít working.
    Also, the ability to remotely log into a customers system, with their approval, to check settings, make adjustments, or change/investigate an issue the customer may have concerns. This is especially important when a customer is upgrading from a older simplistic A/C system and the new system operation can sometimes concerns them because of the longer run times on lower stages to prevent temperature swings and increase comfort levels as well as lower humidity levels which makes it feel much cooler than it did on their previous HVAC system.

    At a recent dealer meeting the were telling use about some future equipment thatís coming out that will tell the contractor the exact part needed when their is a failure and it will also have the ability to tell you when time/date a part is due to fail. Th reason for all of this is becAuse of the serious lack of qualified technicians and the bleak future of filling the positions of a lot of techs that are getting ready to retire in h near future with not enough qualified techs to take their place.

    I blame it on the younger generations work ethics and thinking itís a 9 to 5 job. No thought taken into account about continuing education and that their job is a career which requires work to be done when they arenít at work.

    Iím talking about individual that arenít members of a forum like this.

    Damn. I didnít mean to write that much...my apologies.

    Cheers!


    Sent from my iPhone using Tapatalk

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