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Mixed Heat Pump Application Performance Rating

BTUChaser | Posted in Expert Exchange Q&A on

The NEEP site is a great resource for providing heat pump system performance data (COP, HSPF and capacity at various OA temps).  This data is available for “matched” systems with specific AHRI numbers.  How can you get similar data for mixed heat pump systems?  For example, a single 36 MBH outdoor unit with a 24 MBH indoor AHU and (2) 9 MBH ductless units.  If such performance data is not available, is there a reasonable way to estimate the performance (e.g. look at the performance of the 36 MBH outdoor unit with its matched 36 MBH indoor AHU without the ductless units?

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  1. walta100 | | #1

    Consider not selecting a multi head system.

    Yes, single unit will cost a little more up front but add value over time.

    1 Redundancy, sooner or later your system will have a fault and when it happens the multi head system will be 100% off line and you ate living in crises mode and need to get it repaired today next week is not an option! If you have 3 single systems it is almost a non-event 2 weeks no problem.

    2 Efficiency, the single head units will slow to 10% of their rated output the multi head unit can only get to 30%. Since the multi head system is 300% larger and runs 300% faster at min speed it will be cycling on and off a lot more often loosing efficiency with every start up while id single just keeps humming along.

    3 Serviceability. The most common failure is a leak. The most common leak is at a field made joint. A 3 head system will have 400% more field joint. Identifying the leaky joint is a difficult task the more joints the more difficult the job becomes.

    If you do decide to go with a multi head system, be careful not to let the install oversize the system and put in too many heads. When we read posts about mini split systems with problem invariably the systems is an oversized multi head unit.

    At least ask for a quote for single systems you may be surprised at how small the price difference is.


  2. BTUChaser | | #2


    Thanks for the quick reply and helpful info. In this application, the contractor has actually proposed 1 unit serving an AHU plus 2 ductless units on the second floor and a second unit serving 3 ductless units on the first floor. So i assume it’s not really economically feasible to have a dedicated outdoor unit for each indoor unit. I’m still curious about my question of how to approximate performance with these mixed systems. Based on your comment, performance will be impaired due to the dynamics of oversized operation under a variety of outdoor conditions

  3. walta100 | | #3

    This is starting to sound bad.

    All to often the installers don’t really want to install mini splits so they write a bid 3X oversized bid with a head in every room complete with a I don’t want this job price tag. If you appear shocked by the price, they conveniently have a much lower bid for a more conventional system already written.

    I encourage you to read the archives on this site for people unhappy with mini splits and decide for yourself what most have in common.


    1. BTUChaser | | #4

      Walta...........I will do that........Thx

    2. BTUChaser | | #5 mentioned "the archives" on the exactly do you find them. Not obvious from the visible options.

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