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Heat pump fan speed settings

Steve Grinwis | Posted in Mechanicals on

Hey guys,

Climate Zone 6,  Southern Ontario.  My apologies if the below is too verbose.  Heat pumps excite me for reasons I have yet to quantify,  which was great for thermodynamics class in Uni,  but poor for brevity.

My nearly finished Net Zero house will have a Mitsubishi P-Series heat pump, with a multi-position air handler that feeds three different zones.  The zoning should make the building more comfortable because the main floor has a lot of south facing glass that can easily lead to floor by floor temperature differences with a single zone solution.

They used an AirZone zoning system that knows how to control the Mitsubishi unit by setting a fan speed (1 – 5), and a target temperature for the air handler’s return air temperature.  This controls the speed of the compressor.

The AirZone system is smart enough to increase the fan speed as more zones call for heat and has three self explanatory Modes:  Quiet,  Normal, and Powerful.

If my goal is optimal energy efficiency,  do I want the air handler pushing as much air as possible at all times?  This in theory lowers dT across the coil,  provides better heat transfer from the refrigerant, and lowers pressures on the compressor,  but you do spend some more energy on pushing air around.  Do I perhaps more slower air speeds in the summer for dehumidifcation purposes?  Or perhaps will this not make much of a difference, no matter what I do?

Looking here:

https://www.nrel.gov/docs/fy11osti/52175.pdf

I can see that for A/C on minisplits I definitely want low fan speeds,  but heating looks like a mixed bag.   Anyone have any better data?

–Steve

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    A PVA series air handler doesn't have the same turn down ratio of their wall-coil type mini-splits, but it's still at least 2:1.

    It will use less overall power if the air handler is running at lower speed, and the compressor is modulating it's output to keep the coil temperature & heat/cool output at the optimal level. The duct design will play a large part in just how bad the additional power consumption becomes when running at high speed, but it's not a linear function.

  2. Steve Grinwis | | #2

    There are engineer stamped drawings for the duct work, which is a requirement for whatever Canadian net zero certification program they're using, so the ductwork is decently large, and also very well sealed, every joint taped, etc...

    Also playing into this is the fact that the minimum output of the heat pump IS my design condition heat load, which is making me think that "quiet" might be what I really want anyways. I want the compressor to be just barely ticking over almost all the time...

    It's probably not going to modulate an awful lot except for the coldest day of the year, but it should also be more comfortable than the massively oversized single staged furnaces that most people use.

    We'll see. It's a lot more efficient at its minimum output in stead state than it's max output...

  3. Expert Member
    Dana Dorsett | | #3

    Both maximum & minimum output of the heat pump drops quite a bit with outdoor temperature as temperatures drop below -5C. Those with vapor injection compressors do a lot better on both capacity and efficiency at temperatures of -10C and cooler.

    What is your outside design temperature, and which of the PVA air handler & compressor combination are you using? The smallest "HAxx" cold climate compressor is the 2.5 tonner, which has a lot more capacity at colder outdoor temps than the non-H "Axx" versions.

  4. Steve Grinwis | | #4

    It's the PVA-A30AA7 married up to the pUZ-HA30NHA5 outdoor unit.

    99% Design temperature is -18C.

    --Steve

  5. Expert Member
    Dana Dorsett | | #5

    The NEEP database doesn't have the PVA-A30 + PUZ-HA30 listed, but the PVA-A36 + PUZ-HA36 is. It has the same 18,000 BTU/hr minimum @ +47F as the PVA-A30 + PUZ-HA30.

    https://neep.org/sites/default/files/ColdClimateAir-SourceHeatPumpSpecificationProductListing-Updated9.15.18.xlsx

    The minimum output of the A36/HA36 @ +5F/-15C is 15,000 BTU/hr, so it's likely that the min-output of the A30/HA30 @ -15C is also 15,000 BTU/hr.

    At -25C the min-output is 13,000 BTU/hr, so at -18C it's probably still something between 14-14,500 BTU/hr.

    What is your heat load at -18C?

  6. Steve Grinwis | | #6

    *cough*...

    18,000 BTU...

  7. Expert Member
    Dana Dorsett | | #7

    With a design load of 18K and a minimum output of 15K it's definitely going to cycle quite a bit on zone calls, but the overall duty cycle should be pretty high. It would deliver higher efficiency if operated as a single zone, but that might not be reasonably possible. There may be some tweaking to do to get the minimum run cycle times up. Every time the compressor halts then spins up again a certain amount of energy is lost, and it takes several minutes of run time to approach it's steady-state efficiency.

  8. Steve Grinwis | | #8

    The AirZone system is fully communicating, so I believe it will do opportunistic zoning, i.e. Zone A calls for heat, and zone B is starting to cool off, so it'll satisfy A and B in the same call to get longer run times and fewer cycles. I'm also hoping the thermal mass of the concrete slab in the basement offers a bit of hysteresis. I paid for 2" of foam under it, but it looked like I got more like 4"... I won't complain...

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