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Do Higher BTUs per CFM for Cooling Equal Better Dehumidification

aunsafe2015 | Posted in Mechanicals on

Is it correct to believe that, all else being equal, a system producing more cooling btus per cfm will probably be dehumidifying better than a system with fewer btus per cfm?

Suppose system 1 has a minimum operating capacity of 5400 btu and a minimum air handler output of 310 cfm, which equates to 17.4 btu/cfm.  Suppose system 2 has a minimum operating capacity of 9600 btu and also has a minimum air handler output of 310 cfm, equating to 31.0 btu/cfm.  I understand that if the cooling load is, for example, 5400 btu, system 2 may short cycle compared to system 1.  But at 17.4 btu/cfm, system 1 may not be removing much moisture from the air at all, whereas at 31.0 btu/cfm, it seems that system 2 would likely be doing at least some dehumidification while it is running, even if it is not running as long as system 1.

Any thoughts/insight appreciated!

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  1. joshdurston | | #1

    It depends, but generally yes.. It's true that a colder coil will dehumidify at a faster rate.
    But you can't necessarily determine that the coil temp will be colder based on the fan speed and min modulation alone. The modulation (btu rate) is not determined just by the load.

    Watching my FH09 I think the max coil temp in cooling is some where around 14C at low loads (which is too high to give control over humidity). Under higher loads it will drop into the upper single digits (C) range). Under low loads I've taken to setting a couple magazines on top of the air inlet while monitoring the discharge temp (to restrict the airflow) to get the discharge into the 10-12C range at min fan speed.

    This funny business could be eliminated if they simply would give me a setting to specify the max coil temp in cooling (I'd set it to 10-12C). I'd rather the equipment cycle more often with a lower coil temp, than run constantly with little latent capacity. Dehum mode comes close (it will lower the coil temp), and vary the fan speed to keep the space temp controlled, but it won't shut off completely if it overshoots. It just keeps running at min fan speed.

    There are several control loops. The load generally will determine the fan speed (in auto) and also the target coil temperature. The min modulation is simply the lowest modulation rate the compressor can go to, to achieve the coil temp target. And it may only be running on minimum if the entering air is quite cold. It's possible a unit with a low min BTU could out dehumidify a unit with a higher min if the control strategy keeps the coil colder.

    But they've been playing games with the coil temps to show incomparably high SEER ratings ( I think my FH09 is SEER 30). But if I factored my standalone dehumidifier it's probably closer to 20 in reality for a combined efficiency.

    They will only go to minimum BTU rate if the coil temp target it achieved. Some of the Mitsubishi multi splits have dipswitches on the outdoor unit to select the target coil temp in cooling mode.

  2. charlie_sullivan | | #2

    Up until the point where short cycling starts, it's true that the higher ratio has more dehumidification, but once that unit is short cycling, that might not be true, and in fact I'd say it's more likely that the one that is not short cycling has higher dehumidification.

    I like chilled water systems. You get to set the water temperature where you want it and set the fan speed where you want it.

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