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Community and Q&A

HVAC System for Low-Load Home

HartiganWill | Posted in Mechanicals on

I’m performing a Manual J/S for the first time using Coolcalc and running into some issues now with device selection.

I’m in Arizona (climate zone 2B) and the SHR came out at 0.94 with a total cooling load of 28k BTU. If I’m understanding the principles correctly that makes sense because RH is low and the load is mostly sensible cooling temperature differential. Manual S and CoolCalc are telling me I need ~1350-1400 CFM for this application but when looking through equipment selections I’m struggling to find anything with a CFM/Ton ratio that high.

I’m primarily poking around at the high-efficiency Minisplit VRF systems (full air handlers) with Mitsubishi, Daiken, Carrier, etc., and CFM for 2.5-3 ton units is generally in the 700-1100 range.

How do folks size a system for this or am I doing something wrong? Splitting into multiple HVAC systems doesn’t seem to help much as the CFM/Ton ratio stays consistent.

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  1. Expert Member
    Akos | | #1

    Don't worry about the CFM from coolcalc.

    The CFM/ton is fixed for mini splits, even the ducted air handlers. Use the CFM/ton of your selected unit for sizing your ducts.

    If you are looking to change the SHR, you can always bump up the pressure setting on the air handler (usually programmable with the remote), most can provide well above their rated air flow rate as long as you can keep the pressure low. Check the air handler's data sheet for blower flow curves.

    1. HartiganWill | | #2

      I thought the SHR is a function of sensible cooling and total cooling? How does changing the air handler pressure change this?

      Perhaps I do not understand what the Manual S CFM is prescribed for? I thought that's what is needed in order to distribute the temperature change and or hit RH targets. Am I allowed to be that far off the mark on CFM from Manual S?

      1. Expert Member
        Akos | | #4

        Increasing the air handler pressure setting will bump the speed on the blower, so if your pressure loss is low, you'll get a bit of extra flow. For example your PVA-A36 is rated for 1100CFM on high at 0.8". If you reduce the duct loss to 0.65" but keep the pressure setting at 0.8" you can get 1300CFM. I've done the opposite of reducing the pressure setting to reduce the airflow and increase humidity removal.

        Since you don't need much humidity removal, getting higher airflow will generally improve the efficiency of the system.

        P.S. I would look at the at the M series SVZ/SUZ combination. Much cheaper than the P series which is a commercial unit.

  2. HartiganWill | | #3

    I'll give another update/example here.

    I worked this system in the Manual S speed sheet assuming a required cooling capacity of 28k BTU and sensible 26.5k BTU. By oversizing with a Mitsubishi PVA-A36AA7 (3Ton) unit I get a 116% total capacity and 110% sensible capacity which seems reasonable. However, latent capacity is 245% with a 150% limit.... This approach is ignoring the CFM suggestion and going purely off capacity requirements.

    Does it sound like my sensible cooling requirements are going to need a slightly oversized system and potentially lead to over-dried air with so much latent capacity?

  3. walta100 | | #5

    For cooling oversizing can be a problem in humid locations in that the shorter run times may not remove enough moisture from the air and that does not seem like a problem in AZ. Sizing the unit perfectly might save a few kWh but that not many if we are talking about mini splits with a single head because they can run at such low speeds.

    Years ago, I took a refrigeration class 400 CFM per ton was the standard if you had less than that, things got ugly in that coils started dropping below freezing reducing the air flow more and more as the coil turned into an ice ball. Once the coil iced over liquid refrigerant could make its way back to the compressor. If the compressor tried to perform the impossible task of compressing a liquid they tended to self-destruct. Today systems have better sensors and can protect themselves. The computer running the system are programmed to move 400 CFMs per ton in order to remove as much moisture as possible because many more units are sold in swampy places than in the desert.

    In short I think you are better off oversized than under sized in your climate so long as you are not 300% or more over and use a single headed mini.


    1. HartiganWill | | #6

      Hi Walta,

      This makes sense! When you refer to 300% oversizing I assume you mean the latent capacity specifically, while keeping the sensible capacity on target?

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