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AC Sizing Question

james_talmage | Posted in Mechanicals on

I am remodeling a home in Southern Michigan. With the exception of  3 existing 1st floor brick walls, which we’ve taken down to the studs on the inside, on the 1st floor, it is basically new construction home. Zip sheathing, spray foam, etc. I expect it to be relatively tight.

Load calculations for cooling came back as follows:

Sensible: 24811 Structure + 4698 Ducts + 1461 ERV = 29518 Btuh total sensible (with .95 Rate/swing multiplier – I don’t know what that means).

Latent: 1994 Structure +2899 Ducts + 2206 ERV = 7099 Btuh total latent.

First question: Do those duct losses seem high? They are above the ceiling in an encapsulated attic.

Second question: Using the above numbers, the AC manufacturer is recommending a 4 ton unit. They are dividing the sensible load by 0.7 to arrive at that total load. I don’t fully understand why they are doing that. Are they assuming a 0.7 SHR, and ignoring the calculated SHR? 

Maybe my understanding of how this works is flawed: If a piece of equipment has a 0.7 SHR rating, that means it can remove up to 30% of the load as moisture. If the house has a higher SHR, does the unit need to be oversized? Once moisture is removed, won’t the remainder of the energy be spent reducing the sensible heat? Or is a piece of equipment only capable of removing a fixed amount of sensible and a fixed amount of latent?

I am sure one recommendation would be to find a piece of equipment that better matches the loads, but we are using a high-velocity system for the smaller ducts. So options are limited.


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  1. Jon_R | | #1

    > won’t the remainder of the energy be spent reducing the sensible heat?

    As I recall, Manual S allows 50% of unused latent capacity to be counted as sensible capacity.

    Be careful with SHR - with inverter compressors and variables fans, it often changes significantly with load, sometimes rising to near 1.0. This is likely to be much more of a negative than a little over-sizing.

    > Rate/swing multiplier
    Supposedly this should be turned off and capacity should be adjusted to account for any difference between rating conditions and your design conditions. Ie, a "4 ton" rated unit isn't 4 tons under your conditions.

  2. james_talmage | | #2

    > Be careful with SHR - with inverter compressors, it often changes significantly with load, sometimes rising to near 1.0. This is likely to be much more of a negative than a little over-sizing.

    Why is this? At low loads the evaporator isn't cold enough to condense? It's only a 2-stage, so probably not an issue, correct?

    1. Jon_R | | #3

      My guess is that the poor CFM/ton is to game the efficiency ratings. Perhaps not a factor in a two speed unit.

      1. james_talmage | | #4

        I guess I'm a little confused.

        If SHR rises with variable speed compressors, wouldn't that be even more of a reason not to oversize it? (Less modulating down == maintains rated SHR)

        1. Jon_R | | #5

          The point is to avoid such units altogether. Ie, given:

          a) a perfectly sized unit that has high SHR at lower than rated loads OR
          b) a moderately over-sized unit with a SHR that is nearly constant

          I'd take b).

          1. aunsafe2015 | | #6

            I would also take (b).

            For the record, the Trane XV18/20 series dehumidify very well all the way down to their lowest operating capacity (about 30% compressor speed). So such units do exist.

            Anybody else know any inverter-based units that dehumidify well even at lower operating capacities?

          2. james_talmage | | #7

            Is SHR at various modulations even listed? How do I find that. (Looking at Unico coils).

  3. Expert Member
    Akos | | #8

    If your ducts are in conditioned attic, than both sensible and latent losses are zero.

    4 ton AC is about right sized for a well sealed 6000sqft house, so if your place is less, I would get something better sized.

    You can always adjust SHR on a two stage unit by setting blower speeds at each stage.

    With the high summer time humidity around the great lakes, the best setup is set the low stage for humidity removal so around 350cfm/ton and the high stage for optimal cooling closer to 500cfm/ton. These are easy to set but you'll probably have to fight with the installer to get it done.

    P.S. Make sure to get an ECM blower for your air handler, these consume about 1/2 the power of a regular blower, tend to be much quieter and easier to adjust.

  4. Expert Member
    NICK KEENAN | | #9

    The actual SHR you get is not a property of the AC, rather it is dependent upon what the temperature and humidity of the incoming air and the temperature of the air coming out.

    Here's how SHR is useful: if you want to achieve a certain SHR, what air outflow temperature do you need to do that? Just for the sake of argument I'm going to take your numbers unadjusted, sensible of 29,518 and latent of 7,099. That gives a SHR of just under 81%.

    You don't say what interior temperature and humidity your Manual J assumes but let's say 75F and 50% RH. That air has a dew point of 56F. If you cool that air to 51F and 100%RH/ 51F dew point, that cooling process will remove sensible heat of 2,592 BTU/hr for every 100 CFM of airflow, and 690 BTU/hr of latent heat -- which gives a SHR of 80%. (I'm not showing my work on these calculations, you just need to trust me for a bit.) At 350 CFM the total cooling is 11,487.7 which is just about exactly the 350 CFM/ton that Akos recommends. (500 CFM per ton equates to an output temp of 54 and a SHR of 90%.)

    If the air flowing out is colder the SHR is lower, more drying takes place. If the air flowing out is warmer the SHR is higher. If the air flowing out is above the dewpoint of the incoming air -- 56F in this example -- no dehumidification takes place, there is no latent cooling, and the SHR is 100%.

    So where does the air temperature come from? Inside the AC there's a heat exchanger. The fan is bringing in warm, humid air, and the compressor is pumping in cold refrigerant. They meet in the heat exchanger, and the temperature there will settle where the heat lost by the air matches the heat carried away by the refrigerant. And the way you tune that is by adjusting the blower speed. That's the adjustment most available to the HVAC tech. But the system has to be reasonably sized to begin with.

    The best performance occurs when on the design day the compressor is running continuously, the blower is running continuously, and the air being produced is cold enough to meet both the total cooling load and the SHR. That's perfect sizing. If the compressor is undersized it will be hot and muggy. But if the compressor is oversized that's a problem too. An undersized compressor won't run continuously, it will short-cycle, turning on and off frequently. This leads to poor dehumidification, because any condensation that is on the coil at the end of a cycle evaporates rather than dripping away. With an oversized compressor you end up cold and clammy.

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