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A/C sizing

I would like to ask when siizing a A/C system and the calculations determne that the required total capacity at .70 SHR is 4.6 tons, should the system bi sized at 5 tons or drop it to 4 tons. 

Thank you,



  • meplumbermeplumber Member Posts: 678
    Depends on the applicaiton.

    I have dropped the tonnage when I was doing mostly dehumidification with the unit.  And I have up'd the tonnage when I was concerned about straight cooling.

    I guess I would use building usage to determine my size.  Here on the coast of Maine, I usually drop the tonnage and allow the ahu to run longer, since dehumidification is more appropriate than straight space cooling.  When I was in GA, years ago, I would have tended to go with the 5 ton.

    Good Luck.
  • TechmanTechman Member Posts: 2,144
    edited May 2011
    Which way ?

    Why not go for the 5t ,but ,make it two stage .100% or 67%! .70SHR?
  • Brad WhiteBrad White Member Posts: 2,393
    edited May 2011
    The Range

    This is forever the question. I agree with meplumber to lower the tonnage and favor "running long to dehumidify". This is appropriate for most comfort applications. 

    I also agree with Techman, assuming that your bid can reflect the added cost without penalty, to go to a 2-stage or even variable approach; basically buying the higher tonnage and staging it by the system control or adding a Rawal APR valve. (I really like those). Because either of these will raise your cost, you may offer them as an Alternate and sell the benefits.

    Questions for you, Tim:

    1. Is the application comfort AC for a residence?

    2. Is there a dominant short term sensible gain peak, such as large east-facing windows which may see the higher gain in the AM when it is cooler out? That may favor dropping the size and letting her run long in the afternoon.

    3. What is your confidence level in the calculations? Was it an automated program taking into account the moving sun/rolling peaks? Or was it a manual calculation?

    4. Are the design space conditions (the capacity of the system at which point it is rated), is that at ARI conditions of 80/67 entering the coil (ugh!), or is it corrected for say, 75/63 which is more comfortable? That too can drive the selection at 5 tons to actually perform at 4 tons.

    Just to help you sort this out. Could make the difference.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Tim_5Tim_5 Member Posts: 37
    A/C Sizing

    Thank you for all the responses. I am not sure of the ARI, that is not lsted as ARI on the design Information, it was sized with Wrightsoft from our local HVAC supplier, It is for a home in the Chicago area and the reason i ask is we mostly use it for dehumidification, thinking if the system runs longer it may actually work better at removing humidity. The biggest heat gain i think will be from the West in the afternoon because it is on a lake and there is nothing to block the sun in the afternoon, however because it is on the lake we almost always heve a nice breeze. In the past the only time of the year we actually need AC is in late July and August when it actually gets warm in this area of the country. Because of the usage only a couple of months at best, but i don't want to short change myself either for over the long term is not a tremendous amount in purchase price but operating costs are the trade off sometimes.               

    Thank you,

  • Brad WhiteBrad White Member Posts: 2,393
    edited May 2011
    I would ask

    the person who did the calculations for the back-up, were they "heavy" on glass areas, for example.

    To give you one instance, many people who do calculations, especially if done as a loss-leader service, may "round up" the measured areas, to both save time (time = $) and so that no one can say the system is under-sized.

    I am not accusing, just suggesting from my experience. 

    Here is where a "rounding up" can over-size a cooling system: Take that west-facing glass you mentioned. One might take the rough opening (RO) size of the window to be conservative. But the glass area, the part responsible for the immediate solar gains way ahead of the frame and trim, might be 80% of that area. That is a 25% over-sizing on what would drive the cooling load for that room.

    The other thing I would ask for is the true orientation. A shade more of "north" as in "west-northwest" can save you some solar gains more than "west" or "west-southwest". It is good to be precise here.  Also consider shading.

    Basically, I am of the mind to have you go with the smaller unit absent better information, but want to gather assurance that the sensible gain is more likely to be met. I hope that makes sense.

    EDIT- Oh, ARI, that is just a rating point for industry-comparable capacities. The evaporator should really be selected for the design conditions you will maintain in the space. It is worth asking to see the calculation outputs. If you want to send me a set, off-line, I will be happy to take a look for you, render an opinion if you like.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • TechmanTechman Member Posts: 2,144
    Hold on a sec ,please,

    What is that .70SRH ?
  • Brad WhiteBrad White Member Posts: 2,393
    SHR = Sensible Heat Ratio

    The percentage of total capacity which is dedicated (at the stated operating point) to reducing the temperature of the air but without removing humidity. The remainder is latent capacity which of course is the moisture removal portion.

    So a 5-ton (60,000 BTUH) system with a 0.70 SHR will have a sensible capacity of 42,000 BTUH (0.70 x 60,000).

    The "SHR" slides a lot on a given system, depending on the incoming air to the evaporator. A muggier condition will have a lower SHR because more moisture is there for removal. In dryer conditions, a higher SHR. Think of it as a sliding bar dividing the total capacity.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • John Mills_5John Mills_5 Member Posts: 935

    That SHRs are usually given under very high load conditions. 95° out, 80 db and 67 wb inside. Since our inside conditions are usually lower than that, say 75 db and 59 wb, that .7 SHR is probably lots higher. So before you size a machine critically, see if you can find extended ratings that show the sensible & latent capacities under more reasonable conditions. A/Cs adapt to the load. So a working machine with a cool, dry inside may have a considerably higher sensible than the 42K would have you believe.

    With a 4.6 ton load, doesn't sound to me like 4 tons is close even.
  • Tim_5Tim_5 Member Posts: 37
    A/C Sizing

    The outside db is 97 and the inside db is 75 with a relative humidity of 50%. I am not a technical engineer but the db's seem way off, i don't think we ever get that warm here in Chicago except maybe once a year if were lucky. The relative humidity though may be on the low side.



  • Brad WhiteBrad White Member Posts: 2,393
    edited May 2011
    The outside air DB temperature

    is often stated as 95F and that corresponds to what the condensing unit sees as incoming airflow. This is normally not a problem and if on a black roof, I may go for 105F and de-rate the unit capacity a bit.

    I have attached two Chicago area weather packets (USAF data), which both show for O'Hare and Midway a 1% occurrence of 89 DB and 73F mean coincident wet bulb (MCWB) condition. The 2% occurrence for both locations is 86F with a 72F MCWB.

    EDIT: Heck they are PDF files but they are not "sticking". Write me off line and I will send them to you. B.

    In point of fact the outside temperature means less regarding heat gains than most people think. Glass in sun is the big dog. Insulated walls and roofs in sun do pick up heat during the day, sure. But they release it later in the day or over-night as it works its way to the cooler interior. Time delay you see.  So do not sweat, so to speak, the dry bulb temperatures.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Tim_75Tim_75 Member Posts: 44

    design conditions for Chicago (0.4%) is 91.7 degF db and 74.9 MCWB.

    For those that don't know, what that means is that the high temperature, as read on a thermometer is 91.7 degrees Farenheit, exceeded only 0.4% of the time. The mean coincident wet bulb temperature is the temperature as read via a sling psychrometer, averaged at the same time the high number (db) was recorded.

    As a design engineeer working in northern IL (Rockford) I used 91/75 for design conditions. Some used 95/75. More of a dehumidifying climate than not.

    Not to be used in lieu of calculating real numbers, but a "gut check" is square feet/ton. In a single family home in N.IL with average construction the number will be 600 to 700 sf/ton. If it is WAY off, check the calculation.
  • Brad WhiteBrad White Member Posts: 2,393

    -and that is a qualified statement- comfort cooling is designed for 1% sometimes 2% conditions for most residences and commercial applications. The 0.4% is used for businesses, some medical facilities.  I would not consider that appropriate for a house. You probably would not either! For hospitals we used 95/75 in our area, pretty much same as yours. If we ever see a 78F wet bulb, it would be in the mid 80's dry bulb, so the total enthalpy is similar than at 95/75.

    NIH type labs might use something more stringent, but that is because they use a lot of outside air. (There is no "diversity" -allowance in variable demand- when using a lot of outside air. It trumps solar gains in a walk.)

    The way we use which design conditions varies too.

    We use a higher design wet bulb temperature (not the mean coincident wet bulb temperature) when selecting cooling towers or if there is a lot of outside air. 

    We would use 100 year record temperatures with mission-critical facilities.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • meplumbermeplumber Member Posts: 678
    Since you mentioned enthaply Brad....

    I am going to throw out a question that is a little of track.

    I have been under the impression and observation that reverse cycle heat pumps are limited to operation above 7 btu/lb enthalpy.  So, in the northeast, where those January and Febuary days get cold and dry, it is semi-impossible to wring heat thru reverse refrig cycle out of air that cold and dry.  Am I way off base?  I cannot remember where that 7 btu/lb number came from.  But it matches what I am seeing real world.

    Care to elaborate from an engineering standpoint?
  • Brad WhiteBrad White Member Posts: 2,393
    edited May 2011
    I had to ask around, frankly

    My understanding was pretty much the same as yours regarding the ability of a refrigerant to transfer useful heat at low temperatures. The ability to "cool already cold air" being a limitation.

    I contacted my representative on the VRV type of split systems and was advised that it is less a function of the refrigerant and more a function of the compressors, specifically variable speed ones. We do not do a lot of residential nor air-source heat pumps, so a good time to learn anew what is out there. The brand he represents is Mitsubishi, by the way.

    Here is what he had to say:

    "The refrigerant doesn't

    have as much to do with the ability to heat at low temps as the compressor

    does.  The compressors are designed to be "over sped". The

    frequency can vary from 15 - 120Hz. 

    Our standard units are rated to -4

    deg F, where they get roughly 60% of the full load heating capacity. 

    Definitely a big improvement over traditional heat pumps with single speed


    We also have a high heat unit called the Hyper Heat (6, 8, 12 & 16 tons).  You can

    continue to over speed a compressor to get more heat out of it but there is a

    point where the compressor will burn out.  This unit has a secondary

    refrigerant circuit that is used only to cool the compressor and keep it from

    burning up.  This unit allows you to get 100% capacity at 0 deg F, and is

    rated down to -13 deg F where you still get about 83% heating capacity. 

    This is a very big improvement over traditional heat pumps.  (Most people

    don't believe this is possible the first time I tell them.)"

    So, that is what I learned recently. "Different than we thought we knew". All good.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • meplumbermeplumber Member Posts: 678
    Thanks Brad.

    We have a company up here manufacturing heat pumps that are supposed to work at design conditions (-10).  I have seen a handful installed.  Not performing as advertised.  I have a large number of Fujitsu mini splits running in commercial office buildings.  They work excellent.  We use the radiant slab as primary heat and use the heat pumps in shoulder season.  The heat pumps stop providing good comfort heating down around 5*.  But hey, that is what the radiant is for.

    Thanks for the info.  I learned something new again today.  Amazing how that happens everyday even after all these years.
  • Brad WhiteBrad White Member Posts: 2,393
    Always my pleasure.

    I like your "radiant as base" approach and am impressed that you get useful heat pump heating performance as low as you do. Shoulder seasons add up to huge hours at mild but heat-worthy temperatures, so it is a wonderful thing to be able to take the chill off with a low dose.

    It is a new world out there and it always fascinates me to challenge what I thought I knew and to dig in to the science behind it. Keeps me young!  :)

    Where in Maine are you by the way? I have been meaning to take a road trip to met NRT Rob and you, some time soon. I am a grandson of Maine but that does not give me any birthright, no more than a cat having kittens in the oven makes them biscuits. Ayuh.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • NRT_RobNRT_Rob Member Posts: 1,009
    you mean

    it doesn't slow down as you get older? I'm going to need a second head to store this stuff as time goes on... now I need to go study enthalpy some more just to understand what you guys are talking about.

    to think I thought radiant was "design intensive"... compared to AC, it's a breeze!

    noting your commentary on heat pumps I do have to note that the altherma in our shop does seem to provide 50% capacity at -4 deg F at a COP of about 2... going up from there as temp rises. Here in the Augusta region, that's our design temp and it is working very well for us as primary heat year round and cooling all summer as well, on our 33kbTU/hr design load shop.

    I am sad that other MFG never got its act together, but the true cold climate options are here and are available.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • meplumbermeplumber Member Posts: 678
    I am on MDI.

    I am not a native Mainer either.  No matter how long I live here, I will always be from away.

    It is funny how fast things have changed in the last 15 yrs.  they seem to have changed faster since say 1998 than they did in the previous 10 yrs or so.  When I started in the trade in the late 80's, you only had to have a strong back and a couple of pipe wrenches.  Now, I spend more time in class and use my laptop to interface with BAS controls in houses.  15 yrs ago, these types of systems were confined to institution and large commercial.  Now we are seeing them in high end residencial and small commercial.

    Where are we headed next.

    If you guys ever get up this way, let me know.
  • Eugene Silberstein_2Eugene Silberstein_2 Member Posts: 349
    As was mentioned...

    As was mentioned by a number of others, the answer is... "It Depends". A number of factors will need to be addressed, including the temperature at which the occupant will want to keep the structure. Heat gain calculations typically size systems for a 75 degree indoor air temperature. If the homeowner has his/her heart set on hanging sides of beef in the living room, I would definitely opt to go with the larger system. Otherwise, go with the 4-ton.

    In addition, systems are designed for specific outdoor temperatures based on geography, so the percentage of time that the actual temperature will be above that temperature is relatively low. For example, in the early morning or late at night, the outdoor temperature will be below the design temperature, so the smaller system will be sufficient.

    If it were my house and my system, I would go with the smaller system, assuming that all inputs in the calculations were accurate.
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