Proper Air Conditioning Sizing


I recently received this question from a homeowner who came upon the blog site:

Mike
I read some of your articles online and am asking for your opinion.
I replaced my antiquated system with a new system. I have a 1250 sq. ft. House. I was quoted for a two ton system to properly remove humidity, however was also told that they could only guarantee a twenty degree difference (100 degrees out and only cool to 80 degrees). I live in NJ.
I had a R22 for over twenty years. It worked great. I opted for an American Standard 2.5 ton single stage condenser with 3 ton coil. The furnace is an 80K two stage variable speed. I am experiencing 50-60 % interior humidity and sticking to my leather couch. My condenser averages 6-8 minutes on and 10-12 minutes off. To get comfortable I have to keep lowering the thermostat to chilling levels. Hindsight is 20/20. Can I tweak my new system to run longer to properly dehumidify. Can a two ton condenser  work with a three to coil ? I’m currently using a portable 70 pint plug-in dehumidifier which seems to run non stop. What % of humidity should a house be to truly be comfortable ?
Please advise.  Your help will be greatly appreciated.

The contractor who quoted the system did his homework and followed correct procedures in quoting the 2 ton system for a properly sized unit. However, in order to make the sale, he let the end user choose what he wanted instead of what was correct. Now the homeowner has a problem.  Here is what I responded:

Oversized A/C units will always be a problem when it comes to humidity control.  I always taught to slightly undersize units for the best humidity control. Ideally, an A/C should maintain about 35-40% indoor humidity levels.  The 20 degree temperature drop is an industry standard based on local design conditions.  In Chicago we size for 95 outside and 75 inside. But you need to remember that determines the size.  How often does it make 95? –Rarely , so right off the bat, all A/C units are oversized for much of the cooling season. Now, when you oversize the already oversized A/C, you have humidity control problems.

Short of replacing the outdoor unit, you could try slowing the blower down allowing the air to stay on the coil longer.  Most systems can go down to 350 cfm/ton of cooling so you could go from 1000 cfm where you are now down to 875 cfm and see if that helps.

The bottom line is — Too big or too small is a problem for your heating and air conditioning system. It needs to be sized correctly!

We all have used some rule of thumb before but this does not and should not be applied when it comes to sizing a heating and air conditioning system. This is a science and not a rule and should be treated as such. Make sure you complete a load calculation on the home so you have the right size equipment for your customer’s needs.

A load calculation is an analysis involving the current home structure such as: orientation of the home, window efficiency, the type of doors, building materials, insulation in the attic, insulation in the walls, roof color and where your duct work for your HVAC is located. All of these factors and more determine the correct size of the equipment by giving you the heat loss and heat gain of the space. From there, you can properly size the equipment that is needed. If the system is too big, then it will short cycle and not provide good humidity control and cause wear and tear quickly to your system. If your equipment is too small, it will never keep you comfortable and it will run consistently trying to keep the temperature of the thermostat setting.

Regardless, if you are replacing HVAC equipment or installing new, make sure a load analysis is being completed and install the correct size equipment .

The point I want to make is that A/C needs to be PROPERLY SIZED TO WORK RIGHT. There are no rules of thumbs. When it is not sized correctly, things like what happened to  the person asking the initial question are going to happen and you will have unhappy customers.

Just remember the acronym what I was taught that applied to everything I did and that is — D.I.R.T.Foo.T. (dirt foot) — DO IT RIGHT THE FIRST TIME.

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About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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11 Responses to Proper Air Conditioning Sizing

  1. rjp says:

    I might have offered the homeowner a two stage outside unit to go with his variable speed furnace and used a Honeywell IAQ thermostat with dehumidification control wired to the air handler. Unfortunately too many companies are afraid of apparent complexity and probably don’t understand the options well enough to give a buyer high btu capability with lower capacity and quieter operations most of the time.

    • I agree with the 2-stage option. Dehumidification would be fine with the 2-stage but not with what he has. He could add a whole house dehumidifier and then use the IAQ stat option. With what he has, all it would do is make it colder like he is doing now Thanks for the comment

  2. RCF says:

    Gentlemen, the contractor oversized both the cooling and heating. The homeowner will be uncomfortable as long as this system is in the home. And we wonder why our industry gets bad marks.

    • I agree. I had to deal with things like this as a manufacturer’s rep for over 24 years and it was always the “equipment’s fault” -no.t the contractor. If you noticed in the quoted email — never did this homeowner blame the installer.
      Thanks for the comment.

  3. Dean Shetter says:

    one idea that can help the dehumidication situation as brought, Trane/American standard does have a reasonably priced dehumidication control that can be wired into the control board, it’s recommended to be used only on variable speed blower asmy, I’ve used this on the same situation that has been called “bigger the better” a/c & h/p installed by others

  4. Chip McClain says:

    Would an adjustable setpoint thermastat help with humidity?

    • no — because a thermostat controls to temperature and we want to control humidity. thermostat would get rid of humidity but at the cost of longer run time and getting colder (uncomfortable) in the space

  5. Gino says:

    I agree perfectly well that a system needs to be precisely sized correctly in either for the system to run efficiently for a long time. However, in my humble opinion, it isn’t always the case. Especially, when you reside in an area like Montreal, Canada; many other factors need to be taken into consideration. Typically, the winter season here is very long and bitterly cold. In Montreal, Canada; we generally start heating in the month of September until late April. We usually have temperatures that range from 10°F and occasionally really cold down to -25°F in January and February months. Therefore, AC is really only used less than a couple of months during the summer and having that extra 1/2 ton is really not so much of a big deal. Especially, when your home is built in the 60s. But it indeed does mske a huge difference in the winter months, where you will get more btu/hr in the heating mode and higher savings on your electrical Bill. In our area, electrical is the main source of power, and when the auxiliary heat comes on; so goes the dollar bills out the window. This is why an efficient and reliable heat pump with a good HSPF rating is more of concern than the SEER rating.

    • Gino — very valid point. A contractor needs to know his marketplace, as in your case, heating is used much more than cooling and with a load calculation and design criteria for your area being -25 degrees, “OVERSIZING” the heat pump makes a lot more sense and getting a good HSPF definitely is more important than SEER rating.
      Thank you for sharing this on the site.

      • Gino says:

        Hi there, I was looking at the York,Luxaire & Coleman websites regarding the THJF30S4S3 heat pump unit specifications. All three post a Rating of 9 HSPF when matched with a FC43C3XC1 indoor coil and integrated in a 2.5 ton air handler. However, there is no mention of what climate region is the 9 HSPF for. I’m in Canada, therefore the region is V (5). I know that other manufacturers commonly use a climate region of IV (4), which makes a difference when looking at the savings and ratings posted. I tried calling York directly, unfortunately I wasn’t able to get a clear answer on this. Any chance you can find out?… Knowing which climate region York is referring too, would indeed help to determine what is their true HSPF rating in my area. Typically, I would need to divide region IV by 1.15 to obtain the rating for region V. Thanks, Gino.

      • I am retired from york/JCI. Your local distributor in the Quebec province is Le Groupe Masters.(1-800-361-6805 or 514-527-2301) In Toronto/Ontario you can call York direct at the Oakville, ON (Phone:905-829-1411 Toll Free:800-463-2604) They should be able to assist you with your question.

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