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