Comments on: How to Tell If You Have an Oversized Air Conditioner

By: Scott

Scott — Sat, 27 Sep 2014 22:48:18 +0000

As an HVAC technician with 21
As an HVAC technician with 21 years of experience, I have to disagree with the concept of oversized systems causing high humidity. My home A/C system is 4 tons in the midwest. Manual J would probably call for a 2.5 or 3 ton system. Lazy, cheap contractor didn't do a load calc. It runs 5-7 minutes per "on" cycle on a warm day, never more than 10 minutes. Out design temp is 95 and it will cycle off on a 106 degree day. But It dehumidifies just fine. With a modern 13+ SEER system the evap temp will fall to 45 degrees within the first 30 seconds of running. A 45 degree evap will be collecting full moisture if the dewpoint of the entering air is 50 degrees or more. One caveat: the blower must cycle. With a continuous blower you will need 80% runtime or more to dehumidify most homes due to re-evaporation of moisture during the off cycle. I never recommend a continuous blower for that reason.

By: Charlie Conger

Charlie Conger — Wed, 03 Sep 2014 21:33:47 +0000

Those humidities seem well
Those humidities seem well controlled. We did something very similar in my sons house in Houston. He has a 3 ton AC in 2300 SQ ft new home. It had a sealed attic and the AC ran very little, even when set to 75 degrees. Reducing the ventilation rate on the external ventilation controller and adding a 70pt per day portable dehumidifier made a huge difference in humidity levels ( now about 45% ), comfort, and smell. The monthly bill went up about $20 but the comfort is much better and they don't freeze all the time. This was just an experiment to see how it worked. He will be installing a central dehumidifier which is at least 3 times as efficient as soon as he can afford it.
I am convinced that in humid climates dehumidifiers are almost always helpful. They allow independent control of temp and humidity. This is not the case for AC's except for the Top of the line models that allow much better control using better logic and variable speed blowers and compressors.

Of course all of this is a process and the house should be well insulated and tight BEFORE you do anything else.

BTW I have also had success by INSULATING the temp sensor inside my Honeywell thermostats with a short strip of weatherstrip ( foam tape). This works because most Honeywell thermostats try to control the temperature too tightly often to less than +/-1 degree. While this may be OK for moderate /dry climates it does not work well with oversized AC units that can change the inside temp 1 degree in less than 10 minutes. By insulating the temp sensor it takes longer for the thermostat to react. You can temporarily test this by building a foam box and putting it over the whole thermostat. Alternatively block all the air vents on the thermostat by sealing them with blue removable masking tape.

I would not recommend disassembling your thermostat unless you are very comfortable with such activities.

In the less hot months it made an even bigger difference in his house.

By: Ashok Aiyar

Ashok Aiyar — Fri, 22 Aug 2014 22:55:44 +0000

My 3-ton AC unit is way
My 3-ton AC unit is way oversized for our two-story 1600 sq ft town home in New Orleans. This unit was installed by the previous owner right after Hurricane Katrina. The 3-ton unit replaced a 2-ton unit that was probably closer to the required size.

During the average summer day, the average run time was 5 minutes (and even the 5 minutes was the default setting using a programmable Honeywell thermostat). The AC ran for about 4 hours a day, and even though the temperature was set at 75oF, it felt hot and muggy all the time. As is typical with short-cycling, the indoor relative humidity ranged from 60-68%. Typically, there were about 40-50 AC compressor starts every day.

All that changed after I read several of your (Allison Bailes) posts and a few articles by John Proctor. First, I replaced the Honeywell with an ecobee thermostat because it was simpler to program various installation settings such as the minimum compressor run time. Based on information from both of you, my compressor is now set to run for a minimum 20 minutes at a time. In addition, I have added a small 40-pint portable dehumidifier into the mix that is downstairs (the HVAC return register and thermostat are upstairs). The dehumidifier is set to maintain 50% RH. I also have Vornado fans circulating air throughout the house. After several weeks this summer, I now have the following averages

Sleep cycle (21:30-06:25), Set Temp = 80, Av. Temp = 77.6, %RH = 45.6
Wake cycle (06:30-09:25), Set Temp = 77, Av. Temp = 75.4, %RH = 47.3
Away cycle (09:30-17:25), Set Temp = 80, Av. Temp = 78.6, %RH = 48.3
Return cycle (17:30-21:25) Set Temp = 77, Av. Temp = 75.5, %RH = 47.5

The AC runs for an average of 5 hours a day, with 14-16 AC compressor starts.

The house feels very comfortable throughout the day - even during the Away cycle. With the fans, the perceived temperature is a few degrees lower, low enough that a comforter is needed at night.

Without the dehumidifier in the mix, the %RH ranges between 50-55%, so the tiny dehumidifier certainly helps - I have to empty the 9.5 pint bucket once a day. The dehumidifier consumes about 6 kWh per day (measured using a Kill-a-Watt).

So far, this "Rube-Goldberg" solution has worked well to compensate for an oversized compressor. I'm guessing the longer run times and fewer starts are probably better for life of the AC compressor as well.

Do you think this is a reasonable approach? What are the negatives to doing so (apart from the increased power consumption - however, oddly enough, my electric bills this year are about 15-20% LOWER than last summer - I cannot understand why)?

By: edda harbert

edda harbert — Tue, 19 Aug 2014 23:53:36 +0000

my air conditioning runs all
my air conditioning runs all day once a while kicks off at night. is it normal to stay on allday? WE HAVE IT SET AT 74

By: Allison Bailes

Allison Bailes — Tue, 19 Aug 2014 15:38:21 +0000

Chris M.:
Chris M.: If your HVAC contractor really wants to bring attic air into the return, then you need to find another contractor. Not only will that not solve your problem, your energy bills will go up. 55% RH isn't bad, but if you're regularly getting up north of 65%, (especially if you spend significant time over 70%) then yes, you may want to look into using a dehumidifier. It's too bad you just replaced the AC.

By: Chris Marriner

Chris Marriner — Tue, 19 Aug 2014 06:29:32 +0000

So, My AC system died last
So, My AC system died last spring, and I was prepping to put new windows (impact resistant, low e) in my house… My house is approx 2200 sq.ft in Florida, south of Tampa… I had a 4 ton system, and knew that would be too big… my HVAC tech replace it with what was here and said he could tune it… Now my house averages 55%rh or higher… Up into mid 66-67…

I talked to him about it and he wants to bring some attic air into the return so it will run longer… I think that adding a whole house dehumidifier, like the Williams air sponge, or April Aire would be the best solution… Any feedback??

By: Jessica

Jessica — Tue, 05 Aug 2014 03:36:04 +0000

I can't seem to figure out
I can't seem to figure out what's wrong with my AC situation. We have a 70 year old 1000 sq ft home and recently upgraded our AC unit to a 16 seer 2 stage from a 10 seer single stage. Both are 2 ton. The humidity level in my home will not go below 59%, even if I let the AC run for over an hour at a time. I have set the AC to 1 CPH for both 1st and 2nd stage, and I let the temperature get FREEZING in the house, but to no avail. What gives? Is it because we are a drafty house? Could it be that the attic stairs *next to the return register* are VERY poorly sealed? Please help! I don't know what to do to fix this and I want to do something fast before we get mold or mildew.

By: Bruce

Bruce — Thu, 05 Jun 2014 23:03:11 +0000

Can I adjust this formula
Can I adjust this formula down a degree or 2. In Houston our design temp is 94. If its going to be 93 tomorrow and I set the t-stat to 74 instead of 75 will I get the same results?
will this give a good run time comparison?
Thanks

By: Marie jean

Marie jean — Tue, 18 Mar 2014 11:22:28 +0000

I have a newly installed air
I have a newly installed air conditioning. I am experiencing mold, a/c line is clogged,and the coil is icy. What could possibly be happening.

By: Paul McGovern

Paul McGovern — Mon, 15 Nov 2010 17:12:52 +0000

Katherine ... is your system
Katherine ... is your system outside the building envelope? if so, the culprit could be duct & equipment leakage. Allison's demonstration of the effects of static pressure, using a ball and vacuum (see the Energy Vanguard website) was very enlightening. It shows that moving air has less static pressure than the air around it. When the moving air is confined by ducts, plenums, and equipment, it creates a vacuum of sorts and wants to suck the surrounding air in, thus encouraging system leakage. Sounds to me like the system might be sucking in more humid air than the coils can dehumidify. I am no HVAC expert & would like to hear from David Butler or others as this situation is not uncommon.