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  1. #1

    Humidity Spike After New A/C Unit Installed

    When we built the house, our original HVAC contractor undersized the furnace and A/C (they calculated for 2,000 square feet and the house is 4,000 square feet).

    The HVAC company blamed the builder, and builder blamed the HVAC company, and, well, you get the idea. It was going to cost more to pursue the claim than to simply live with it for awhile and upgrade at our own expense.

    So, I installed inexpensive supplemental heat and then when the fan motor went out on the furnace I simply had an efficient, properly sized unit installed. I also received a tax credit.

    The A/C finally went out, and though it was a minor repair, I decided to upgrade to a properly sized unit. I received 3 estimates, and chose the company that had installed the new furnace because their price was in the ballpark and they had done satisfactory work previously. The recommended unit was a 14 seer 3 ton Lennox. The old unit was only a 2.5 ton.

    The installers screwed up a few things, but to the HVAC company's credit, they were out the next day and resolved the issues at no extra charge. The new unit cools much better. The whole house feels better even at the same temp reading as when we had the old unit.

    The bad news is that I have had a humidity spike. While the basement, which was previously stuffy, now is very cool, the humidity spiked from 43% to 53%.

    Before I talk to the installers, I want to determine if this is normal and acceptable.

    I understand that a larger ton unit will typically run less, but it has been 100 degrees since the new unit was installed and it has been running much of the time, so the humidity spike shouldn't be because the unit is "oversized".

    I have a $$$$ Santa Fe Classic dehumidifier for the basement which runs full time. We also have a slightly musty smell in the basement, which we did not have when the humidity was lower.

    Any thoughts?
    Last edited by jpsmith1cm; 07-18-2012 at 07:42 AM. Reason: Pricing

  2. #2
    Join Date
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    50-55% are ideal conditions. That being said, did they upsize the system just because or was a load calc done to confirm that size?
    Quote Originally Posted by k-fridge View Post
    The laws of physics know no brand names.

  3. #3
    Join Date
    Jun 2005
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    SW Wisconsin
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    When U reduce compressor capacity to get higher SEER numbers, it can't pull the pressure & temperature of the indoor coil down to as low a temp at same conditions as the larger capacity compressors.

    Colder coils tend to condense more moisture...

    Is the indoor coil up-sized?

    Is the blower set for 350-CFM per Ton of cooling?

  4. #4
    Quote Originally Posted by udarrell View Post
    When U reduce compressor capacity to get higher SEER numbers, it can't pull the pressure & temperature of the indoor coil down to as low a temp at same conditions as the larger capacity compressors.

    Colder coils tend to condense more moisture...

    Is the indoor coil up-sized?

    Is the blower set for 350-CFM per Ton of cooling?
    Yes, they changed the interior coil to match the exterior unit, or at least they billed me for that. I didn't actually witness the installation.

    The blower is multi-speed and they billed me for whatever was required to coordinate the blower speeds with the exterior unit. I can tell you that the air flow from the vents is significantly stronger than before.

  5. #5
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    Quote Originally Posted by ThatOneGuy View Post
    Yes, they changed the interior coil to match the exterior unit, or at least they billed me for that. I didn't actually witness the installation.

    The blower is multi-speed and they billed me for whatever was required to coordinate the blower speeds with the exterior unit. I can tell you that the air flow from the vents is significantly stronger than before.
    During high cooling loads, the inside %RH is dependent on the cooling coil temperature. If you want 75^F with <50%RH, you need <50^F indoor cooling coil temperature. Reduce the air flow through the coil reduces the coil temp and removes more moisture. So get the coil cold enough to get the indoor %RH you want during high cooling loads. Operate the a/c blower in the "auto" mode to max the moisture removed by the a/c. The Santa Fe will maintain <50%RH in the basement.
    Keep us posted.
    If there is any question about the Santa Fe, catch the condensate from the Santa Fe for a hour or day.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  6. #6
    Join Date
    Jan 2009
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    Keokuk, IA
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    If the basement was stuffy before and is cooler now, the temperature may be lower, but the dewpoint the same so the RH is higher. At the cooler temps, the dehumidifier may have lower capacity because it goes into defrost more often.

    As mentioned above, the previous unit may have had a 2.5ton coil, while the new one possibly a 3.5 ton or 4 ton coil for a higher SEER rating and with the shorter run times, it is removing less humidity. Also, the newer 13 SEER coils have have more surface are and run higher coil temps than the previous 10 SEER coils.

  7. #7
    Quote Originally Posted by SoFlaDave View Post
    50-55% are ideal conditions. That being said, did they upsize the system just because or was a load calc done to confirm that size?
    Yes, each of the 3 companies from whom I obtained estimates did a load calc. The company that I chose recommended 3 ton. One company recommended 3.5, while the other recommended 4 ton.

    I'm not saying that my humidity is necessarily too high, I'm just saying that the reading jumped from 43 to 53 percent and now our finished basement has a musty odor that wasn't there before. Also, those of us that understand basic math recognize that an increase from 43% to 53% is a 23% increase in moisture in the air, just as an increase in a tax rate from 43% to 53% would be a 23% increase in nominal dollars paid.

    Again, I'm not an HVAC expert, but 23% more water in the air and a musty smell that didn't previously exist gives me some concern. If such a spike is normal, or if the humidity was too low before and now at 53% it is back up to the "ideal" range, then I won't call them back. I just assumed that my humidity levels would not increase with this new system since no other variables have been changed.

  8. #8
    Join Date
    Jan 2009
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    Keokuk, IA
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    RH is relative, if the downstairs temperature went from 74F to 69F, with the same dewpoint, you'd go from 43% to 53%RH. It's the exact same amount of water in the air, just the temperature has dropped in the case above.

    Now, why is the basement colder?

    Load calcs still follow the basic law of "garbage in = garbage out". So if the contractor uses inaccurate values for insulation, windows, shading, etc... and then worse uses the incorrect design temperatures, then you get an oversized system. For example, in most parts of the country, a perfectly sized system at 100F should be falling behind. The only exception in hot dry desert areas and most parts of Texas. The design temp is lower pretty much everywhere else.

  9. #9
    Join Date
    Apr 2012
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    new york
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    I would be looking at what udarrell said what is the blower cfm per ton
    you said the air flow has gotten significantly stronger which leads me to belive that compared to the old system's cfm per ton this one could be set alot higher

  10. #10
    Join Date
    Dec 2010
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    Quote Originally Posted by ThatOneGuy View Post
    Yes, each of the 3 companies from whom I obtained estimates did a load calc. The company that I chose recommended 3 ton. One company recommended 3.5, while the other recommended 4 ton.

    I'm not saying that my humidity is necessarily too high, I'm just saying that the reading jumped from 43 to 53 percent and now our finished basement has a musty odor that wasn't there before. Also, those of us that understand basic math recognize that an increase from 43% to 53% is a 23% increase in moisture in the air, just as an increase in a tax rate from 43% to 53% would be a 23% increase in nominal dollars paid.

    Again, I'm not an HVAC expert, but 23% more water in the air and a musty smell that didn't previously exist gives me some concern. If such a spike is normal, or if the humidity was too low before and now at 53% it is back up to the "ideal" range, then I won't call them back. I just assumed that my humidity levels would not increase with this new system since no other variables have been changed.
    I assure we use a great deal of math on a regular basis

    As Moto said, we're talking about relatives and not absolutes so there is not a 23% increase in water content. What that number means is that at a given temperature the air is holding 53% of the water it can hold before the water molecules condense. The lower the temp is the closer the water molecules get and the closer they are to condensing. As an example, when fog forms the air is holding over 100% of the water it can at a given temperature. Those of us who understand basic physics get it
    Quote Originally Posted by k-fridge View Post
    The laws of physics know no brand names.

  11. #11
    Join Date
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    We are the right track. Every degree that the temp drops increases the %RH on the air. Consider that under carpeting or anything covering concrete in the basement which is much as 10^F cooler than the air in the space. As the %RH at the concretes surface approaches 80-90%RH, biological growth produces the musty smell. Make sure that the basement is 50%RH. The a/c will not beable to do this during part load cooling conditions. A good dehumidifier is part of maintaining <50%RH during the seasons of the year in green grass climates.
    Hard to believe this draught will end.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  12. #12
    Quote Originally Posted by SoFlaDave View Post
    I assure we use a great deal of math on a regular basis

    As Moto said, we're talking about relatives and not absolutes so there is not a 23% increase in water content. What that number means is that at a given temperature the air is holding 53% of the water it can hold before the water molecules condense. The lower the temp is the closer the water molecules get and the closer they are to condensing. As an example, when fog forms the air is holding over 100% of the water it can at a given temperature. Those of us who understand basic physics get it
    Dave, no offense intended. If you'll read my post a second time with a less defensive posture you'll see that I said those of us. I was posting in this forum, to you, and said "us". My point was that certain trades do math, and those of us that do math understand that when all other variables remain constant, and a reading moves higher, then you can quantify the increase.

    Your point is appreciated and well-taken, i.e. you're saying that I do not understand that the humidity reading is a relative not absolute reading, and that I do not understand that the temperature reading on my thermostat being the same doesn't mean the temperature reading in parts of the house isn't different than before. In fact, I agree with you completely now that you've explained that since the basement feels cooler, it doesn't matter that the thermostat reads the same, the basement probably IS cooler and since humidity is relative, the moisture in the basement might not be higher simply because of the higher humidity reading.

    So, I've confirmed that the basement is cooler with a temp gauge near the humidity gauge. Right now, the basement humidity is 53%, and the temperature is 75-76 (it fluctuates, but it definitely is cooler and definitely feels cooler). The main level temperature is 77. Prior to the new unit installation, when the main level thermostat was at 77, the basement humidity was at 43%. I fully understand your point that this is a relative reading. Let's assume that my basement temp was 77 before the change to the new A/C unit.

    In your view, should I be concerned about the humidity reading change when the temp is 1 to 2 degrees lower, and the "old wet sock" odor, or should I accept this as the price of a correctly sized unit? The Santa Fe Classic unit is running full tilt 24 hours a day and can't get the humidity below 53%, when before it easily kept it in the low 40's.

  13. #13
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    Dec 2003
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    Where are you located?

    I ask this because you said that it has been 100 degrees is that normal?
    If you are in an area where the design temperature is 92 then your unit could be sized 100% correct but it is doing all it can given the load placed upon it.

    It's like getting a truck that says 20 MPG but that is designed for certian circumstances. Put a ton of cement in the back and tow a trailer and the truck will still move, but you won't get 20MPG.
    Quote
    “Engineers like to solve problems. If there are no problems handily available, they will create their own." Scott Adams

    "We can't solve problems by using the same kind of thinking we used when we created them."
    Albert Einstein

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