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  1. #1
    Join Date
    Feb 2014
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    Air handler CFM speed question.

    I live in a 3 bedroom, 1474 sq ft home in the Tampa Bay Area. Original HCAV unit was a 3 ton Lennox, and the humidity was always very high. Took a look at the blower settings, and it was set for maximum CFM, when the size of the AC at 3 tons called for a bit less. I changed from high to medium high and that helped a great deal.

    The unit recently failed, and I now have a Goodman GMES800805CNAA furnace with a CHPF3642C6CB evaporator unit. This cools the house very quickly, but the humidity has been staying at over 60% throughout the house. The furnace was set to the factory default setting of 2202 CFM, which sounds way high for a 3 ton unit. I have the choice of lowering that to 1352 which I tried first. This helped, but I'm still having to use a dehumidifier to keep things where I want them.

    The next lower setting is 822 CFM which seems a bit low for a 3 ton AC. I just changed to that to see how things go. The humidity levels are dropping to around where I want them, but I don't want to damage or constantly freeze the evaporator coils either. The Manual for the furnace wants a 16 x 25" filter and I have a return grill filter that is 18x30. It's a straight and short run from the furnace to the return grille.

    As I like to keep my home somewhat warm (77-78 during the day in the summer), will I be ok leaving the blower at the 822 CFM setting, or should I go with the 1352 setting and continue to use the dehumidifier as needed?

    Side note, do installers really not bother to check these settings? I almost understand for the builder that installed all the units in the development, but I was quite surprised to discover the factory tape covering the dip switches on this new unit.

    Thanks.

  2. #2
    Join Date
    Feb 2014
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    34
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    Thread Starter
    One other thing to add, there was a fair amount of liquid (seemed like water) that was draining from the condenser around where the line set is connected to the condenser. I have not checked to see if this is continuing to happen after changing the blower CFM setting. It is no where near the evaporation drain line, and the old one didn't do this. They seemed to do a good job of insulating the line set though, far better then what was on the old one. Looks like a foam tube coated with mastic.

  3. #3
    Join Date
    Jan 2019
    Location
    Vero Beach, Florida
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    497
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    You are oversized

    How was the new unit sized?

  4. #4
    Join Date
    Jan 2012
    Location
    Missouri, USA
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    1,503
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    Usual whenever the airflow is adjusted the refrigerant charge needs to be checked also.

  5. #5
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    9,725
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    Quote Originally Posted by jacount View Post

    I live in a 3 bedroom, 1474 sq ft home in the Tampa Bay Area.

    Original HVAC unit was a 3 ton Lennox, and the humidity was always very high.
    Took a look at the blower settings, and it was set for maximum CFM, when the size of the AC at 3 tons called for a bit less.

    I changed from high to medium high and that helped a great deal.

    The unit recently failed, and I now have a
    Goodman GMES800805CNAA furnace with a CHPF3642C6CB evaporator unit.

    This cools the house very quickly, but the humidity has been staying at over 60% throughout the house.
    The furnace was set to the factory default setting of 2202 CFM,
    which sounds way high for a 3 ton unit.
    I have the choice of lowering that to 1352 which I tried first.
    This helped, but I'm still having to use a dehumidifier to keep things where I want them.

    The next lower setting is 822 CFM which seems a bit low for a 3 ton AC.
    I just changed to that to see how things go.
    The humidity levels are dropping to around where I want them, but I don't want to ...

    Side note, do installers really not bother to check these settings?
    I almost understand for the builder that installed all the units in the development,
    but I was quite surprised to discover the factory tape covering the dip switches on this new unit.

    Thanks.
    Your mechanical contractor Obviously provided you with HvAC K dis-service.
    3-Ton should operate close to 1,080 CFM.

    I presume your residence is < 25 years old.

    A/C should run > 12 hours / day June through September.
    _ " ___ " __" > 9 hours / day May and October
    _ " ___ " __" > 6 hours / day April

    ___ Cooling Degree Days : 3,600
    Annual runtime estimate: > 2,400 & < 2,900 hours

    A 3-ton, 2-stage system would likely run on
    First stage > 86% of the time
    First Stage equivalent: ~ 2.1 Ton

    Tampa: should have a Heat Pump so match up of evaporator & condenser is more appropriate.

    Furnace at 80,000 BTU/HR / 80% is at least 3 X more than needed for heating.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  6. #6
    Join Date
    Feb 2014
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    34
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    Thread Starter
    There was no measuring done. The fan failed, the repair person determined the system needed to be replaced, and took pictures of every sticker on each unit lol.

    The home was built around 04, and there are two large wild oaks that probably weren’t there when the original was installed. It’s also a huge development so I doubt there is much model variation on these units.

    The old system must have leaked pretty badly as I have some Awairs to monitor my IAQ, and average CO2 levels have about tripled with the new system.

    I was thinking of putting a ~MERV 13 4” filter to restrict flow a little and bumping the blower to the next highest speed.

  7. #7
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    9,725
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    Quote Originally Posted by jacount View Post
    There was no measuring done. The fan failed, the repair person determined the system needed to be replaced, and took pictures of every sticker on each unit lol.

    The home was built around 04, and there ...

    The old system must have leaked pretty badly as I have some Awairs to monitor my IAQ,
    and average CO2 levels have about tripled with the new system.

    I was thinking of putting a ~MERV 13 4” filter to restrict flow a little and
    bumping the blower to the next highest speed.
    What was the corresponding change in Ambient ( outside) CO2 levels?

    What was the old CO2 level ? 300 ppm
    _____________ __ New: ? 800 ppm

    https://ohsonline.com/articles/2016/...y-control.aspx

    What is your current Mechanical Ventilation rate?
    ____ How does that value compare to ASHRAE 62.2?

    __What is Your Natural Ventilation rate?

    ______ Blower Door Test
    _______ https://www.energy.gov/energysaver/blower-door-tests

    --
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  8. #8
    Join Date
    Feb 2014
    Posts
    34
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    Thread Starter
    Quote Originally Posted by dan sw fl View Post
    What was the corresponding change in Ambient ( outside) CO2 levels?

    What was the old CO2 level ? 300 ppm
    _____________ __ New: ? 800 ppm

    https://ohsonline.com/articles/2016/...y-control.aspx

    What is your current Mechanical Ventilation rate?
    ____ How does that value compare to ASHRAE 62.2?

    __What is Your Natural Ventilation rate?

    ______ Blower Door Test
    _______ https://www.energy.gov/energysaver/blower-door-tests

    --
    No way a blower door test was done. Usually mid day CO2 was 500-600 with the fresh air blower turned off between 11 AM to 9:30 PM, and using 41 watts. Now I am struggling to get below 800 ppm with the blower at maximum, about 73 watts. I doubt the outdoor amount changed much during the 6-7 hours it took to swap out.

    The plus side I suppose is that I am adding to the heat load a bit.

    Unfortunately for me, the lowest Awair triggers for CO2 using IFTTT is 1000 ppm when my target is below 700. Now it's hard to get there at all.

    On a funny/sad side note, my Ecobee thermostat refused to work when they wanted to start testing. They traced 3-4 unused wires and the blue C wire but could not make it work. That evening I was able to install it by untangling the rats nest of wire, cutting, re-stripping it all, and replacing the Ecobee. When I was doing this on the furnace side, I noticed that they jammed something in the door interlock switch that caused it to keep running with the blower side door off. Needless to say I was quite happy I noticed that before changing any DIP switches.

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