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  1. #1
    Join Date
    Jun 2007
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    Vermont
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    179

    Central AC with too much air?

    You guys have been great answering everyone's questions (along with a few of mine). I have a duct type question I would like to get some opionions on. I am in a condo, unit above me, below me, and behind me. I face west, lots of windows in Vermont. The units on West side of buiding have 2 tons of cooling (1200 sqft). Pretty sure it is oversized from what I read on here. For example when it does reach 90 here, unit never runs more than 15 minutes. Also, I really feel the air a lot. I can never seem to get a good setting on the tstat. Sometimes feels to hot or too cold. I got a good vision pro 8000 and that helped a lot but still, I can surely feel and hear it when it's on.

    Anyway, I have one metal duct that runs through the center of the apartment. It is roughly 28" by 7 " in an enclosed soffit (8 foot ceiling in the apartment). The duct is roughly 20 feet long and all registers come off that one main duct. I have 6 registers.

    2 7" by 6"
    2 9" by 6"
    1 6" by 18"
    1 13" by 18"

    All are rough measurements of the outside of the grill. In all cases the actual opening is slightly smaller. My fan coil is mounted in a small closet not ducted return. The fan coil on a pedestal and there is simply a return grill in the side of the closet. The return grill is 17" by 9". Also, the door to the closet has a 1 inch opening around the entire parameter so it also functions as a return. Roughly another 150 sq inches by my calcs. So after reading on here looks like my return is undersized. Problem is if I open the door to the closet, essentially making my return size 1026 sq inches.. HUGE. I didn't see any difference in the supply air from the registers. So that led me to believe I am starved on the supply side. So I decided to take the grills off and I found a removable back piece that allows you to shut the vent. I took all of these off. See pic.

    I then had tons of air so I thought I did a good thing. I then saw a difference when opening and closing the HVAC closet door. Problem now is I was getting too much air!! You can now see the leaves on the plants on the other side of the room blow quite a lot. Also, I got lots of duct noise. Like the duct metal was flexing. Also, as you guys realize, my humidity level increased a little bit. It was around 55% when it was in the 60's outside and raining.

    So I then went into the air handler and set the CFM adjust to low. As I am sure you know, that allows the air to be reduced by roughly 10% from nominal. I still had a lot of air coming out of the vents, but it was a little better. Still had duct noises.

    So, now I have put the metal pieces back on the grills. I call them restricters. My own termonology since they do seem to restrict the air. The fan coil is still on low setting. Now the AC is much quieter, no more duct noises and I feel the coolness without feeling the breeze if you know what I mean. It feels as it should. I have lived in houses that have central air and I know shouldn't feel the AC so much as I did. It nows feels better than it ever has. The temp seems more consistent too.

    I know you guys are going to tell me they should have done a calc and the ducts should be done correctly. This condo is 2 years old so none of that is going to happen. The ducts are buried in sheet rock soffits so they aren't changing.

    My question is IF, and I do mean IF, I never get a freeze up of the coil, am I ok with the way things are? I will check the coil for the next few days and see if I get low air flow. If there is ice, I will go back to nominal and leaves things alone. If I don't get ice, then is there a problem running partially starved of air? I am thinking maybe early motor failure? I think just the motor can be changed so if that goes out in 5-6 years I think it's worth it for the quiet and confort.


    Any thoughts? Am I crazy?

    Here are some pics to help you guys.
    Attached Images Attached Images       

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,292
    Where you might get in trouble is with your compressor. An iced up coil involves more than just a bunch of ice crystals blocking air flow and eventually stopping the system from cooling. It can also take out a compressor, due to liquid refrigerant flooding back to the compressor.

    I never change blower speeds on an a/c without checking the refrigerant charge afterward.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  3. #3
    Join Date
    Jun 2007
    Location
    Vermont
    Posts
    179
    A few weeks ago I had a low ambient kit installed (fan cycler and crankcase heater). It will cycle off the condenser if pressure builds to high. I know this is meant for low ambient temperatures, but would it possibly help stop liquid from ending back up at the condenser? Maybe I am totally off.

    Will I know if liquid ends up back at the condenser? Wouldn't I need to see ice before that happens?


    Thanks for reading my long long post.

  4. #4
    Join Date
    Jun 2007
    Location
    Darby, PA
    Posts
    531
    A low ambient kit will cycle condenser fan on or off or modulate condenser fan speed to maintain an adequate head pressure in the system for optimal operation, if adjusted properly. You don't necessarily need to see ice to have liquid refrigerant back to the compressor. Sounds like you need a tech to analyze your system and correct the issues your having. Could be airflow, mischarged system or other causes that need to be addressed.
    Psalm 51:10, 12

  5. #5
    Join Date
    Jun 2007
    Location
    Vermont
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    179
    Quote Originally Posted by ldmth44 View Post
    A low ambient kit will cycle condenser fan on or off or modulate condenser fan speed to maintain an adequate head pressure in the system for optimal operation, if adjusted properly. You don't necessarily need to see ice to have liquid refrigerant back to the compressor. Sounds like you need a tech to analyze your system and correct the issues your having. Could be airflow, mischarged system or other causes that need to be addressed.
    With the fan speed on nominal it works fine. No problems as long as the grill backs are on like they are now. So now everything is like when I bought the place. I just like the lower fan speed for comfort and noise. Maybe I should put it back to normal and live with the slightly higher noise level.

    FYI info, it has just been running for around 10 minutes. Inside temp set at 73 degrees, outside temp is 68 degrees. Kinda humid out that is why the AC is on.

    The small copper pipe is still very warm. So I think I don't have liquid going to the condenser as long as that pipe stays warm, not cold. I can easily check that pipe for the next few days since all I have to do it open the closet door.

    I'd hate to call a tech because it is working fine. I just like the lower fan speed.

  6. #6
    Join Date
    Jun 2007
    Location
    Darby, PA
    Posts
    531
    One can't assume a system is running fine just by the sensed temperature of the liquid or suction line. There are too many variables to consider to be sure that a system is running properly. If there are any doubts, you should have system checked by a pro.

    I would personally feel better if my auto mechanic checked my emmissions system on my car other than relying on "feel" that my engine is fine and making adjustments myself, since I know a bit about an engine but not all that is required to make it run right continuously without proper training.
    Psalm 51:10, 12

  7. #7
    Join Date
    Jun 2007
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    Quote Originally Posted by ldmth44 View Post
    One can't assume a system is running fine just by the sensed temperature of the liquid or suction line. There are too many variables to consider to be sure that a system is running properly. If there are any doubts, you should have system checked by a pro.

    I would personally feel better if my auto mechanic checked my emmissions system on my car other than relying on "feel" that my engine is fine and making adjustments myself, since I know a bit about an engine but not all that is required to make it run right continuously without proper training.
    Ok. Thanks. I guess I am kinda confused because there are two jumpers on that fan coil. One, lists the compressor size that the coil is connected to. For example, mine is clearly on 024 which is the model of compressor I have. The HP/AC adjust, can be adjusted for low, nominal or high which is -10%, nominal, or +15% to allow for comfort settings (ie noise and humidity removal). It says this right in the install manual.

    So I took that as being I can move the low, nominal, or high as long as I don't touch the more important compressor choice jumper.

    I am actually on the association for my building and I was instrumental in getting a contract for our HVAC company to inspect all our fan coils twice a year and compressors once a year. So I do understand the importance of getting a pro to look at it, I just didn't think this minor change was am issue.

    I switched it back to nom, where it was, and I will call them tomorrow. They know our systems very well since they are always here. Hopefully they can let me know over the phone. In my experience they don't like to come out unless something is broken. They are very busy all the time.

  8. #8
    Join Date
    Jun 2007
    Location
    Darby, PA
    Posts
    531
    Peace of mind knowing it is done right is better than trying something yourself and screwing it up. Even I had to learn that the hard way, we all eventually do. Good luck!
    Psalm 51:10, 12

  9. #9
    Join Date
    Dec 2008
    Location
    caribbean
    Posts
    76
    the manual will tell the min cfm for ur ahu, but u deffinily have to check the system 's pressures. the compressor could be flooding with liquid freon and not necesary icing up at the evap.

  10. #10
    Join Date
    Jun 2007
    Location
    Vermont
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    Quote Originally Posted by ticotech cayman View Post
    the manual will tell the min cfm for ur ahu, but u deffinily have to check the system 's pressures. the compressor could be flooding with liquid freon and not necesary icing up at the evap.
    You guys are making this much too dificult on me. I simply wanted to lower the fan speed. Just kidding.

    I have to say I always thought HVAC was rather simple until reading this forum. Also, this is residential type stuff. I was out in Vegas a few weeks ago and saw the HUGE cooling towers behind every hotel to keep all that sq footage cool. It is quite impressive. I can't even imagine all the issues with keeping those systems running in 120 temps.

    Thanks for all your help. I might just leave my unit on high and live with the slightly increased noise and air. If I am still in my apartment when the HVAC needs to be replaced, I am going for a 1.5 Ton unit.

  11. #11
    Join Date
    Jan 2009
    Location
    Keokuk, IA
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    5,520
    Quote Originally Posted by smoss View Post
    Also, this is residential type stuff. I was out in Vegas a few weeks ago and saw the HUGE cooling towers behind every hotel to keep all that sq footage cool. It is quite impressive. I can't even imagine all the issues with keeping those systems running in 120 temps.
    .

    Actually, the cooling towers work very well in the hot dry desert air. Cooling towers require regular maintenance to keep them clean nad the proper chemical levels. The cooling towers however, are just 1/2 of the system. What you didn't see are the numerous chillers located inside the building... located in large mechanical rooms along with the boilers and numerous pumps. The chillers are basically really big A/C compressors, that use water instead of air to cool the condenser coil. The heat is removed by sing evaporation in the cooling tower to cool the water.

    They are pretty cool. At my work, in 1 building we have 5 chillers, 2 big onces and 3 smaller ones that are staged depending on the heat load in the process. It seems like at least 1 or 2 are always down for maintenance or tripped out on some safety or another. Probably installed incorrectly by our engineering group.

  12. #12
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,292
    Quote Originally Posted by smoss View Post
    A few weeks ago I had a low ambient kit installed (fan cycler and crankcase heater). It will cycle off the condenser if pressure builds to high. I know this is meant for low ambient temperatures, but would it possibly help stop liquid from ending back up at the condenser? Maybe I am totally off.

    Will I know if liquid ends up back at the condenser? Wouldn't I need to see ice before that happens?
    A low ambient kit is for when outdoor temperatures are below the normal operating envelope of an a/c condenser. It works by keeping head pressure high enough so the right amount of refrigerant will flow through the evaporator, allowing interior cooling to continue normally. Low ambient kits are typically found in commercial HVAC systems that must operate for most or all of the year, such as office buildings with air cooled equipment, or computer rooms.

    It is still possible to freeze an indoor coil that has a low ambient kit installed on the outdoor unit. I've seen it happen on variable speed air handlers used in VAV applications where the hot gas bypass was not adjusted correctly. For a residence, if everything about the system is normal and the coil is freezing, there simply is not enough heat load in the house to keep the coil surface temperature above freezing. Running the a/c to dehumidify a house during cool, damp weather can put a coil at risk for freezing over.

    Being you live in a condo, you could have moisture entering your unit from untold sources, plus the moisture you create by showering, laundry, cooking, etc. Have you ever measured what the humidity runs at in your unit on a day to day basis? Do you base your need to dehumidify (when it is cool outside) because the air in the condo feels "stuffy"?
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  13. #13
    Join Date
    Jun 2007
    Location
    Vermont
    Posts
    179
    Quote Originally Posted by motoguy128 View Post
    Actually, the cooling towers work very well in the hot dry desert air. Cooling towers require regular maintenance to keep them clean nad the proper chemical levels. The cooling towers however, are just 1/2 of the system. What you didn't see are the numerous chillers located inside the building... located in large mechanical rooms along with the boilers and numerous pumps. The chillers are basically really big A/C compressors, that use water instead of air to cool the condenser coil. The heat is removed by sing evaporation in the cooling tower to cool the water.

    They are pretty cool. At my work, in 1 building we have 5 chillers, 2 big onces and 3 smaller ones that are staged depending on the heat load in the process. It seems like at least 1 or 2 are always down for maintenance or tripped out on some safety or another. Probably installed incorrectly by our engineering group.

    Actually, where I work we have several test floors. On the test floors are large testers that have 200 amps service each. There might be 5-10 of these in one room. So as you can imagine, we have big chillers on the floor to keep things cool. When a chiller fails, the test floor rises to 90 degrees within 30 minutes. Also, the chillers natrually run all winter long (even in Vermont in winter). I heard recently we are using outside air to cool instead of the cooling towers when ambient air is below 30 degrees. Sounds like a good idea sine almost all winter it is very cold.

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