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  1. #1
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    Confused Unexplainable High Humidity in Apartment with Central Building HVAC

    Dear HVAC-Talk,

    I write to you in desperation, and in hope of figuring out this riddle. It looks like you're the smartest community in all of Google, and I doubt anybody else can help. Please bear with me and this little wall of text.

    I have a massive humidity problem (RH 80-85% at 73.4 F) in a one-bedroom apartment. The building is centrally cool and Im having a disagreement with the building maintenance contractor about it. Theyre not able to figure it out why humidity is so high, and theyre denying some mold growth on supply registers.

    Ill write down the facts and discoveries so far, and I hope you can help me by at least pointing me in the right direction so I have something to talk to with the maintenance contractor. Im not an HVAC engineer (but an engineer nonetheless) and I was forced to learn a lot about humidity and HVAC systems just to talk to the contractor.

    Here goes:

    - I live in a deserty climate. In the summer, outdoor humidity can be 75% for a few days, then falls to 15% for a few days more. The average temperature is 100-110 F.
    - Ive lived in my apartment for 5 years. First 3 were absolutely fine (~55% RH), the last 2 are absolutely miserable (~80% RH).
    - The building has centralized cooling, but is not humidity-controlled. I have a single FCU in the apartment, and its been maintained properly every 6 months by a specialist company.
    - When the trouble started and some mold appeared on the supply vents, I called an A/C and mold specialist. They cleaned the coil and the filter, check the drain tray, replaced the moldy chilled water piper insulation, and scraped the ducts. Then they applied some chemicals to prevent mold regrowth (though its coming back now).
    - My neighbors on the same floor have the same high humidity, which is 70-85% RH depending on the apartment. All tested at 74 F. The neighbors one level above are completely fine (55% RH). According to the maintenance contractor, both levels are controlled by the same FHU.
    - They check the fresh air supply coming into my apartment. It was coming at 45% RH / 74 F.
    - They checked for any leakage in the shafts, and plugged any little holes. (It had almost no effect at all)
    - They tested the temperature of the chilled water pipe. They said its 45-50 F. Theres no way I can confirm it, but the FCU is cooling perfectly fine.
    - They tilted the drain tray to make sure that no water stays there. I checked, its completely dry.
    - I set the airflow on low or medium. Neither changes the humidity.
    - The corridor areas have weak chilled water pipe insulation. They said that has no effect because the insulation is just as week one level above, and theyre fine.
    - My indoor humidity is always affected by outdoor humidity, and is usually 10% higher than outdoor humidity. (When A/C worked well, it was barely affected by outdoor humidity)
    - I got a dehumidifier that does 24 L/day, and I throw away the water every 7-8 hours. Itreads 61-70% at all times, and its constantly on. I dont think its able to cope with the humidity the A/C is introducing.
    - One thing I cant understand is when I set my thermostat to 77 F, my RH rises by about 10-15%. Its counter-intuitive. Shouldnt RH fall when the temperature is higher?


    I cant for the life of me find an explanation why this is the case. Ive researched this subject to death. I brought multiple specialists to the apartment and my neighbors apartments and they couldnt find the problem. I dont think legal action would fix it either, because if engineers couldnt find the problem, I doubt lawyers can.

    Im also not able to prove to them that whats forming on the supply registers is actually mold. Theyre saying its just dust, but I think I know the difference. Aside from lab testing, is it possible to tell if its mold or dust visually?

    Please ask me any questions, maybe that will help me find the problem.

    Id really appreciate any advice. Im getting desperate, and seriously baffled. I started my research here 3 months ago, and now have nowhere else to turn but you. Im sure the maintenance contractor would like my calls to stop as much as I do.

    Thank you in advance.

    P.S. I'm not sure if I posted in the right sub-forum. If I didn't, let me know and I'll re-post elsewhere.

  2. #2
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    My first guess, if they have sealed up the ductwork, is a neighbor has a grow room or boils a lot of water. How did you set the fan speed to low? Is there a setting or did you open up the unit and change it?

  3. #3
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    Thread Starter
    Maintenance contractor said that all apartments are completely separated from each other.

    I went around the neighbors and asked if they either cook too much or something, and none of them seem to do anything to raise humidity levels. Also, if the neighbors were the cause, I'd guess that my humidity levels won't be affected by the outdoor humidity.

    The thermostat has 3 fan settings. I didn't need to open the unit.

  4. #4
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    Get a laser thermometer, and record temp at your coil (actual coil) and if you can, get the same reading of your neighbors that are @ 55% rh.
    Eliminate your hvac, and start tracking down the source (infiltration).

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  5. #5
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    Maybe your coil is slightly clogged internally affecting water flow which will affect dehum.

  6. #6
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    Thread Starter
    Quote Originally Posted by kamersoutdoor View Post
    Get a laser thermometer, and record temp at your coil (actual coil) and if you can, get the same reading of your neighbors that are @ 55% rh.
    Eliminate your hvac, and start tracking down the source (infiltration).
    That's a good idea. They haven't checked the actual coil temperature, but they measure the air coming out the supply vent, which isn't very far from the FCU (it's a small 1 bedroom apartment). The reading was 52 F / 85% RH. Which seems normal.

    I forgot to mention that in the post above. I did try to find any infiltration, so I made sure all the windows and doors are insulated, and the contractor said they found some small holes in the shaft and plugged all of them.

    If apartments are isolated, can an infiltration affect the entire floor? I know that 6 of my next-door neighbors are having the same humidity issues in their apartments.
    Last edited by curious_owner; 07-23-2015 at 05:16 PM. Reason: Added quote

  7. #7
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    Quote Originally Posted by toocoolforschool View Post
    Maybe your coil is slightly clogged internally affecting water flow which will affect dehum.
    Done that too. They put a pump/compressor and made sure it's not clogged. They've done the same thing to the drain pipe for good measure.

    Most of the companies who came over said there's nothing left to do in the apartment, and we should start looking in the building. The building's contractor isn't able to find the problem yet, but they agree with the companies that my apartment seems fine.

    Btw, how would bad water flow affect dehum? Is it because the coil won't be cold enough to condense the water that the fan is blowing?

  8. #8
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    Air blow in the "on" or "auto" mode? Should be "auto" mode.

    What is the cold air supply during the wet times of weather?
    We need <45^F coil temp, slow air which your test indicated you have. Check while the indoor humidity is high.
    Bigger dehumidifier is nothing else can be found. Very interesting problem.
    Keep us posted. How much water removed by dehumidifier in 24 hours?
    Regards Teddy Bear
    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"

  9. Likes kamersoutdoor liked this post.
  10. #9
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    Quote Originally Posted by teddy bear View Post
    Air blow in the "on" or "auto" mode? Should be "auto" mode.

    What is the cold air supply during the wet times of weather?
    We need <45^F coil temp, slow air which your test indicated you have. Check while the indoor humidity is high.
    Bigger dehumidifier is nothing else can be found. Very interesting problem.
    Keep us posted. How much water removed by dehumidifier in 24 hours?
    Regards Teddy Bear
    Cold air supply coming from the vents is around 52 F / 85% RH. I haven't checked the temperature of the coil directly, but the building maintenance guys check the return water temperature from the chilled water pipe, and it was around 45-47 F. So they're saying my coil temperature should be fine.

    The thermostat in my apartment is a Honeywell T6575B. It doesn't have an "on/auto/off." Since this is central-controlled cooling, I think "on" is the same as "auto," but I'm not sure.



    I have two dehumidifiers, one for the bedroom (small room) and one for the living room. Together they're 7.5 L, and I empty them 3 times a day. So I empty around 22.5 L / day. I always set them to stop at 60% RH, but since my indoor humidity never reaches that, they're always on.

    There's something that I think is strange and I can't quite explain it. When I set my thermostat to 74 F, I get 75-85% RH. When I set the thermostat to 65 F (essentially making it work non-stop), the humidity drops to around 65% RH. This really confuses me. I thought when the air is colder, RH should increase. What am I missing?

  11. #10
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    It appears the key is that the problem is floor wide, and not just your apartment.

    So logic would dictate that readings would be taken from each floor and compared to each other. Logic would also say that something changed.

    You said "both levels are controlled by the same FHU". What does FHU stand for?

    You said there is a fresh air supply? Has this been completely blocked off to see if there are any changes to the environment? Somehow, I would guess that this has been tried, but you never know. At a minimum, you want to eliminate [or confirm] the obvious first.

    After the fresh air, what else is common? Seems to me it would be the chilled water. So what has changed? I would be comparing the supply and return temps for the chilled water loop on each floor.

    Let's try thinking just a bit outside the box. Could it be a balance problem, flow's not to design anymore? What if your floor is getting too much water flow? I know that sounds odd, but check out this thread. Only the people here could give me the correct answer to a problem I found [and corrected] many years ago.

    http://hvac-talk.com/vbb/showthread....ed-to-This-Day

    So if each floor uses its own common loop, how are the flows controlled in those loops? Could have been a simple mistake by someone adjusting a valve. Or replacing a valve or circulation pump. Or a strainer restricted; or too fine of a mesh of screen inserted.

    Just because the water is cold, doesn't mean the system will work well. Maybe some food for thought.



    Quote Originally Posted by curious_owner View Post
    Dear HVAC-Talk,

    I write to you in desperation, and in hope of figuring out this riddle. It looks like you're the smartest community in all of Google, and I doubt anybody else can help. Please bear with me and this little wall of text.

    I have a massive humidity problem (RH 80-85% at 73.4 F) in a one-bedroom apartment. The building is centrally cool and I’m having a disagreement with the building maintenance contractor about it. They’re not able to figure it out why humidity is so high, and they’re denying some mold growth on supply registers.

    I’ll write down the facts and discoveries so far, and I hope you can help me by at least pointing me in the right direction so I have something to talk to with the maintenance contractor. I’m not an HVAC engineer (but an engineer nonetheless) and I was forced to learn a lot about humidity and HVAC systems just to talk to the contractor.

    Here goes:

    - I live in a deserty climate. In the summer, outdoor humidity can be 75% for a few days, then falls to 15% for a few days more. The average temperature is 100-110 F.
    - I’ve lived in my apartment for 5 years. First 3 were absolutely fine (~55% RH), the last 2 are absolutely miserable (~80% RH).
    - The building has centralized cooling, but is not humidity-controlled. I have a single FCU in the apartment, and it’s been maintained properly every 6 months by a specialist company.
    - When the trouble started and some mold appeared on the supply vents, I called an A/C and mold specialist. They cleaned the coil and the filter, check the drain tray, replaced the moldy chilled water piper insulation, and scraped the ducts. Then they applied some chemicals to prevent mold regrowth (though it’s coming back now).
    - My neighbors on the same floor have the same high humidity, which is 70-85% RH depending on the apartment. All tested at 74 F. The neighbors one level above are completely fine (55% RH). According to the maintenance contractor, both levels are controlled by the same FHU.
    - They check the fresh air supply coming into my apartment. It was coming at 45% RH / 74 F.
    - They checked for any leakage in the shafts, and plugged any little holes. (It had almost no effect at all)
    - They tested the temperature of the chilled water pipe. They said it’s 45-50 F. There’s no way I can confirm it, but the FCU is cooling perfectly fine.
    - They tilted the drain tray to make sure that no water stays there. I checked, it’s completely dry.
    - I set the airflow on low or medium. Neither changes the humidity.
    - The corridor areas have weak chilled water pipe insulation. They said that has no effect because the insulation is just as week one level above, and they’re fine.
    - My indoor humidity is always affected by outdoor humidity, and is usually 10% higher than outdoor humidity. (When A/C worked well, it was barely affected by outdoor humidity)
    - I got a dehumidifier that does 24 L/day, and I throw away the water every 7-8 hours. Itreads 61-70% at all times, and it’s constantly on. I don’t think it’s able to cope with the humidity the A/C is introducing.
    - One thing I can’t understand is when I set my thermostat to 77 F, my RH rises by about 10-15%. It’s counter-intuitive. Shouldn’t RH fall when the temperature is higher?


    I can’t for the life of me find an explanation why this is the case. I’ve researched this subject to death. I brought multiple specialists to the apartment and my neighbors’ apartments and they couldn’t find the problem. I don’t think legal action would fix it either, because if engineers couldn’t find the problem, I doubt lawyers can.

    I’m also not able to prove to them that what’s forming on the supply registers is actually mold. They’re saying it’s just dust, but I think I know the difference. Aside from lab testing, is it possible to tell if it’s mold or dust visually?

    Please ask me any questions, maybe that will help me find the problem.

    I’d really appreciate any advice. I’m getting desperate, and seriously baffled. I started my research here 3 months ago, and now have nowhere else to turn but you. I’m sure the maintenance contractor would like my calls to stop as much as I do.

    Thank you in advance.

    P.S. I'm not sure if I posted in the right sub-forum. If I didn't, let me know and I'll re-post elsewhere.

  12. #11
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    RH should drop the longer the unit is cooling. The cold coil will condense the moisture in the air, resulting in condensation. Slowing the fan speed down will aid in humidity removal.


    What is your supply air temperature/RH, return air temperature/RH, entering supply water temperature , leaving supply water temperature.

    To clarify, does every apartment have there own individual unit or is there a central unit serving the individual floors?

    As for identifying the "fuzzy stuff", I would bring in a professional or take a sample that can be sent to a lab. They can better explain what is dangerous or not and what are considered safe levels of growth.

  13. #12
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    Quote Originally Posted by curious_owner View Post
    Cold air supply coming from the vents is around 52 F / 85% RH. I haven't checked the temperature of the coil directly, but the building maintenance guys check the return water temperature from the chilled water pipe, and it was around 45-47 F. So they're saying my coil temperature should be fine.

    The thermostat in my apartment is a Honeywell T6575B. It doesn't have an "on/auto/off." Since this is central-controlled cooling, I think "on" is the same as "auto," but I'm not sure.



    I have two dehumidifiers, one for the bedroom (small room) and one for the living room. Together they're 7.5 L, and I empty them 3 times a day. So I empty around 22.5 L / day. I always set them to stop at 60% RH, but since my indoor humidity never reaches that, they're always on.

    There's something that I think is strange and I can't quite explain it. When I set my thermostat to 74 F, I get 75-85% RH. When I set the thermostat to 65 F (essentially making it work non-stop), the humidity drops to around 65% RH. This really confuses me. I thought when the air is colder, RH should increase. What am I missing?
    Fan auto runs the blower when the t-stat calls for cooling. On runs the fan non-stop. Auto is best setting to avoid rapid re-evaporation moisture back into the space.
    If that 52^F, 85%RH reading is right, you have 48^F dew point supply air. That is good and should get you dry


    68^F setting lowers the temp and removes more moisture. But the temperature in space drops which raises the %RH. The question is does the dew point of the air in the space drop or not. The dew point should drop. Do you recall the space temp/%RH when you had the 52^F/85%RH supply? If 74^F, +60%RH, @ 300 cfm air handler 2-3 L per hour. My guess is that the cooling part does not run much because the adjouning space is cool with the exception of the outside wall. If so dehumidification is the only way out.
    With 22 L per day from the dehus plus the a/c moisture ??, this indicates a tremendous of moisture from some place. Outside most likely and some from occupants. WOW.

    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"

  14. #13
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    Just did some research about my inaccurate RH statements- shows you what I know. Still much to learn.

    I'm still would be curious as to questions posed. Any chance you have the unit info as well?

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