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  1. #1
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    Question What HVAC process has a byproduct of increasing OUTSIDE humidity?

    I have a concept-level question regarding what happens to the OUTSIDE air with rooftop units. I am dealing with three rooftop HVAC units on a decent sized church that seem to be saturating the shingles on a pitched roof adjacent to the HVAC units. We have identified the HVAC units as being the source of the excess humidity (the humidity on the roof immediately downwind / upslope of the units is higher than anywhere else on the roof. Shingles on this patch of the roof are staying more water-saturated in this downwind area than anywhere else on the roof).

    We are not looking for a problem with the HVAC units but rather questioning how an HVAC process might have a byproduct of increased outside humidity. The units are Lennox make, one is a 7200 CFM (serving a multipurpose room) unit and the other two are 3600 CFM units (serving offices and an open space). The actual exchange of air (inside for outside) is minimal on these units, so the excess humidity is taken to be a byproduct of one of these processes, not from internal humid air being released to the outside. There is no apparent malfunction with the units and condensate / coil drains appear to be working properly.

    I have done some basic research but have only been able to find information on inside humidity levels and on collection of condensation from inside air being cooled, but not on the effect of HVAC on outside air.

    From my basic understanding, there are three scenarios under which the HVAC units might be producing the excess humidity: 1) running the AC, 2) warming the building by means of a heat pump, and 3) heating the unit by means of firing the burners. This facility is located in Alaska, so temperatures rarely make it into the 90’s.

    1) Running the AC. Focusing on the outside air, here is my understanding of what is going on: Outside ambient air is drawn across the unit’s external coils, is warmed (cooling the refrigerant), and blown back out the top of the unit into the outside ambient air. When warmed by the coil, this pocket of air decreases humidity / increases ability to hold moisture. When this warmed pocket of air is blown out the top of the unit, it comes in contact with the cooler outside air and cools back down to its original temperature. Technically this pocket of once-warm air decreases its ability to hold moisture from its previously warmed state when it is cooled. But does this actually increase its humidity to greater than the surrounding air? Or does it just return to previous levels, matching the outside ambient air? No more moisture was introduced to the air while it was warmed. Would air off the AC coils have the possibility of increasing the humidity downwind of the units? Enough to saturate roof shingles over time?

    2) Warming the inside air by means of the heat pump. As I understand it, this is basically running the AC in reverse. Similar to the AC mode but with the opposite temperature effect, outside air is drawn across the external coils, cooled (refrigerant is warmed), and then blown out the top of the unit. This cooled pocket of air now has less ability to hold moisture / is at a higher humidity level. Might this pocket of more-humid air have a significant effect on the humidity downwind of the units, enough to keep shingles saturated? Or would the temperature of the cooled air return to match the outside air temp once again and humidity levels balance quickly?

    3) Heating the inside air by means of the units’ burners. These units are natural gas, so exhaust that is produced when the burners are running is laden with moisture and is hot. When the exhaust exits the units, it is cooled by the outside air. Consequently, humidity goes up. Unlike the AC and the heat pump, burners running actually introduces an additional source of moisture rather than just altering the temperature one direction and back again.

    Is there one that you would suspect over the others? Is there one of these three scenarios NOT likely to produce increased humidity immediately downwind of the units? The source of the excess humidity seems to be cumulative and we have not been able to distinguish which time of the year it mainly occurs during.

    If you have an idea which of the three scenarios may be the main culprit for increased outside humidity, please weigh in!

    Thanks!

  2. #2
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    water doesnt like to flow UPHILL so I would guess that it's either windblown water from heat pump defrost or perhaps a problem with the ducting under the roof line (leaking air in the attic/roof line causes ice/snow to melt only in the area)

  3. #3
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    are the affected spots on the north side of the units? ie, wetter due to no sun exposure driving off moisture?
    Col 3:23


    questions asked, answers received, ignorance abated

  4. #4
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    You may have relief air with higher humidity (in winter) being blown onto the roof but in cooling mode I don’t think any process will raise the outside humidity.

    My guess is that you have a condensation issue not a high humidity issue. The roof is cooler in the area of the issue than the dew-point of the air causing liquid water to form on the roof. Could be North exposure/shade like billygoat mentioned or poor insulation or wet insulation from a roof leak, or perhaps cold air leaking from duct work in that area.

  5. #5
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    OK, thanks! I will have to look more into where the heat pump defrost dumps its water on these particular units. Is there a decent possibility that moisture from the heat pump defrost could be blown into the air by the coil fans? There is a ledge between the flat roof where the units are located and the adjoining sloped roof that is staying saturated. Any moisture getting to the roof from the units has to "jump" this ledge.

    The area of roof affected is above a large multipurpose room and all ducting is along the sidewalls of the room, none is in the attic space. We have not observed any ice dams or uneven melting in the winter, so our assumption is most of the moisture accumulation is from spring or fall, which I guess is when the heat pump would probably be operating most.

  6. #6
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    I meant to post #5 in response to HVAC_marc (post #2). Still learning the forum format setup. Thanks!

  7. #7
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    Quote Originally Posted by billygoat22 View Post
    are the affected spots on the north side of the units? ie, wetter due to no sun exposure driving off moisture?
    The units and the affected area of roof are both on the north side of the building. However, the HVAC units are on a flat roof, and the roof area in question is is an adjacent sloped roof to the east (downwind) of the units. The area staying saturated is not the whole north-facing roof but rather is fan-shaped, spreading out downwind from the units for roughly 50 feet. The fact that the affected area of roof basically makes an arrowhead pointing directly upwind at the units and that the affected area is elevated a few feet (angled 0' to 12') above the flat roof is what has us considering moisture in some form being blown into the air from the units.

  8. #8
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    Quote Originally Posted by hvac n-j-near View Post
    You may have relief air with higher humidity (in winter) being blown onto the roof but in cooling mode I don’t think any process will raise the outside humidity.

    My guess is that you have a condensation issue not a high humidity issue. The roof is cooler in the area of the issue than the dew-point of the air causing liquid water to form on the roof. Could be North exposure/shade like billygoat mentioned or poor insulation or wet insulation from a roof leak, or perhaps cold air leaking from duct work in that area.
    Thanks for your input on it probably not being a cooling mode issue. Per my scenario #1 in my original post, I know cooling changes the temperature of the outside air around a bit but wasn't sure if that really did anything with the humidity levels.

    I know that being north-facing is not helping out and may be keeping it from drying out completely, but there being only a fan-shaped area that is saturated (not the whole north-facing roof) is making me wonder how it is getting saturated in the first place. We looked into insulation and vapor barrier and all of that seems to be done tightly and evenly. All duct work for the area below the roof is below the insulation / vapor barrier, inside the heated envelope of the building. I definitely hear you on those points but it doesn't appear to be the root cause in this situation.

    The fan-shaped pattern on the roof downwind of the units that I described a bit in my response to Billygoat22 (post #7), and the roof in question being a bit higher than the units (about level with the top of them) is what got me to originally thinking about humidity because I wasn't sure how else the moisture might be "jumping" to the adjacent sloped roof from the units. I am totally open to it being more of a condensation issue as you suggest, but am not too familiar with the boundary between humidity and condensation. Could condensation be blown by wind / coil fans to the neighboring roof? The roof in question is about 4 feet away from the largest unit and is about level with the top of the unit.

  9. #9
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    Im having trouble picturing all this. Get another 2 posts and you should be able to put up some pictures

  10. #10
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    OK, I tried to attach a picture to this post. Hopefully it works. If not, I will try again later to get a picture up. The units are on a small flat roof area, but the rest of the building has a sloped, shingled roof.
    Attached Images Attached Images  

  11. #11
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    The place was just re-roofed in the photo, so you cannot see the saturated fan-shape in the shingles. But there was a distinct pattern starting at about the four coil fans in the large unit pictured that fanned out along the length of the roof (downwind).

  12. #12
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    what do the fans along the wall do? maybe they are picking up water and slinging it because the air from them blasts against the side of the RTU and has no place else to go but over the roof.

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  14. #13
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    You have talked about 3 different scenarios, all three could be possible, however one requires a heat pump the other requires a gas burner. Could you narrow this choice down for us as in is it a heat pump, gas furnace, or dual fuel.

    From the picture I would guess that it is a heat pump and when it goes through defrost the "steam" coming off the coils that gravitates through the fans is wind blown across the roof. This would be going on any time the outdoor temperature is below 40°

  15. Likes Juan Madera liked this post.
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