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  1. #14
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    Those fans you mentioned are an economizer off the return air from the building to the unit (lets excess air through only when needed). I have been told by one of the installers that the economizer opens a minimal amount / less than 10% on this unit, and I have never seen the fans run any of the times I have been on the roof. Yes, the economizer is a bit tucked in behind the unit I agree.

  2. #15
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    Quote Originally Posted by BNME8EZ View Post
    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°
    The unit is gas/electric with AC coils and gas burners. I had originally been under the impression that it was dual fuel, running as a heat pump until cooler temperatures when the gas burners kicked in. However, after looking into it further it appears to have NO heat pump mode. The condenser coil fans have not kicked on since it snowed this year (snow still sitting on the fan blades), there appears to be no reversing valve, and after I finally got a hold of tech support from the manufacturer, they stated the serial number for this unit does NOT show in their records as having a heat pump mode. I was slightly disappointed when I found this out, as I had just been coming to the conclusion that air off the coils during heat pump operation was the most probable cause.

    Not having a heat pump mode removes my original scenario #2, leaving me with the following: 1) air off the coils from AC mode, 3) Exhaust out of the flue when the gas burners are firing, and now the new 4) Inside air exiting the economizer on the return ducting right before the unit (located between the wall and the unit as pictured above). One of these (or a combination) is presumably providing moisture to keep the area of roof downwind from the HVAC units more moist than anywhere else on the roof.

    Here's some quick info and questions I have regarding each scenario:

    1) Air off the coils in AC mode: I know some atmospheric air is warmed up by the condenser coils then cools back down as it exits the coil fans and mixes with atmospheric air again. Does anyone feel that this would change the humidity / cause condensation at all, or would temperature and humidity of the warmed-then-cooled air just return to the same levels as the atmospheric air it started out to be? There is no additional source of moisture from what I can tell. I am thinking the likelihood of this creating a moist environment such that would saturate shingles on the adjoining roof is not very likely, but do not want to write this possibility off too quickly.

    2) The units do not have heat pump capabilities, so is no longer being considered.

    3) Exhaust out of the flue when the gas burners are running: the flue is a small 2" x 4" port on the side facing you in the picture above (post #10), The predominant wind in this area would carry the moisture-laden exhaust plume over the HVAC unit and onto the roof in the background where the issue is. There is a rather broad temperature difference between the hot exhaust and the cool outside air when the heat is running, so I am thinking that as the exhaust cools drastically it could reach its dew point and the moisture (byproduct of combustion) would have a good chance of dropping out onto the roof. Does anyone see a hole in my logic on this one, or does the exhaust seem to be a sure source of moisture for this roof saturation issue?

    4) The economizer off the return ducting, located under the eave of the pitched roof, pointing at the side of the HVAC unit / the supply ducting (see the picture in post #10). Those that I have talked to that were around when these units were installed and programmed say that the economizer is not intended to run often and only feathers a portion of air when called to. They say it runs less than 10%, and is primarily to limit the amount of cooling required in AC by letting out hot inside air and replacing it with cooler outside air. The small amount of air that does come out of the economizer would undoubtedly channel up between the large unit pictured and the roof eave, then blow across the sloped roof where the saturation issue is. But if the air coming out of the economizer was laden with enough moisture to keep shingles on the roof soaked, I would think there would be moisture issues with the wood wall, eave, and fascia right above the economizer. All this wood is perfectly intact, which makes me initially think there is not much moisture coming out of the economizer. But would it be possible that the warm air does not cool down to the dew point until it gets past all the wood above the economizer, then cools to the dew point once it gets above the shingles? How probable of a moisture source is the economizer in y’all’s opinion?

    The condensate drain off the evaporator coil is out the North of the unit (Left as pictured above), nowhere near the condenser coil fans or the return air economizer, so I do not think this source of moisture is adding to the problem at all. I also recently looked at the roof and all snow is of even depth on the roof, showing no signs of melting in the area that has been wet during the summers. It is continuing to appear as an issue that occurs primarily during the spring / fall.

  3. #16
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    Was the sheating under the shingles rotted or was it just a stain/discoloration on the shingles?

  4. #17
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    Quote Originally Posted by jtrammel View Post
    Was the sheating under the shingles rotted or was it just a stain/discoloration on the shingles?
    Moisture went down through the saturated shingles and tar paper into the sheathing. Vapor barrier and insulation below was fine (foam insulated roof, no attic space).

  5. #18
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    Quote Originally Posted by AKkid View Post
    Moisture went down through the saturated shingles and tar paper into the sheathing. Vapor barrier and insulation below was fine (foam insulated roof, no attic space).
    So it was actively wet when the roof was replaced or just evidence of being wet?

  6. #19
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    Was still partially wet when the roof was opened up. Shingles seemed to have been constantly moist/saturated long enough that they dissolved. The roof was dried all the way before re-roofed and Ice and Water put back on rather than tar paper to keep moisture from penetrating in as it had before. But still trying to figure out where the moisture might be coming from. The area that was wet being a perfect fan shape down-wind of the HVAC units is what got me looking into how they could be "generating" excess moisture.

  7. #20
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    My bet would be on the exhaust gases. Most economizers are not equipped with a barometric relief and the ones that are don't move that much air to cause your problem.

    A simple solution may be to direct the exhaust fumes to a few inches above the unit. The fumes would them be higher as they cross the roof. As I see it now the fumes come out low on the unit, rise along the unit then are taken by the wind across the roof. By ducting teh fumes above the unit the fumes will be blown upward so by the time the wind takes it over the roof it will be high enough to not be a problem. I believe most manufacturers make a kit to direct these fumes above the unit. I had a unit that when the wind was in the right speed and direction the fumes would blow to the economizer on another unit that was close by. It cured that problem.

  8. Likes AKkid liked this post.
  9. #21
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    Quote Originally Posted by BNME8EZ View Post
    My bet would be on the exhaust gases. Most economizers are not equipped with a barometric relief and the ones that are don't move that much air to cause your problem.

    A simple solution may be to direct the exhaust fumes to a few inches above the unit. The fumes would them be higher as they cross the roof. As I see it now the fumes come out low on the unit, rise along the unit then are taken by the wind across the roof. By ducting teh fumes above the unit the fumes will be blown upward so by the time the wind takes it over the roof it will be high enough to not be a problem. I believe most manufacturers make a kit to direct these fumes above the unit. I had a unit that when the wind was in the right speed and direction the fumes would blow to the economizer on another unit that was close by. It cured that problem.
    Thanks for the input! That is the conclusion I was coming to as well but wanted to make sure I was on the right train of thought. I have already found a source for the flue extender kits for these units and plan to install them soon. I've seen quite the moisture cloud coming out of the units and drifting over the roof the past few times I've looked at the units, so feel elevating the flue should go a long way in reducing how saturated the roof gets. If the problem persists after raising the flues I might look into the economizer more, but glad to know you don't think it's that likely of a culprit.

  10. #22
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    Dec 2013
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    Hi AKkid,

    What you are missing from a "conceptual" standpoint is that when natural gas burns, H20 is one of the byproducts. So no, it isn't the heating of the air which causes the large amount of water in that air, it is the byproduct of the combustion process. This is why you will sometimes see water coming out of car tailpipes and why condensing furnaces have to have a drain, for all the excess water.


    BNME8EZ is on the right track with ducting the combustion exhaust away from the roof.

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