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  1. #1
    Join Date
    Jul 2012
    Location
    Los Angeles, CA
    Posts
    4

    Do Under Ground Ducts greatly affect air Temperatures?

    I'm a Home Inspector, neither a Tech nor a Homeowner, hopefully this is the right forum. I'm trying to confirm what an HVAC Tech told me to make sure it's correct and I currently don't have an HVAC source that I know and trust. I recently inspected a House where the ducting is below the slab. The air temperatures that I measured at the various vents dropped off significantly as I got further from the Unit, in some cases as much as 30 degrees cooler when heating and 12 degrees when cooling (it's a big house with 5 zones). These measurements were taken after running the system for 20-45 minutes. I figured that the ducting was original to the house (40 years or so old) and may be rusted out. The tech said that the ducting is buried in the ground 3-4 feet and it takes 3-4 hours for the temperatures to equal out. In ground ducting is fairly rare around here so I don't see enough of them to have a good feel for this. Is the tech right or is this cause for concern?

    If there's any question about the integrity of the system what if any tests can be done to determine the health of the ducting?

    Is there any fix for in ground ducting if it is leaking or rusted out?

    Thanks very much for any help that you experts can offer with this, I don't want to report something like this as a problem if it is not.

  2. #2
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    6,719
    A pipe scope camera would let you see what the integrity of the duct is. I would imagine the duct staying fairly cool being underground. ~50*F if there is no insulation then a lot of the heat would transfer to the ground. The further you get away from the unit the longer the duct takes to get up to temperature. This a lot of times the reason for hot/cold rooms in a home. With an oversized unit the temp near the tstat will satisfy while the rooms further away the duct has not even had time to get warmed/cooled so it makes the room hot/cold.

  3. #3
    Join Date
    Mar 2003
    Location
    Santa Clarita, CA
    Posts
    36
    What methods are you using to check the air temperature? 30 degrees seems like an awful big temperature swing.

  4. #4
    Join Date
    Jul 2012
    Location
    Los Angeles, CA
    Posts
    4
    Quote Originally Posted by servpro View Post
    What methods are you using to check the air temperature? 30 degrees seems like an awful big temperature swing.
    I'm using an infrared thermometer and also an infrared camera to measure the temperatures at the vents. I use the thermometer to check the accuracy of the camera, the camera shows what's going on but is unreliable for temperature readings. The cameras accuracy seems to depend upon how long it's been on. I know it's a really big difference in temperature, that's why I reported the problem. The homeowners HVAC guy says that the ducts are 3-4 feet under ground and it takes a long time for them to heat up or cool down and even out. He has an excellent rep according to someone I know so I'm trying to confirm if this is typical of such an installation.

    I would have thought that underground ducting would be at least as well insulated as above ground, is it not?

    I have seen this issue before but see so few houses with in ground ducting so I've always presumed it to be an issue when I see big temperature swings, as it is with normal, above ground, ducting. I'm hoping that someone here has enough experience with such systems to advise.

  5. #5
    Join Date
    Sep 2012
    Location
    Central Florida
    Posts
    880
    An infrared thermometer will be reading the temperature of the duct and not the air. They give a surface temp not air temp. If the duct has had enough to time to equalize with the air running through it then it may work, but then your always asking, has it had time yet.

    about your ducts being buried in the ground....... I don't have any real world experience but if the ground is significantly colder the then air being run through the duct, or warmer for cooling, then you will have heat transfer to it. This is actually how geothermal units work, though the process uses water and not air. By either transferring heat to the ground, or pulling heat from it. The ground is a very slowly changing thermal mass.

  6. #6
    Join Date
    Aug 2012
    Location
    willowick ,oh
    Posts
    261
    Ducts buried in the ground can cause many issues. Trust me. I have seen areas of duct with water infiltration causing this and huge temp differences in certain areas. In ohio this is very common in slabs, and there are a ton of them. I also see huge humidification issues. Water in ducts plus warm air going across it. Thats a home made humidifier. I have also noticed that homeowners in general have more illness in these style homes, probably from mold spores i would assume. Many of them are old clay tile that hold up well until the ground freeze/thaw gets the best of it. The spiral pipe ive seen in the ground holds up well until the plastic and mastic fails. I can go on all day about this. lol. Ive had a ton of calls for humidifiers not working properly and the windows are dripping wet and the humidifier isnt even running. I have heard of a process where they sleeve the actual existing piping in the ground to eliminate these issues, but i wouldnt put my name on the warranty for it. Bottom line is that you probably have more issues than you can see. get the ducts inspected with a camera. srry for rambling.

  7. #7
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,626
    Slab homes are much less common around here than homes with basements. But there are pockets of existing slab homes and some (rare) new homes with slabs.

    I've done both. I do not know of a case where I've seen the duct 3-4 feet deep. I do not know of anyone that would do that much digging when it's not necessary.

  8. #8
    Join Date
    Jul 2012
    Location
    Los Angeles, CA
    Posts
    4
    That's nearly correct, I'm measuring the temperature of the vent, not the air. I used to use analog thermometers stuck in the vents years ago but when I got the infrared thermometer I tested it against them and found that it only takes a couple minutes for both ways to read the same. I've always run the systems for at least 20 minutes before testing so it's a non issue. Infrared is a lot quicker and I don't need dozens of thermometers to do a large house.

  9. #9
    Join Date
    Jul 2012
    Location
    Los Angeles, CA
    Posts
    4
    What I basically need to know is if the ducts in the ground are "healthy" would one still expect it to take a long time for them to more or less equalize or catch up to temperature?

  10. #10
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,183
    In slab ducts are common around here. The discharge air temp normally is slow to catch up but should be able to do so within 30 minutes. If there is water in the ducts it will affect air temps also.

    We normally end up sealing them up and running new ducts in the attic.

  11. #11
    Join Date
    Nov 2012
    Location
    Georgia
    Posts
    13
    A blower door test would give you an idea of how much air leakage you are getting. Around here in North Georgia, I have only seen one system with underground duct and it was PVC.

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