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Thread: why does my furnace have two 3/4 npt female coupling?

  1. #1
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    why does my furnace have two 3/4 npt female coupling?

    Bought a house built in 1969. The central heat/air unit is newer then that. No makers name, no data plate visible. It is downflow. On the front of the unit near the bottom it has two welded in 3/4 pipe couplings. It looks like a piece of tin was taped over them to cover them over. that fell off. Any thoughts on what they may be. The condensate drain pipe is right below them and drips into the crawlspace.
    I do not think this unit is new enough to be a high efficiency condensing unit. It does have a small fan and motor that appears to be forced combustion air or something not the main blower fan and motor.
    Bil lD.

  2. #2
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    Sounds like a unit that can be installed sideways and those two would be the primary and secondary condensate drains.

  3. #3
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    My ac/heat pump has fittings in the same location for heating hot water

  4. #4
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    I think Bruce has it nailed. It has two sets of two. The two I will plug on the side and two at the bottom. One of the bottom ones is the drain line plumbing and the other is just open. I wonder if conditioned air is being blown out through the drain line all the time?
    Can I plug the unused lower one. It looks like rusty water has seeped out of it from time to time. Maybe it is when the condensate drain plugs?
    Bil lD

  5. #5
    Quote Originally Posted by Bill Dufour View Post
    I think Bruce has it nailed. It has two sets of two. The two I will plug on the side and two at the bottom. One of the bottom ones is the drain line plumbing and the other is just open. I wonder if conditioned air is being blown out through the drain line all the time?
    Can I plug the unused lower one. It looks like rusty water has seeped out of it from time to time. Maybe it is when the condensate drain plugs?
    Bil lD
    Agreed; it certainly sounds like the pri/sec condensate drains - one pair for veritical install, the other for horizontal. You'd have to look inside the unit to determine which is which (unless they somehow got marked for this?). Condensate should reach the primary first, as the name might imply. Primary should go to your DWV / septic /sewer connections; secondary should drip where you can see it. Lot of attic based secondaries around here drip from the soffit near the compressor, or over a window. If it drips, the primary is deemed to be plugged.

    Some new, small, window-sized units will drip the (evaporator) condensate on the condenser coil - helps cool the coil and the water evaporates to outside air. They never drip - - in theory.

    Perhaps humidity isn't an issue in your area and you get virtually no condensate ...?? If not, I'd recommend you re-route at least the primary drain. I'd not want it under the house. (I'll spare you the horror story of my neighbors.) You could leave the secondary as it exists - not ideal, but at least you'd see it at filter changes??

  6. #6
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    I think the primary goes into the crawlspace and comes out the side of the house. There are actually two 3/4 pvc pipes that look like condensate drains to me. one near the ac unit and one 45 feet away but maybe a water heater, dishwasher or washing machine drain.
    I have not crawled under the house to look yet.
    It is dry here in summer so not a huge amount of condensate.
    Bill D
    Last edited by Bill Dufour; 03-02-2021 at 1:42 PM.

  7. #7
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    A properly plumbed condensate system has:
    Primary piped with a slope downhill to exterior or to a sump pump. Never to a sewer system because even with a trap in the attic for example it will dry out and sewer gas will escape and enter the house via the Hvac unit.

    The secondary port on the unit can be seen by looking into it for the dam or baffle that keeps water from coming out unless it is deep enough.

    Secondary piped in one of three ways:
    To a safety pan that has its own drain pipe or its own water level switch to turn off unit. This is the best method.
    Or to a visible place outside so it will be noticed dripping water to alert homeowner that the primary is clogged. The problem with this last method is that very few ever notice it.
    A safety pan must be under the unit if leaks will cause damage.

  8. #8
    Quote Originally Posted by Bruce King View Post
    A properly plumbed condensate system has:
    Primary piped with a slope downhill to exterior or to a sump pump. Never to a sewer system because even with a trap ...
    Perhaps codes are different? In 14-odd houses over my adult home-ownership life, all but one HVAC system was in the attic, and all those primaries have drained to a vent passing thru the attic; secondaries to the soffits. My brother builds homes, and all his are so constructed. ...I've never heard anyone howl over sewer gas in their cold air??

    Mr. Dufour, my apologies. My read of original post and 'drips to crawlspace' led me to believe it was just dripping in the dirt there.
    Last edited by Malcolm McLeod; 03-02-2021 at 7:52 PM.

  9. #9
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    I think the secondary is just dripping into the crawlspace. There is hardware cloth around the floor so combustion air can come from the crawlspace. The secondary has rust stains running down so it has had water overflowing above the primary drain. I will clean the rusty water stains off and see if they reappear this summer. I may even hit it with some white paint if it does not clean up enough.

  10. #10
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    I've only saw one house like that. I doubt it’s allowed anywhere now unless they are using an older code cycle. Codes do differ but not all building inspectors go into the attic and if they do it’s much more limited than what a private inspector does.

    Quote Originally Posted by Malcolm McLeod View Post
    Perhaps codes are different? In 14-odd houses over my adult home-ownership life, all but one HVAC system was in the attic, and all those primaries have drained to a vent passing thru the attic; secondaries to the soffits. My brother builds homes, and all his are so constructed. ...I've never heard anyone howl over sewer gas in their cold air??

    Mr. Dufour, my apologies. My read of original post and 'drips to crawlspace' led me to believe it was just dripping in the dirt there.

  11. #11
    Quote Originally Posted by Bruce King View Post
    I've only saw one house like that. I doubt it’s allowed anywhere now unless they are using an older code cycle. Codes do differ but not all building inspectors go into the attic and if they do it’s much more limited than what a private inspector does.
    For reference, following is code requirements from Dallas TX site; they are 'the elephant in the room', and most regional municipalities tend to track what they require. (I did not analyze the local amendments...I like to be right, but work beckons. - Sorry.) I GC'd my last house in an exo-burb in 2010 - local Code Compliance checked the attic and it's 2 HVAC systems quite diligently. I've owned 5 homes just in this 4-county metropolitan area, built between 1929 and 2010. Some, including current, got major re-work during my occupancy, all followed this drain plan. My brother builds ~100 homes/year in multiple code jurisdictions; he confirms same drain scheme.

    The secondary soffit drain on my current house blows a tiny amount of air when HVAC fan is on, so I assume the primary is the same. They ARE in the fan plenum, so this makes sense. Seemingly this would push sewer gas outside - if any intruded to either drain?


    CHAPTER 16: 2015 International Fire Code, including Appendix J, with Dallas Amendments (effective October 1, 2016)​
    The Fire Code amendments include adoption of the 2013 Edition of the following Standards: Sprinklers, NFPA 13, 13D, 13R and Fire Alarm, NFPA 72.
    CHAPTER 53: 2015 International Building Code with Dallas Amendments
    CHAPTER 54: 2015 International Plumbing Code with Dallas Amendments
    CHAPTER 55: 2015 International Mechanical Code with Dallas Amendments
    CHAPTER 56: 2017 National Electrical Code​ with Dallas Amendments (effective March 1, 2018)
    CHAPTER 57: 2015 International Residential Code with Dallas Amendments
    CHAPTER 58: 2003 International Existing Building Code with Dallas Amendments (effective July 1, 2004)


    CHAPTER 59: 2015 International Energy Conservation Code with Dallas Amendments
    CHAPTER 60: 2015 International Fuel & Gas Code with Dallas Amendments
    CHAPTER 61: 2015 International Green Construction Code with Dallas Amendments

    ****************
    Codes vary. A great deal, apparently?
    Last edited by Malcolm McLeod; 03-03-2021 at 8:30 AM. Reason: ugly formatting

  12. #12
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    Data point from SoCal, not sure how codes differ from further north. House was built in 1976 and has a first-floor vertical updraft unit.

    My relatively new system has four fittings on the AC evap unit, two each for vertical and horizontal (which are plugged). On mine, the lower one goes to the drain, which goes down through the plenum into the slab, exits the nearest exterior wall the other side of the kitchen. There is no secondary drain as such: the "upper" fitting (maybe 1/2" higher) has a widget screwed into it that detects if it fills with water and is wired back to disable the whole system if that occurs.
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  13. #13
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    this blowing conditioned air outside kind of puts the lie to a sealed house saves energy. Do all you can to seal air leaks then use a powered blower to get around all that sealing.
    Bill D

  14. #14
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    That’s an extremely small pipe for an air leak but the fix that is only on a few houses owned by Hvac techs and home inspectors is to put p-traps on those and fill the secondary trap with mineral oil and prime the primary if it drys out over the winter.

    The houses that are not built to newer codes which is a low percentage of existing houses have a combined air leakage to equal to a 6 to 10 inch pipe.

    Some of new homes that are very well sealed have a fresh air intake pipe about 8 inches diameter. It is piped to the Hvac return usually and pulls air in only while the unit is running. This is usually not programmed to shut off in humid weather and causes a humid house in the lower half of most of the country. Not all codes are right.
    Last edited by Bruce King; 03-03-2021 at 10:15 PM.

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