Water running out fresh air inlet pipe

Discussion in 'HVAC Heating & Cooling' started by DaveHo, Jul 26, 2011.

  1. DaveHo

    DaveHo Member

    Messages:
    53
    Location:
    SE PA
    I had a new condensing gas furnace & central A/C installed last fall. The furnace/evaporator are located in my basement. Lately, it's been very hot & humid in my area. The other day I noticed a small puddle of water by the furnace. Near as I can tell the the hot humid air from outside is condensing in the air inlet pipe & running back to the furnace.

    Due to this pipe running perpendicular to the floor joists & existing my basement via the rim joist it is not possible to slope it toward the outside. I could plug the pipe at the outdoor exit point, but I really would like to avoid a "solution" which would render the furnace inoperable should I forget to remove the plug prior to the heating season. Currently most of the water is running out of the pipe where it meets the collar on the furnace. Only a few drips are actually making it inside & nothing electrical is getting wet, but still not a good situation. My initial thought is to put a drip leg somewhere on the intake pipe to catch the condensation. Can anyone offer up a better solution?

    The install is currently under warranty & I should probably make this the installer's problem, but I thought I'd get some opinions first.

    Thanks.

    -Dave
  2. Runs with bison

    Runs with bison Member

    Messages:
    897
    Location:
    Midwest
    Same problem here. It is just a few drops for mine and is hitting the top of the water heater. I remember the installer saying they are not allowed to glue/seal the inlet air pipe at the joints. It is the elbow joints that are dripping on mine.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,273
    Location:
    New England
    This may be normal, but it seems that your supply/return ducting isn't balanced as the a/c shouldn't be sucking air in through the exhaust port on the furnace! In fact, there should not be an air leak between the burner assembly and the distribution air system, at least one big enough to pull enough air from outside to cause condensation. Unfortuneately, while a supply duct might be sized properly, it is not uncommon for the return to be wrong. This puts the house on a negative pressure situation, and the system will then pull air in from anywhere it can. It might be acceptable to put a damper on the exhaust duct that should minimize this problem.

    Note, it is not uncommon for the fan speed to be higher in the summer during a/c operation than it would be in the winter during heating, so this negative pressure situation may not be as bad or exist then. But, if it does, it would be sucking exhaust fumes into the house...make sure you have a good CO detector.
  4. Runs with bison

    Runs with bison Member

    Messages:
    897
    Location:
    Midwest
    I can't speak for his system, but I know that the returns on mine are insufficient. The main supply return ducts are matched in size but there are serious choke points on the individual return runs; I can tell that the builder's original HVAC contractor didn't understand how to keep things even close to equal area throughout a run--probably never occurred to them. This is particularly an issue with the stud cavity returns from upper floors where there are 90's. I had the seams taped on my new install, but the original runs elsewhere are leaky and I can't get to most of them. With most of that being between floors and connected to the utility space it draws some there.

    However, in my case it's not much air entering through the PVC supply duct from what I can tell. If I glued it, this leakage would stop. I haven't tried bagging the external snorkel and rigging to measure flow rate. I actually think that it is more a factor of the elbows/pipe sweating internally, then dripping out the joint rather than pulling that much air in. The temp of the walkout/utility room space is below the dew point of the outside air this summer (and with this line closest to the supply ducting it's likely even colder than that right where the drip is occurring.)

    It's not a CO problem on mine in winter. I put in several new CO meters in the last two years and have never registered anything. I checked this closely when the new furnace was installed.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,273
    Location:
    New England
    On mine, the joints are glued except for the one that connects to the boiler itself (condensing boiler/furnace, should be the same issues). It also slopes back towards the boiler where it is picked up by the trap and drain (which goes to a condensate pump). I've never noticed any leaks on the pipes anywhere. Because mine is a boiler, there's no path between the air handler and the heat exchanger, so that part is different. The instructions on mine say to have it slope back towards the unit, and not outside.

    For there to be condensation in the quantity you indicate, there has to be a fair amount of air moving as otherwise, it would reach equilibrium and stop fairly quickly. You may want to button up that space, and if no pad on the ground, add a plastic moisture barrier. There could be a fair amount of ground moisture coming in (in addition to the common foundation air leaks).
  6. Runs with bison

    Runs with bison Member

    Messages:
    897
    Location:
    Midwest
    We might be talking about somewhat different things. My duct system is far from ideal with return runs being insufficient so I know that the blower will draw some vacuum into the utility space, and that utility space includes most of the return duct runs--so every seam in them is drawing air.

    Not sure how you mean that is different unless you are using a radiant system with no air handler, or air handler for AC in a different room. The furnace side and air handling side are physically separated by the heat exchanger walls in mine as well. But any openings in the air handler or supply duct in the space could draw from any opening in the joints of the combustion air supply (or anywhere else in the direct vent combustion that were leaking or not fully sealed.) Home ducts are typically not very tight...unfortunately.

    Not sure what you mean there about the source. The source for the moist air is through the combustion air supply to the burner. Water from the ground, slab/wall air leakge etc. would have no impact on the drip. It's not that the room itself is humid and producing the drip. While I'm all for tightening the building envelope, ironically it would tend to have the opposite effect in the space: if the space is tighter, then the differential of the room pressure to the outside air will be greater so it it would draw some additional quantity of air through the combustion air supply joints.

    My back of the envelope estimate for my unit is that it will take about 200 cu. ft. of air every 12 hours to produce about 1 drop/min (higher than what it appears I'm getting) with about 1/4 of the ambient humidity outside condensing on the pipe wall. That's an air flow 0.28 CFM or 0.0046 CFS. Seems a reasonable air leakage rate. And not all of that air would have to enter as there is typically a breeze along the exterior wall where the combustion air piping is introduced.
  7. DaveHo

    DaveHo Member

    Messages:
    53
    Location:
    SE PA
    Looks like I need to clarify some things. I am not talking about supply or return ductwork. I am not talking about the 2" PVC combustion exhaust pipe which does have drain which goes into a condensate pump. I am talking about the 2" PVC fresh air INTAKE pipe which feeds the combustion chamber. This pipe is sealed everywhere except where it meets the collar on the furnace. As near as I can tell, the combustion chamber door is sealed, so apparantly I'm getting enough air movement past the collar that it is allowing the humid outdoor air to condense in the pipe. I'll try sealing the collar connection & see it that improves things.

    Incidentally, since the heat & humidity died down earlier this week, I'm not seeing the problem right now.
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,273
    Location:
    New England
    If the furnace isn't running, there should be NO air moving in the intake air pipe. The fact that there seems to be indicates a problem. Since the only link between the conditioned air and the air supply duct pipe is through the combustion chamber - that is where I think you may have a problem.

    Try an experiment - when the a/c is running, take a piece of paper and place it over the air intake on the outside. See if it gets sucked to the pipe. Or, if the shape of that opening isn't easy to do that, take a smoking candle or something like that to see if it is drawing air in. resolve that, and you should resolve the problem.

    Also, get some metal foil tape and seal all of the seams on the ductwork you can reach. If you can take them apart (often not the case) they make some mastic that does a better job.
  9. Runs with bison

    Runs with bison Member

    Messages:
    897
    Location:
    Midwest
    Most likely incorrect. The combustion chamber is not likely to be the issue, probably one of the least likely causes. Any negative DP with respect to the outside air will pull air through any opening in the pipe--that includes any unglued fittings. Doesn't take much air flow to cause a slow drip if the dew point is in the 70's.

    The air handler cabinets are not that tight on the suction side, nor are many filter housings. Add to that any duct seam leaks etc. and one should expect some negative pressure around the air handler. No, it isn't the ideal, but that seems to be the norm for installations as well as the air handler designs. Add to this utility rooms with a standard gas water heater flue and or a clothes dryer pulling air from the room and there are additional ways to create the negative pressure. Note also that many installers aren't keen on tightening up the penetrations where the gas line/condensate lines/etc. enter the furnace. Any openings there provide a path for entry of outside air.

    I used a rubber band to put a clear bag over the outside inlet pipe on mine 10 days ago. Haven't had a drop of water since even in record shattering heat and humidity. My system has two 45's and two 90's to thread it past other ducts, vents, and pipes. That along with the three seams from the standard reducer and inlet connection add up to 11 internal joints.

    The one thing to be wary of is a situation where outside air is sucked through in such a fashion that water is condensing and dumping into the furnace rather than draining from a joint. For that reason I'm probably going to cover the inlet piping each year when I go to cooling only--I might do the discharge side as well for the same reason, even though it does have a condensate return leg. The presence of condensation in the furnace is fairly easy to check by opening the front panel of the furnace during a time when drips are observed from the inlet piping.
  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,273
    Location:
    New England
    The air intake to the furnace should ONLY go to the burner, and not be able to be affected by any other system. If there's a path to the conditioned air, that could lead to CO poisoning since the intake and exhaust from the burner are obviously connected as well.

    A sealed combustion system should be just that - sealed.
  11. Runs with bison

    Runs with bison Member

    Messages:
    897
    Location:
    Midwest
    Doesn't differ from non-sealed systems in that regard...pretty much the norm for the past several decades since they are open to the air in the home. The CO trouble in the "sealed" systems would come from an ID fan failure since they aren't set up for natural draft.

    Fully sealed is a great theory, but that isn't how furnaces are built or installed from what I've seen, and it has been an eye opener for me. "Sealed" is an approximation of how they are actually set up. The basic cabinet design is nowhere near that tight and the supply/exhaust are not vacuum/pressure tested for leaks. If it isn't pressure tested, there is no certainty that a system is tight. The exhaust piping is glued, plus the exhaust side of the furnace is enclosed within the suction side of the furnace housing, so the CO laden stream is unlikely to be a problem. But the combustion air supply side is not as tightly sealed and does not contain condensate traps.

    And even if it was completely sealed inside the home, there would still be some moisture laden air circulating in both the inlet and exhaust lines unless they are capped during summer--particularly with the winds we get here. That could still produce a stagnant liquid leg in an otherwise sealed chamber. I've had similar issues in chem plants with dead legs accumulating condensate (be it water or organic condensibles or even two liquid phases) and even clathrates (solids...in addition to two liquid phases and a gas phase present in the legs as well.) In those very tightly sealed systems the only way to keep the DP legs clear in winter was with steam (or in some cases warm condensate using condensate temp valves) or electrical tracing.

    The quality of the basic seal is not anything like what I'm accustomed to in process plants where we don't want to let anything out. I'm accustomed to working with syngas (CO & H2 mix) industrially at pressures from several hundred psig up to about 1200 psig. I've worn CO monitors while process gas sampling and leak checking, and I keep an eye on the CO detectors in my home. Mine never showed anything at home, even when the initial install of my cabinet was less sealed--to the point that I could just detect a whiff of nat gas/exhaust when it was igniting.

    I'm not saying that one shouldn't attempt to tighten and seal things as best possible, I'm saying that the design & fabrication of these sealed systems is not as tight as one would anticipate based on the name and that one should assume there will be air leakage. That is why I anticipate bagging my inlet/exhaust each summer.
  12. DaveHo

    DaveHo Member

    Messages:
    53
    Location:
    SE PA
    Well, the installer's service department is coming out to take a look at things tommorow. When I explained the issue to him over the phone he seemed baffled. Not very reassuring. I agree that there is air movement which shouldn't be happening, but I'm hesitant to seal the pipe at the collar on the furnace. As was mentioned elsewhere, if there are other sources of air movement, then all that condensation will end up inside the furnace. As of now, only a few drips are making their way in and nothing important is getting wet. The easiest fix would be to block the intake on the outside, not the solution I'd really like. I'll see what the service guy comes up with.

    -Dave
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