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    Power vent condensation problems (20 Posts)

  • wml52 wml52 @ 5:49 PM
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  • icesailor icesailor @ 8:01 PM
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    Power Vent:

    I don't believe that I have ever quite seen anything like that.
    It looks like an attempt at "sealed combustion" with an oil burner. They are known to do that, condense like the fog on a hot , humid summer day.
    There is some sheet metal around the air band on the oil burner but I don't see any pipe. There is a 4" smoke pipe take off on the exhaust that goes down the side of the furnace, But I can't tell where it goes.
    The fire side of the HX must be in tough condition. The exhaust gas temperature must be very low to condense like that. Like 200 degrees.
    Get that sealed combustion set-up removed pronto. I've not seen anything quite like that. It can't be normal.
  • billtwocase billtwocase @ 9:59 PM
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    looks like

    a mess. Not a direct vent, not a power vent, but where does that flexible conduit go that's thru the plate?
  • don don @ 6:04 AM
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    condensing

    The swg series power venters are design to bring in combustion air as well as exhaust it.I see no test hole for checking draft or stack temps.
    I am going to say it under-fired along with draft issue.It may even be the blower is pushing to much air which is good for efficiency but not for keeping her out of condensing mode.Any test numbers from the company that been out before?
  • wml52 wml52 @ 6:49 AM
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    condensation problem

    Unfortunately no numbers from the HVAC contractors that came out. There is a test hole for measuring draft n temp just difficult to see in photo.
  • don don @ 7:38 AM
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    quick check

    For you to do if you have a thermometer.You want to drill a hole in supply plenum on at the furnace make note of the temp.Also take another reading at the return going into the furnace.the difference of the two temp is what we call temp rise.Post that number and we might be able to point you to it being under-fired or if you are getting too much airflow across the system.
    I would suspect more then likely she is under-fired but i have seen too much airflow in some cases with furnaces that have a 5 ton fan drive on them.
  • wml52 wml52 @ 9:19 AM
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    condensation problem

    I'll be on the road until Thursday. I'll take some readings with my thermocouple and give you the numbers. Appreciate the help.

    Bill
  • Jim Davis Jim Davis @ 9:44 AM
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    Condensate

    Underfiring could add to a condensate problem but the actual problem is the there is improper post-purge on the venter.  Having sold hundreds of power venters over the years for gas and oil, I found that oil required at least 8 minutes of post-purge to eliminate condensation, after-drip and smells.  I found in many cases 5 minutes was never enough for oil but just fine for gas.
    Field Controls offers many post-purge kits for their venters because of my input over the years.  Just find a contractor that knows what they are doing.
  • Bob Harper Bob Harper @ 12:11 PM
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    post purge

    I agree with Jim 100% on the post purge. I would add that if you're tired of replacing single walled galvanized chimney connector, you could replace it with type L vent, which has a stainless steel inner liner and warranted for 10-25 yrs depending upon mfr. and is listed to UL 641 for oil. Otherwise, pp. You can measure the time required simply by jumping out the exhaust so it runs constantly after the burner has shutdown and measure the stack Rh%. to chart them and see how long it takes to dry out. You can also disconnect the pipes after one minute intervals and manully inspect for presence of moisture condensed on the walls of the pipe or not. Also look at your CAZ Rh%. If your basement is soggy then no amount of post purge may dry it all out. Look to your gutters and leads away from the house, ground water, sump pumps, and indoor sources of moisture vs. ventilation. A $30 meter from Radio Shack can tell you your Rh% close enough.
  • wml52 wml52 @ 12:57 PM
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    condensation problem

    I can tell you that during the winter months the temp in the basement averages about 43 degree F and the RH ranges from 51% to around 62%. Post purge is currently set at 4 minutes.
  • Jack Jack @ 3:24 PM
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    Guys

    Do you think an oversized furnace short-cycling like crazy is a part of the problem? I would look down on the HX right away. It will not last with the current condition. As proposed, Double wall vent connector is always a good idea. Your double wall pipe is hitting the exhaust gases with whatever temps are outside pushing the stack temps below dew-point temps. Disconnect the cold air intake side for a while, making sure your set-up is correct and I'll bet things will improve. As noted, this is an attempt to create a kinda/sorta direct vent system with a single wall penetration. I'm thinking you are going to have short life from this furnace HX.
  • billtwocase billtwocase @ 5:50 PM
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    gotta disagree

    a warm air furnace, an 8 min post purge pulling outside air will most likely create a condensation problem. A boiler with about 5 min would be OK. With the furnace blower and the power vent running together on the burner off cycle, post purge on that should be about 3-4 min. I also see no spillage switch, and where is the CK-61, 62 located? I would also eliminate the combination air intake/exhaust. You are probably getting cross contamination. Also, what are the breech and over fire draft readings?
  • icesailor icesailor @ 7:28 PM
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    Hints:

    That's sort of what I was hinting at.
    Looks like cross contamination to me.
  • wml52 wml52 @ 7:23 PM
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    Condensation problem

    See attached photo for location of CK-61
  • wml52 wml52 @ 5:09 PM
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    Condensation problem

    Just an FYI regarding the makeup air intake off of the power vent. This problem existed prior to installing the intake for the makeup air, it was installed after speaking with Field Controls who suggested it. So whether or not the make up air is supplied from the outside or  from the basement air it did not make any difference, the condensation problem was the same.

    Bill
  • Jim Davis Jim Davis @ 8:41 AM
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    post-purge

    Post-purge can be trial and error on most installation.  When I sold 90% condensing furnaces with 100% outside air, we had problems with the factory installed 5 minute post-purge.  Went to 8 minutes and problem was gone.  Are there some short flue situations that 5 minutes might work?  Sure!  It has to be long enough to stop the white powder or condensation from developing.
    Cross contamination could lead to sooting but it would not contribute to condensation.  I don't like cold air for combustion air when it can be avoided.  Cold air makes equipment the least efficient and harder to adjust especially because the temperature of outside air is not constant.
  • Jean-David Beyer Jean-David Beyer @ 11:19 AM
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    I don't like cold air for combustion air when it can be avoided.

    I am not trying to sell anything here.

    But it is interesting that the design of the Weil-McLain Ultra boilers use an uninsulated heat exchanger. The burner is inside the heat exchanger.

    That boiler is mainly that heat exchanger inside a box. The combustion air comes into the box at the top, and the intake for the actual burner comes from a little below the middle of the box. So any heat lost from the heat exchanger serves to warm up the incoming combustion air.

    Now the temperature of the heat exchanger is probably fairly low. I have not tried touching it, or measuring it, while the thing is running, but I may measure it sometime. It should not be much more than the temperature of the supply water coming out.

    I do not know just how much heat is lost from a heat exchanger that usually puts out water between 75F and 135F. So I do not know just how much it warms the combustion air. But that is what it does.

    Once the call for heat ends, the circulator(s) run for 30 seconds up to 120 seconds (depending on where the heat was going) and that pretty much empties the heat exchanger of hot water, so the heat is not wasted.
  • wml52 wml52 @ 9:32 PM
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    condensate issues

    Sorry it's been a while since I last posted, but had a family emergency that took priority.

    After reading the replies I took a few measurements and these are the results. Keep in mind that the readings were taken weeks ago so the outside ambient temp was still rather mild.  I didn't get a chance to post them due to the emergency I had.

    * Basement temperature: 58.9 deg F RH 55%


    * Combustion temperatures @ 7 minute interval: Combustion temperature at burner inlet 61.5 deg F @ inlet of power vent 424 deg F
     
    * Combustion temperature @ 8 minute interval: Combustion temperature at burner inlet 62.4 deg F@ inlet of power vent 426.5 deg F
     
    * Combustion temperature @ 10 minute interval: Combustion temperature at burner inlet 63.7 deg F@ inlet of power vent 432 deg F

    * Post purge run time 00:02:30, temperatures after post purge run time:

     
    * Combustion temperature at burner inlet 60.8 deg F @ inlet of power vent 125.4 deg F     

    Currently the basement temp was 49 deg F and the RH was at 43%, which is about where it remains during the winter months.

    Hope this was helpful it trying to determine a fix.

    Thanks to all.

    Bill
     
         
  • Jim Davis Jim Davis @ 11:51 PM
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    Post-purge

    Still have flue gasses in the chamber after 2:30 minutes.  Increase post-purge to 8 minutes and you will be fine.  This is based on actual field situations that I was involved in and ask to solve the problem.  I wish I knew why oil takes that long to purge all the moisture but it does.  It also helps prevent delayed after drip.
  • wml52 wml52 @ 8:10 AM
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    condensation problem

    Hey Jim,
    Thanks for the input, I have increased post purge time to 8 minutes. Will let you know the long term results.

    Bill
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