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  1. #14
    Join Date
    Dec 2007
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    USA, Midwest
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    35

    CO, venturi effect, now I'm getting confused.

    Quote Originally Posted by DavidInAustin View Post
    Excellent question my friend Gus - Venturi effect... (The blower motor is far stronger/powerful air displacing/moving effect than the inducer motor). Venturi effect will pull some CO from the tube ( more or less effected by location of hole in respect to cross airflow) as the air from the blower passes by the exchanger at 1000 - 2000 CFM - if it has a leak.

    If your not familar with "Venturi" - Picture a glass of water with a straw in the glass. If you stick the straw near your lips and blow hard across the straw tip - the water will lift from the glass through the straw and move upwards. If enough cross air is produced - the water will come out of the top of the straw.

    In earlyer times - One would remove the burner assy - wet the hand - stick your hand in each exchanger as the blower was running - If your hand felt cooler in one exchanger compared to the ajoining exchanger - bingo...theres a leak (Note: A very small leak can be detected this way, even today!!) - also Venturi effect!
    DavidInAustin ...

    Yep, I get the venturi effect, like primary air being pulled into a burner.
    Where I'm getting lost is when the system's static pressure is taken into account.

    If a furnace blower generates .1" WC static pressure on the supply side and has an a/c coil sitting on top of the furnace, wouldn't the system static pressure kill any venturi effect?

    I can see where the velocity on a 3-ton system (for example) might be around 1,000 fpm which would certainly be fast enough to create a venturi effect.

    However, with .1" WC pushing on the outside of the heat exchanger, in all directions, how can the venturi effect take place?

    Whenever I've encountered holes/cracks in heat exchangers, there's always air blowing into the heat exchanger creating a flame disturbance.

    I once had a downflow furnace with a 4-cell heat exchanger where the tops of the cells had almost rusted through. You could see a million small holes all over the top of each cell. There wasn't a CO issue (based on the customer's description, CO wasn't measured) but there was a lot of flame disturbance in every cell. Not to mention a large accumulation of rust in the bottom of the cells.


    OK, last question. Why were the burners on the job you discussed generating 400ppm CO, and what was the location of the measurement? I've only seen readings that high when burners were physically misaligned and were directing the flame against the side of the heat exchanger. But, I haven't been checking every job, so I might be missing something,

    Regards,

    Gus

  2. #15
    Join Date
    Nov 2007
    Location
    Austin Texas
    Posts
    54

    My understanding.

    First of all - .1"Wc STATIC is nothing in force compared to the venturi of 2000 cubic feet of air per minute across an exchanger. (or any other environment outside of the furnace casing - including "THE EXCHANGER") Dont be confused with how air is affected in ductwork outside of the furnace as compared to inside the furnace - It is two different environments.

    just like water holding up in the drain pan untill the unit turns off. The air is being sucked into the furnace casing and moving downstream to the furnace - the same happens in the exchanger because its environment is not the same as the environment inside of the furnace casing - How do you think humidifiers like april-aire work? Is it the suction of the return? Pressure of the supply? or venturi? or all three?

    ***Keep in mind - It can do both - suck in and blow out - this would be determined by where the hole is at physically. In front of the stream of blower air - to the side, or even behind. - even in inducer motor units ***


    This is my understanding of the physics of the matter.


    Here is some supporting data on the matter - As follows - will find more for you.


    jrupert

    Yes it is possible to have carbon monoxide leak into the home from a heat exchanger in a inducer draft fan furnace. The pressure within the heat exchanger is still greater then that out side it until the blower kicks in and even then the risk of pulling C02 in to the house air is present.

    Yes deaths have occurred here just a few
    http://www.dailystandard.com/date/20.../headline2.htm (http://www.dailystandard.com/date/20.../headline2.htm)
    http://www.carbon-monoxide-poisoning...y.com/news.htm (http://www.carbon-monoxide-poisoning...y.com/news.htm)
    http://www.homesafe.com/coalert/cofacts.htm (http://www.homesafe.com/coalert/cofacts.htm)
    http://www.coheadquarters.com/COEpidem/coepidem02.htm (http://www.coheadquarters.com/COEpidem/coepidem02.htm)

    --------------------------------------------------------------------------------

  3. #16
    Join Date
    Dec 2007
    Location
    USA, Midwest
    Posts
    35

    I think it's my understanding that might be faulty

    David,

    Thanks for the repy and for sticking with the topic.

    I checked out your CO incident links. It's situations like these that have prompted my concerns about heat exchanger cracks.

    I'm having trouble reconciling my experience with what happens in "theory". Experience shows that cracks in heat exchangers produce air disturbances from air entering the exchangers through the cracks or holes.

    I've used smoke bombs trying to trace air flow by lighting 4 at one time, one in each cell of a heat exchanger. I've had smoke coming out at the burners or simply going up the flue (atmospheric burners), but I've never seen smoke make it into the blower air stream or into the house.

    I've also flooded the cells of a heat exchanger with R-22, kicked on the blower and tried to detect refrigerant in the home with a GE H10 leak detector. Nothing was detected until I went into the basement and opened the blower door for a couple of seconds. Then, I could here the H10 screaming, so I know it was working and picking up ambient refrigerant. But it took opening the blower door to get the refrigerant into the air stream, just the holes in the heat exchanger didn't do it.

    As a side note: I've also seen round branch take-offs on a rectangular supply duct either deliver NO air, or create a slight negative pressure. That was due to where the take-off was positioned relative to a turn (elbow) in the main trunk duct. So I know the "venturi effect" is alive and well, and it "can work in mysterious ways."

    April-air humidifier ... it's simply air flow, not venturi effect. Bypass style humidifiers use a supply duct piped in one end, while the other end is connected to the return duct. It's an apples/oranges comparison. The humidifier depends on receiving dry warm air which is routed past a drip-panel or a mist nozzle. The air picks up moisture via evaporation and the air is then routed back to the return duct so it can be distributed throughout the house. Power humidifiers have a fan that pulls in some supply air and discharges the humidified air back into the same air stream.

    What I would like to try, but never had the resources or courage to attempt, is to light-off a high-output, purple or red signal flare in a furnace with a cracked heat exchanger and see how much color actually gets into the air stream.

    I've always thought the grey or black smoke bombs were getting dilluted, and that's why I couldn't see it.

    Back to the question on the 400 ppm furnace. What was causing the burners to put out 400 ppm? I thought that a CO reading that high would be causing soot issues.

    As far as CO deaths due to heat exchanger cracks, I'm not so sure. Every situation I'm familiar with has had extenuating circumstances that actually caused the CO.

    The incidents have had blocked flues, damaged flue pipes or backdrafts caused by other appliances or conditions. The common element has been loose return-air ducting that has allowed the blower to pull air with high levels of CO into the living area and the fact that the products of combustion were routed back into the heat exchanger and reburned.

    In every situation there have been at least two problems that contributed to CO production. Unfortunately, nobody seems to be tracking and reporting the other items. Everyone always turns their attention to the furnace or space heater as the culprit.

    Regards,

    Gus

  4. #17
    Join Date
    Nov 2007
    Location
    Austin Texas
    Posts
    54

    Thank you the same.

    One more very important point to be made (Most may know this but still dont really understand it) - Carbon Monoxcide (Mixed) is slightly more heavy than air(this is key!). IT BUILDS UP in a confined and simi confined area ( like water dripping in a glass taking the place of the air - in a short time a drip becomes a gallon). What might be 10PPM in one hour will be 40PPM 15 min later - and so on...This is how it kills more often than not. It (CO) does not mix with the air like you might think - It does not break down and deminish in a confined area. (most often overlooked and explained incorrectly by the tech to the consumer) - (ie.. "its a small leak(or just rust even) but will get larger in time" - you might need to replace this sometime!") This is a problem!!! The CO builds up from a small leak and becomes a killing ground in confined areas!!!!! The return air back to the furnace may not carry a large portion of the CO back through the system depending on the physical location of same ( return in hallway - bedroom doors closed - return high) That i have learned through research. If there is a suspected hole - Find it and condem the furnace - even if it is rust!!! Condem it. No life of any child or even those nasty adults is worth less than even 1 million brand new furnaces! All the points you have made is common knowlege among most people in the field. That being said - Nothing is impossible and people have been killed in many different situations concerning fuel burning machines. I have taken the view that anything can happen when the situation arises and presents itself - often more than not - there are many situations that come together to make an end result happen. This you can be sure of. Like it or not, Physics is a part of this field in many ways. To understand more than a picture shot of a situation is the difference in being a good tech and a great tech. When it comes to furnaces - Its imperative to learn and teach. If your not teaching the lessor skilled techs - Its a crime. It doesnt matter who has been doing it longer - WE all learn if we keep our pride low and be humble. We all can contribute.

    One interesting thing to me is - More people are killed by improperly sealed blower cabinets and flue problems than any other cause - Funny how many techs cut out the door switch and wire past it even today!!!! And if people were completely triuthful - The tech more often than not does not even look at the flue beyond whats coming out of the furnace! (The three screw rule has a reason!)

    Kills me!

    D

    There are many other points to be made that are so important, unfortunately I cant write a book here! Thank you so much. I have total respect for you. Good day.

  5. #18
    Join Date
    Oct 2007
    Posts
    232

    huh?

    DavidInAustin
    I’m confused about the information you posted it sounds like you are saying if you find rust you should condemn the furnace?

    Doesn’t carbon monoxide mix with the air currents and will likely be higher up in the room since it has been heated from combustion?

    And that TP7 you mentioned in a earlier reply is that a combustion analyzer or a CO detector?

  6. #19
    Join Date
    Nov 2007
    Location
    Austin Texas
    Posts
    54

    Ok - Here we go...

    Rust explination - Understand this...(I mean -not- light surface rust). Often when you take an awe and poke at rust on an exchanger - you will find it has penetrated far beyond the surface. I have even surprized myself, because you cant tell by looking at it! you have to probe it. The exchanger is designed to heat up (EXPAND) and cool down (Contract) so many times over the life of a furnace. The exchanger is in constant movement. When the metal rusts ( I am speaking about paper thin high efficiency exchangers at this point) The rust does not expand and contract at the same rate ( acually it separates and breaks down) unlike unrusted metal. It begins to separate (molecularly) and tear.

    Case in point concerning my point - From the time of your checking it and the next time (maybe 2-3-5 years) with some customers - the damage could be much more significant. It is our job to inform the customer of this hazzard. Give them a choice to head off the impending situation. To attempt to correct its causes and slow it down if possible. ( if you dont know what causes it in most cases - keep reading)

    It even becomes more active in destruction with alkalines caused by inefficient burn mix.(air - fuel - spark) Even the wrong size or poorly designed duct attached to the furnace can destroy the exchanger much quicker - Mainly ( hotter, more expansion and contraction beyond design and life) Dirty (repeated neglect of consumer) filters - clogged (even partially) evaps / ie..TD


    I have to reread the rest of your question to answer what was asked - I hope I have helped a bit.

    D

    PS _ TP-7 Analyzer
    "Doesn’t carbon monoxide mix with the air currents and will likely be higher up in the room since it has been heated from combustion?" - Yes and No, it contain's much moisture from combustion and is heavy - It cools and falls. ( keep an open mind - some may travel and mix with the "air" - conditional stratification would be conditional - Mechanical mixing (furnace) will affect when it is running. This depends on infiltration of home/Cold Air Conditions - .In fact CO detectors should be put or located at different heights and locations throughout the home for this reason.

  7. #20
    Join Date
    Aug 2003
    Location
    Central Kentucky
    Posts
    6,248
    Carbon monoxide is not heavier than room air but carbon dioxide is, this is where the problem arises. Combustion air displacement being a result of this in many cases.
    The only time that I am aware of that carbon monoxide is heavier than air is at freezing temperatures.

    If a furnace has rust in it the problem with it should be located and identified as it is most likely related to a venting issue.
    Shutting a rusted furnace down and replacing it does not guarantee the conditions that created the rust in the first place were corrected.

    Don't make the mistake of thinking that a furnace is safe because it is not producing soot, it takes thousands of PPM of CO to produce soot.

    Could not count how many furnaces I have tested running high CO with no signs of soot and blue flames.
    Have you set up a Google alert for Carbon Monoxide yet?
    Click here to find out how.

  8. #21
    Join Date
    Nov 2007
    Location
    Austin Texas
    Posts
    54

    hmmmm

    Let me get the proper data and I will show you why...brb

  9. #22
    Join Date
    Aug 2003
    Location
    Central Kentucky
    Posts
    6,248
    Carbon Monoxide has a molecular weight of 28.011
    Carbon Dioxide has a molecular weight of 44.01
    Oxygen has a molecular weight of 31.998
    Nitrogen has a molecular weight of 28.0134
    Have you set up a Google alert for Carbon Monoxide yet?
    Click here to find out how.

  10. #23
    Join Date
    Nov 2007
    Location
    Austin Texas
    Posts
    54
    This is my reasoning and from what I have read must be taken into account. In real world conditions (not pure CO, or Pure Oxygen) The connection and answer is in the mixes friend...

    For mixed gasses just take a proportionate average:
    Air is 80% N2 + 20% O2 .
    air = 0.8(28) + 0.2(32) = 29 (exactly neutral, by definition)

    So pure carbon monoxide is actually about 3% lighter than air.
    But usually it is mixed in with the normal combustion products: CO2 - (CO2= (12+16+16)= 44, heavy)
    and H2O. Which are always mixed with the 80% Nitrogen that never participates in burning.


    Hmmmm....Think about it! Gn

    D

    PS - If I am wrong about this please let me know.

  11. #24
    Join Date
    Oct 2007
    Posts
    232
    I agree with everything davidr said.


    DavidInAustin
    Quote Originally Posted by DavidInAustin View Post
    PS _ TP-7 Analyzer
    That’s what I thought you were referring to but it threw me off when you said that it read 0 ppm on most of the PM’s you’ve done over the last 6 months. It’s extremely rare to come across a furnace that is safely running at 0 ppm. What is the accuracy of this meter? I would question all these 0 ppm readings you’re getting because to me it sounds like it isn’t working.

  12. #25
    Join Date
    Oct 2007
    Posts
    232
    Quote Originally Posted by DavidInAustin View Post
    This is my reasoning and from what I have read must be taken into account. In real world conditions (not pure CO, or Pure Oxygen) The connection and answer is in the mixes friend...

    For mixed gasses just take a proportionate average:
    Air is 80% N2 + 20% O2 .
    air = 0.8(28) + 0.2(32) = 29 (exactly neutral, by definition)

    So pure carbon monoxide is actually about 3% lighter than air.
    But usually it is mixed in with the normal combustion products: CO2 - (CO2= (12+16+16)= 44, heavy)
    and H2O. Which are always mixed with the 80% Nitrogen that never participates in burning.


    Hmmmm....Think about it! Gn

    D

    PS - If I am wrong about this please let me know.
    Another way to explain this is to compare it to smoke from a fire. Visible smoke from a fire is a particulate which is heavier than air, but it rapidly rises to the ceiling because of the heat. The same applies for CO spilled from an appliance. It will rise to the ceiling and will always be at a higher concentration near the ceiling.

    Molecular weight of:
    Carbon Monoxide----->28.01 Lighter
    Nitrogen----------------->28.0134
    Air------------------------->28.975
    Oxygen------------------>32.00
    Carbon Dioxide------->44.01 Heavier

  13. #26
    Join Date
    Nov 2007
    Location
    Austin Texas
    Posts
    54

    Thank you

    Goodnight all...

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