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Condensing heat exchanger,outside the box?

Discussion in 'Home Power and Microgeneration' started by James Storm, Nov 30, 2003.

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  1. James Storm

    James Storm Guest

    You can do it but you must prepare your self for the consequences of your
    invention. Such as increased back pressure, incomplete combustion and water
    from condensation.
  2. Gunsmoke

    Gunsmoke Guest

    I understand the principle behind the hi efficiency furnace (home use) where
    they capture the
    last 10-15% of heat from the exhaust, but I have seen nowhere on the net and
    in discussion with furnace installation tech
    types that someone has added an extracting system in the exhaust portion of
    a mid-range efficiency one.
    That seems bizzare that something could not be cobbled up to do externally
    what the exchanger does inside the box.
    The secondary condenser seems to be just a manufactured chamber that is
    installed, independant of the primary exchanger, inside the unit, so why
    could this not be done after the flue?
    Anyone had any experience with this sort of thing?
  3. Bob Peterson

    Bob Peterson Guest

    The reason it is not done is that the increased backpressure would seriously
    affect the draft of the system making it probably unsafe. You would
    probably need to add a fan, and controls for that fan, and a lot of other
    stuff I can't think of off the top of my head.
  4. Steve Thomas

    Steve Thomas Guest

    There used to be commercial units available for oil fired furnaces. I
    don't know it they are still available. The one I looked at (in about 1986)
    was approved for several models of forced air furnace. Oil burners have
    forced air combustion but still require chimney draft for proper operation.
    Both of these are adjustable, so maybe that is how they were able to get
    them approved. Anyway, I didn't buy it. One of my objectives at the time was
    to eliminate the need for a conventional chimney, and that would not do it.
    Also, it required a water hook up, I don't recall why. There probably is
    still someone making such a device. Have you done any Google searches?

    Steve Thomas
  5. Gunsmoke

    Gunsmoke Guest

    My ex just had her f/a upgraded to a mid-e, and the first thing I saw was
    the extra exhaust fan mounted after the primary
    exchanger, still in the box, but just before the exhaust pipe. The one that
    I have just acquired has the same thing (Trane), and the one that my friend
    just got, the high-e one, Ditto.
    So it would appear that the back pressure question might be addressed with
    the fan,....anyone.?
    I understand the need for a water wash, as the gas that we burn (Ontario
    Canada) is high in sulphur, and as a result would
    condense out as Sulphurous (H2SO3) and Sulphuric (H2SO4) acid, which would
    eat the chimney liner, brick or alluminum.
    I'm sure there is other nasty stuff in the exhaust as well.
    So if the high efficiencey ones vent the condensate down the drain via pvc
    pipes, (questionable environment act) I am still (ignorantly) miffed as to
    why this is not a viable "after the box" possibility.
    Anyone want to start a business... :)
    Thanx again gents, all input very valuable
  6. Eric Tonks

    Eric Tonks Guest

    I find your statement about gas in Southern Ontario being high in sulphur. I
    worked for one of the large utilities for 40 years and saw many lab reports
    of the gas contents but don't remember much being in the reports about high
    sulphur. What is your source.

    A note about where Southern Ontario gets its gas. Most comes through the
    TCPL from Alberta, some comes through a pipeline from the US via Detroit and
    a small amount comes from gas wells in and around Lake Erie. Surplus gas is
    stored in Tecumseh township in old underground oil/gas caverns.

    Many people are surprised to find that Ontario has oil and gas wells other
    than near Petrolia. Until a couple of years ago there was a working oil well
    just north of Toronto near Keele St. on Langstaff Road. There is a capped
    gas well in downtown Toronto, under the building on the south west corner of
    Yonge and King. Many others are scattered across the province, with the bulk
    under Lake Erie and to the south west end of the province.
  7. Gunsmoke

    Gunsmoke Guest

    Eric, the info I received directly from Union Gas, a few years ago. No data
    to back it up, but the tech I spoke with
    insisted that I would suffer dire chimney wear if I proceeded with my
    concept of reclaiming the exhaust heat.
    I have a friend that is a commercial heli pilot, (as I am) and spent a lot
    of time servicing the (capped) gas wells in Lake Erie
    and he mentioned that the service techs occasionaly talked about the sulfer
    Your data looks far more relavent to the reality of what is being used in
    our furnaces than my info,, but it begs the question that if the
    sulfer content is indeed not, (or at least a small) factor of the exhaust,
    then the only concern to reclaiming the waste heat is material that can
    take the heat in a post primary exchanger. I think I shall take the extra
    step to find out the details of what is in the gas we use.
    Very interesting anecdote on the capped wells. I missed an opportunity to
    buy farm property near Dunnville Ontario, with two
    capped wells on about 100 acres.
    Thanx for your input.
  8. daestrom

    daestrom Guest

    In older style furnaces, it was the temperature of the flue gas, through a
    highth of one or two stories that provided the draft for the firebox. With
    that set up, the flue was always at a lower pressure than the house
    (something like an inch of water IIRC). So minor cracks/leaks in the flue
    piping to the chimney, or chimney itself wasn't much of a danger for CO

    If you cool the flue gasses a lot, you will need to 'mechanically induce'
    the draft needed. If your fan fails, you probably want an interlock to
    shutdown the burner before you asphyxiate your family.

    With hi-eff. furnaces that vent directly outside, a blower is used (mine is
    on the intake forcing air *into* the fire box, but others may vary). So the
    exhaust piping has to be air tight so no 'nasties' leak into my home.
    My hi-eff unit has a stainless-steel heat-exchanger to protect against
    corrosion problems.

    But if you don't smell sulphur (rotten eggs) in you current flue, I doubt
    that the sulphur is very high. But nevertheless, the condensate is probably
    corrosive (maybe NOx compounds that form nitric acid??).

  9. Steve Thomas

    Steve Thomas Guest

    I live and work in southwestern Ontario.
    The building where I work is quite large and is heated with natural gas.
    Most of heating units have no heat exchanger.
    The combustion products from the burning gas are blown into the building
    along with the heated air.
    The code requirement is that there be active air exhaust from the building
    at least equal to 80% of what is pumped in.
    Yup, I was surpassed too, but this is apparently perfectly safe and there
    have been no ill effects as far as anyone knows.
    Under the circumstances it is hard to believe that there can be much sulfur
    in the gas beyond the mercaptan added for flavour.

  10. Gunsmoke

    Gunsmoke Guest

    Just looked this up...

    I have access to IATA and UN codes so will check into further.

    huge snip
  11. Jeff

    Jeff Guest

    The water spray is to help condense out the moisture in the furnace exhaust.
    That water vapour contains about 10% of the combustions energy (it takes a
    LOT of energy to vaporize water).
  12. daestrom

    daestrom Guest

    While I agree there is a lot of energy in the water vapour, wouldn't
    spraying water in the exhaust just heat up the water spray? If the water
    starts out cool (say 15C), then warming it up to 20 or 25C from absorbing
    the heat of vaporization out of the exhaust doesn't really do much good.
    Unless you have a real need for 20 to 25C water ;-)

    Even if your 'spray water' is already pretty warm, and you warm it some
    more, will it really be warm enough for any use? And it would be at furnace
    exhaust pressure (near atmospheric), so you'd have to collect it and pump it
    *somewhere* to be of any use.

    A secondary heat exchanger might be the better way to go. Then the water
    can be in a circulating loop. Or you can use it to warm air directly.

  13. Nick Pine

    Nick Pine Guest

    News implies it might be 54 C. This could be a very cheap "condensing
    heat exchanger."
    It might collect in a chimney bend and run back into a plastic drum with
    some sort of pump. The water might clean particulates from the smoke and
    reduce the acidity that attacks chimney parts.

  14. daestrom

    daestrom Guest

    That could work. But like I said, now you have a 'bucket' of warm water.
    You still have to pump it somewhere and clean out the 'gunk'. An ordinary
    heat exchanger might work better than spraying water and collecting the
    resultant, albiet warm, mess.

  15. Jeff

    Jeff Guest

    The cold water condenses the vapour, thus heating the water spray, which
    runs down the heat exchanger.
    IIRC the water gets heated warmer then that. The heat exchanger is the first
    one in the cold air return. The furnace is pretty much a standard furnace
    other then the extra heat exchanger.
    The heat is used to heat the cold incoming air the furnace.
    It goes directly into a heat exchanger (flowing exhaust gas and water
    vapour). The water gets drained off in a standard residential drain. The fan
    forced exhaust is so cold in the model I saw that a plastic pipe was used to
    direct it outside!
  16. daestrom

    daestrom Guest

    What purpose does the water serve? If the vapor is directed to a heat
    exchanger, where the cold air return of the house is on the other side of
    the heat exchanger, then the cool air will cool the flue gasses and condense
    the water vapor, without any spray.

    My gas furnace works this way without any 'water spray' in the exhaust
    pathway. Exhaust is cool to the touch and vented through PVC. This is
    *not* a new design or anything, been around for years.

    The OP was talking about spraying water into the exhaust flue. Just don't
    see that this is necessary or helpful.
    Why mess around with water spray? There are furnaces out there right now
    that can direct-vent through PVC. Uses air-air heat exchangers to condense
    the water vapour in the flue gas. Seems like spraying water into the flue
    just leaves one open to all sorts of new problems. Scale build up,
    corrosion, plugging of nozzles. And any warm water going down the drain
    will carry away heat from the system. Better to use current design of
    air-air and minimize how much warm water that goes down drain.

  17. Guest

    A water spray can have a very large cheap heat exchange surface...

  18. daestrom

    daestrom Guest

    True. But with the 'caveates' I mentioned before still stand. Now you have
    warm, 'dirty' water at atmospheric pressure. Filter it and pump it 'up' to
    where you use it. It will most likely be acidic, and unless your whole
    heating system can withstand it, you'll need to raise the pH. On-going
    costs and maintenance won't be the same as a regular surface heat exchanger.

    Probably better in the long run to have a surface heat exchanger. Higher
    initial cost, but lower operating cost and less 'hassle'. But hey, the OP
    can try it if they want. Maybe they can post there experience/results after
    a few months.

  19. Jeff

    Jeff Guest


    The one I saw continuously uses domestic water, which then goes down the
    drain. Sounds like a bit of a PITA.
    I agree. Might be much larger although. Perhaps the water based heat
    exchangers were used to retrofit original designs.
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