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Do you get more efficiency with longer vents ????? HELP ME

Post in 'It's a Gas!' started by dattjacobs, Sep 7, 2009.

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  1. dattjacobs

    dattjacobs New Member

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    New to this site and gas stoves. I just got a jotul 600 dv II. Will be installed soon. The question I have is. In the jotul book it says '' 87 % efficiency with maximum vent configuration ". What does max mean. My installer wants to go min with 2 ft up and out. If you go higher do you get more efficiency out of the stove. Is it the longer the pipe the cold air has time to warm coming in the house? Does the pipe give off that much heat in your house? I want to put a 5 or 6 ft pipe up than out. Will the efficiency be higher.

    Thank you very much. Todd cold in michigan..

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  2. DAKSY

    DAKSY Patriot Guard Rider Staff Member

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    Your set-up will work just fine.
    I see no reason to add more venting to the GF600.
    Mine is set up similarly & heats the area I want it to.
    You're gonna like the heat from it.
  3. Corey

    Corey Minister of Fire

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    That is an interesting statement from Jotul - but leaves quite a bit open from an interpretation point. What is the "maximum vent configuration"? Is the efficiency higher or lower if some other configuration is used? By how much?

    I would tend to think the best efficiency would come with the minimum vertical rise and maximum horizontal run. This would slow the flow of air through the firebox and the long horizontal run would give a lot of time for thermal transfer from the exhaust to intake air. I don't think you'd get much heat from the pipe itself as most DV pipe seems to have a pretty small clearance to combustibles - ie it just doesn't get that hot on the outside. If efficiency is of great concern, it might be worth a call to Jotul tech service to see what the heck they actually mean.
  4. Wood Heat Stoves

    Wood Heat Stoves Minister of Fire

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    probably wont notice much diff outside of lab, but yes makers can skew thier numbers by measuring efficiency with excessive stack, means less heat escapes to outside, means stove is "more efficient", lol
  5. R&D Guy

    R&D Guy New Member

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    It is true - the longer the vent run the more efficient the system becomes because all the hot flue gases heat up the incoming combustion air. So the flue gas temps at the termination are quite a bit less. The most efficient way is to have a good vertical run that is choked down with a intake and exhaust restrictor. I say vertical instead of horizontal because with horizontal all the hot air travels along the top of the pipe, whereas with vertical runs, the heat is distributed evenly along the outside of the pipe.

    If you think about it, direct vent pipe is really just a long heat exchanger and the longer the run, then more surface area you have for heat exchange resulting in hotter combustion air temps.

    Another thing to keep in mind is that efficiency ratings are not regulated in the industry meaning that a mfg can put about anything they want in a brochure. As I've joked before, the US efficiency tests (if its even run) is done by the mfg behind closed doors with a drunken sailor and hungry salesman running the test.

    About the only one that is witnessed by an independent lab is the Canadian P4. Your unit, if I'm reading the correct name is listed in Canada as having a 69.8% efficiency which isn't bad. The P4 test is done at minimum flue.

    http://www.oee.nrcan.gc.ca/residential/business/manufacturers/search/fireplace-results.cfm

    So yes, your stove would be more efficient if you had the maximum amount of pipe, but with the additional amount you are taking about (3 or 4 feet) you won't notice more heat standing in front of the unit and I don't think you'd notice a difference on your gas bill either. Just keep the blower on and it will heat your home fine.
  6. Wood Heat Stoves

    Wood Heat Stoves Minister of Fire

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    quote]almost/maybe= if exhaust is overcooled the result is condensation in the pipe at which point the unit would figure so & burn more gas to compensate, as dictated by some heat sensor i'd guess.[/quote]

    wtf?? what gas stove has such a "sensor"??
    condensing furnaces exist that vent with pvc pipe they are so cool, but not hearth appliances generally
  7. DAKSY

    DAKSY Patriot Guard Rider Staff Member

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    Hey Todd...

    <>In the jotul book it says '' 87 % efficiency with maximum vent configuration ".<>


    Where & in what Jotul Book did you see that statement in?
    I looked thru the manual, the brochure & even at the rating
    plate on our floor model & cannot find that statement...
    'Course I'm old & that may have something to do with any oversight...
  8. Wood Heat Stoves

    Wood Heat Stoves Minister of Fire

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    just guessing=theoretical but i do know that if u cool the exhaust enuff it'll condense in flue, & what happens if flue is all wet especially at the top of the flue where freezing temps are present
    maybe gizmo aint built to pook standards yet,eh?[/quote]

    i urge you to go and design such units for hearth applications. once you do you'll see they are too expensive to compeete even with higher efficiencies. plus the need for ac power makes such units poor options for back up heating...
  9. Wood Heat Stoves

    Wood Heat Stoves Minister of Fire

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    well, look at sales figures for models like the condensing Monitor.....
  10. dattjacobs

    dattjacobs New Member

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    In the jotul gas stove brochure on page 22." up to 87 % efficiency at steady state" with maximum vent configuratoin is in small pint on the bottom of the page.
  11. dattjacobs

    dattjacobs New Member

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    I have a high efficiency furance, and the vent does not frezze on that.
  12. Long Burn

    Long Burn New Member

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    Todd,
    The Jotul owners manual has a chart for various venting options and setting the exhaust resrictor for maximum efficency.
    I don't think the difference between you and the installers recomendations will make any difference.
    LB
  13. R&D Guy

    R&D Guy New Member

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    Correct condensation in the pipe is a potential problem although I've never heard of an actual problem out in the field. It's my understanding that its a corrosion issue even though the pipe is aluminum.

    Z21.88 and Z21.50 both have a minimum flue pipe temp at discharge. I want to say it's 180° F after 18 minutes, but I don't remember the temp off the top of my head. This is often a challenge at the max/max run (maximum vertical, with maximum horizontal), but if you don't make the temp then all is not lost. The other way to get past it is to do a dew point calculation since you are already taking flue gas samples and you can prove that even though the inside of the flue pipe didn't make temp condensation would not form.

    If the mfg spec's a stainless inner flue then the appliance is exempt from the minimum flue temp requirement.

    I hope this helps.
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