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fluctuating draft with power vent + power gas

installed my own steamer, copying steamhead's combination of smith g8 and carlin power gas burner.  Imitation is the best form of flattery.


I fired it up today for kicks.  I'm trying to find a decent professional to come double check the burner setup before i call it quits for the winter.  Anyway........


This has a Fields CV-4 power vent with outside air intake.  The intake isn't connected yet.  When firing, the draft fluctuates about .05" H2o, and i can't get it stabilized.  The exhaust also has a pretty mean basslike hum, and i'm getting some type of pulsations. 


I figure its because i used a 90 right off the boiler, and 2 more down the line.  Not that that exceeds specifications for maximum flue length, but i'm getting some pretty mean turbulation going on in there I think. 


anybody have experience with this?  No matter what i do, i can't really get a stable draft reading over the fire, or at the breech.  Damn power vents.  I should have built a chimeny.   I'm using Bacharach 125 digital analyzer. 


Furthermore, i WILL consult a professional before i consider this set-upcomplete, and I have CO detectors, and I did stay at a Holiday Inn last night.


see pics


beautiful Conshohocken PA


  • TomM
    TomM Posts: 233
    combustion number

    sorry i forgot to post combustion numbers;

    after run 10 min:

    air set to 20 on burner.  105,000 BTU

    CO      30ppm                                ok

    O2      3.9                                      ok

    stack  410F                                   ok

    eff      81.2                                    meh

    Co2    9.7                                      ok

    breech draft-    -.15wc to -.02 mostly.  sometimes i get positive numbers



    after run 40 min:

    CO      66ppm                               meh

    02       3.8%                                 ok

    stack    421F                               ok

    eff       81.3                                 meh

    CO2    9.7%                                 ok

    breech draft     -.20 wc to sometimes positive.


    i listened around with a hose, and it seems most of the basslike noise is coming from the 90 right off the boiler.  when i remove the plug for the breech draft testing port, it pulls air AND pushes air.  ie, my meter is not broken, i'm definitely getting positive pressure at the port.


    is there a better way to set up this exhaust?  something is not correct.

    beautiful Conshohocken PA
  • SWEI
    SWEI Member Posts: 7,356

    Why the neckdown at the chimney entrance?

    Is that other vent capped?
  • TomM
    TomM Posts: 233
    thanks for the reply

    the neckdown is because i am using a Fields Combovent 4", rated to 170,000 BTU.  6" is coming off the boiler.  The power vent allows 65' of 5" at max btu, so my lengths are ok.  The Combovent is only offered in 4" and 5".  the 5" is up to 290,000 btu.  My boiler is 105,000. 


    no, the intake is not capped.  I will cap it and report my findings.


    the power vent internal diagram is attached
    beautiful Conshohocken PA
  • TomM
    TomM Posts: 233
    getting somewhere

    ok i think i have the following design problems:

    -the power vent sucks way too much air, and my design causes a ton of internal exhaust turbulation, making the draft regulator swing excessively, causing fluctuations in my draft over the fire and at the breech.


    I think a redesign with longer straight lengths and less 90's + two draft regulators should be in order.


    man this power vent is a pain in the butt, the best reading i can get over the fire is about -.10 wc.  if i get any closer to spec, there is not enough weight on the draft regulator, and it starts swinging causing fluctuations.   If i turn the internal damper on the power vent less than 35 degrees open, then it whistles like a train.  Motor speed controllers can't be used because the motor is capacitor start. 


    turns out this exhaust design is proving tougher than my fancy schmancy steam header.
    beautiful Conshohocken PA
  • icesailor
    icesailor Member Posts: 7,265
    Draft Regulators:

    You need two draft controls and the PV is probably too big.

    Where I work, you need a PV that has enough nuts to overcome 50+ MPH winds so you need two RC's.
  • Jim Davis_3
    Jim Davis_3 Member Posts: 578
    Power venter

    Most power venters should have a damper control to adjust the venter because of all the different lengths of flues.  The draft between the venter and the barometric should be no more than -.06"W.C.  You can make a slide plate damper or neutral pressure adapter to adjust this but the venter should have its own damper.
  • TomM
    TomM Posts: 233
    damper control

    The power vent has its own damper.  Any setting less than 35% open starts whistling.


    i get it now.  I need to set the power vent up to -.06 wc before i do anything else.  Then, when the power burner is on, it should be simple fine adjustments to the draft regulator.  If i can't get the reading between the power vent and the regulator down to -.06, then i need to add another regulator, and get the reading between the regulators down to -.06, (with the regulator closest to the power vent wide open, and the other one shut).   At least that makes sense in my brain.  Reality is another story.


    i'm going to redesign for more straight lengths and less 90's, possibly add another barometric, and report back.


    beautiful Conshohocken PA
  • Jack
    Jack Member Posts: 1,046
    Intake roar

    Not sure if this will be an issue on your system but years ago I was in a lab where a Carlin power burner was running. The overall system was quite loud. The engineer said, "watch this" He put a 1' long piece of 4" pipe on the burner inlet and it quieted the system right down. That was a long time ago and the current burner may not require it, but it is worth a look.
  • TomM
    TomM Posts: 233
    well well well

    whaddya know.  by moving the draft regulator a little farther down the line, so the exhaust gases are more stable, and turning the power vent damper all the way down to 10%, i can get a stable draft.  I guess the turbulence was making it swing, making me pull my hair out.  Whats left of it anyway.

    -.01-.03 over the fire

    -.04-.05 at the breech.


    made a HUGE difference in acoustics too.  purrs like a kitten.  Thanks!  tuning combustion this weekend and we should be good to go.
    beautiful Conshohocken PA
  • RJ_4
    RJ_4 Member Posts: 484

    what about that single wall flue pipe ?
  • JStar
    JStar Member Posts: 2,752

    Don't you just love the nuance of every little detail? Being forced to visualize something that's invisible (air) will make you question your sanity.
  • Steamhead
    Steamhead Member Posts: 15,901
    edited December 2012
    A clear

    If the draft is excessive, it can pull the flame right off the burner head. This is called "loss of retention" which can cause the flame to roar, and on an oil burner can cause a smoky flame unless you add way too much excess air. We had that situation on an oil-fired steamer some time ago, and when we got it straightened out the lady's oil consumption dropped by a third.

    So with that installation and such a nice piping job, the whole system should purr.

    I would, however, double-check that you have enough clearance between the wood framing and the flue pipe. If it's even close to the recommended minimum clearance, I like to use double-wall as an added safety measure.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
This discussion has been closed.