Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.

If you've found help here, check back in to let us know how everything worked out.
It's a great way to thank those who helped you.
Need to contact us? Visit

Flue Draft

RichtdowRichtdow Member Posts: 22
I can't get the draft differential between above the fire and the flue to be less than .400. I've disassembled the boiler and cleaned it. I've also checked smoke and CO2 which are both fine. What else can I do to get the differential down? I believe the differential should be much tighter and it's currently getting too much draft, increasing flue temperature to around 600 degrees F.

Current readings: -.400 (flue), -.001 (over fire)
Boiler: Weil Mclain WGO-2
Burner: AFG50MBAS
Flue diameter: 8"

Thank you!


  • BDR529BDR529 Member Posts: 105
    Didn't even know thay made a 2-section WGO.

    If it runs good leave it alone.
    Wait till a storm rolls over the house or the dryer is running,bathroom fan,vent in the kitchen when the wife is burning you up some dinner.
    Nothing said about barometric installed and distances?

  • RichtdowRichtdow Member Posts: 22
    Yes, sir. Two sections. I've got a small house and even when the 2-section is set at 170F it heats the home effectively and doesn't cycle more than 3 times per hour once at temp.

    There is a barometric dampener installed. I set it such that I have slightly negative pressure over fire.

    The chimney flue is about 20' tall.
  • STEVEusaPASTEVEusaPA Member Posts: 4,193
    Do you have -.04 at the breech or -.4? Honestly if you're getting any negative over the fire you should be happy.
    -.02 differential is fine. If it's actually -.4, not good.

    Even though you cleaned the boiler it's probably not clean enough.
    Did you also open the chamber and brush up in both directions?

    -Nozzle and pump pressure?
    -Rest of combustion numbers?
    -Is your chimney exterior exposed 3 sides going up the wall.

    Describe your flue pipe from the boiler to the chimney base, something like this:
    Comes out of boiler with a 90°, up 3', 90° into chimney base...
    Or some pictures of the flue pipe, and outside where the chimney terminates.
  • JStarJStar Member Posts: 2,752
    Did you take stack draft before or after the barometric damper?
  • HVACNUTHVACNUT Member Posts: 3,412
    > @Richtdow said:
    > Current readings: -.400 (flue), -.001 (over fire)
    > Boiler: Weil Mclain WGO-2
    > Burner: AFG50MBAS
    > Flue diameter: 8"

    Do you mean ?
    -.01 O.F.
    -.4 breach.

    I know the early WGO boilers are pretty tight.

    8" flue? The 3 and 4 section WGO is 7". Why 8"?

    I know I didn't give any recommendations. Just want to know if the numbers you posted are correct.
  • RichtdowRichtdow Member Posts: 22
    edited December 2019
    STEVEusePA - I'm getting -.4 at the breech and -.01ish over fire (slightly negative). I brushed in between the nipples in multiple directions but I don't disagree that I could've missed something. It's my leading candidate for the problem at the moment even though I felt that I gave it a good cleaning.

    The nozzle is clean and it is the one for the burner and boiler. I have not checked the pump pressure as the burner is only two months old. If it was a pump pressure thing, could we be thinking high or low as the problem?

    The chimney is not three sides up a wall. It actually goes right through the center of my house.

    The flue comes from the back of the boiler with a 90 then the T with barometric damper, and another 90 into the chimney. So it would look like a S from the side.

    Combustion numbers are as follows:
    CO2: 11.5%
    Smoke: No trace after adjusting back from a trace of 3.
    Interpolation states an efficiency of 78%. I'm certain I could do better than this.

    JStar - I took the draft before the damper.

    HVACNUT - You are correct, about -0.01 OF (I adjusted it to be slightly negative OF) and -.4 at the breech.

    You are also correct with the flue diameter. I thought 8" but it is 7" as I matched the factory flue collar which is 7".

    Thank you everyone for your help!

  • STEVEusaPASTEVEusaPA Member Posts: 4,193
    edited December 2019
    When cleaning pin boilers, I swing open the door or take off the front.
    I stick my drop light in the combustion chamber at the bottom, and clean from the top. This way I can see how clean I am getting it.
    Then when I'm done at the top, I go into the chamber and brush up on the angles that were inaccessible from the top.
    Then recheck with light in chamber, looking in at top to see.
    The pump pressure should be on a sticker attached to the burner, by the nozzle line, or it's usually assumed to be 100 psi.
    What nozzle is on the burner? Should be a .75x70B @100 psi.

    I don't think I've ever seen that much of a differential, even on a WGO. I think if the boiler were plugged, you'd be positive over the fire, and not much negative at the breech. Which draft gauge are you using?
    What are the draft numbers when you shut the burner off, specifically over the fire (be careful when you turn that burner off that you don't have the inspection door wide open)?
    Edit: I posted this last week but somehow it ended up back in my 'Drafts'.
  • RichtdowRichtdow Member Posts: 22
    Thank you and I apologize for the late response. I haven't verified the nozzle myself but the factory says they put that one on there and this burner is brand new. It's also clean as of last week.

    I cleaned the boiler once again last week and didn't get too much soot. My flashlight shines light right into the fire box from the top. I agree that a plug seems less likely the problem.

    I'm using the Bacharach 13-3000 draft meter. I have not thought to check the draft with the burner off but I do feel a significant draft into the chimney when the flue is removed from the chimney from cleaning.

    I'll try to grab you some burner off numbers this evening.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,084

    If your stack temperature is 600 that sounds high to me Your draft sounds fine.

    I would check nozzle size and pump pressure and look towards reducing the firing rate. Many think you need to fire at maximum firing rate. I don't agree.

    That's what will get your efficiency up. Lowering stack temp is the best way to gain efficiency
    RichtdowSuperTechrick in Alaska
  • RichtdowRichtdow Member Posts: 22
    I was messing around with the fuel valve nearest to the pump and noticed that it was not full open. After opening it to full open I redid my calculations and came up with:

    CO2 readings: 11.5%
    Draft: Same as before
    Temperature: 500F

    I'm not sure how much the valve was restricting fuel flow prior to adjusting it last week.
  • RichtdowRichtdow Member Posts: 22
    I did the burner off drafts. The burner had just shut down a minute or two before but I got:

    OF: -.045
    Flue: -.035

    So there was less draft at the flue than OF.
  • captaincocaptainco Member Posts: 451
    I have tested many pieces of equipment and the only time draft is - .4" the flue is close to 100' tall. Draft at the breach creates draft over the fire so it can't be lower. Something is interfering with your readings or your draft gauge is bad. Could be some type of leak in the heat exchanger or gaskets.
  • RichtdowRichtdow Member Posts: 22
    Thank you. I'll approach my troubleshooting from that perspective by checking my test equipment and the heat exchanger itself.
  • EdTheHeaterManEdTheHeaterMan Member Posts: 621
    edited January 13
    That draft gauge you are using is calibrated by holding it level, the gauge should be on 0 if level. Now without changing the calibration of "Level" move it to the port you want to sample. (I find this mostly impossible to get that accurate with that device.) Next error is the 4 on the gauge means 4 hundredths of an inch water column, expressed 0.04"WC

    Your original post says .400" WC which is read as 4 tenths inches WC

    So your draft loss through the boiler is either .03" WC or .039" WC and I don't think that is too much for a maximum fired WGO-2 boiler. I would try a .50 GPH 70° solid or semi-solid (W) at 145 PSI pressure. This will drop the firing rate by about 20% from .70 GPH to .60 GPH. the fuel will atomize to smaller droplets and burn with less excess air, an air adjustment and smoke test will be needed
    this will lower the pressure in the combustion chamber and you may find the stack temperature lower with .01" or less over fire and .03 at the breach. Try it and let me know how it works out.
  • EdTheHeaterManEdTheHeaterMan Member Posts: 621
    If you get good combustion results but you find that you are not getting sufficient heat output from the boiler on the coldest nights, you can try a .60 GPH at 120PSI pump pressure. This will reduce your firing rate by about 8%. I'm assuming this is your own equipment and you have time to tinker. If this is for a customer I go with the factory settings... and you get what you get!
  • RichtdowRichtdow Member Posts: 22
    Thank you, EdTheHeaterMan! You are correct and I was wrong about the draft. It is indeed 0.040" WC. It is my boiler so I can tinker. So far it's been doing really well on putting out heat but I can see some value in reducing the fire rate. I may wait to make this mod during the off season when I was planning on changing the nozzle out anyway. I'll keep you informed.
  • EdTheHeaterManEdTheHeaterMan Member Posts: 621
    edited January 14
    Reducing the firing rate will put less expansion of liquid fuel into vapor, By increasing the pressure, the oil droplets will atomize in smaller droplets creating more surface area for the fuel to change from liquid to vapor. Since there is more surface area, the fuel burns microseconds faster. Not a lot but enough to reduce the combustion air needed to burn the fuel so less excess air is required. Now with less fuel vapor and less air by volume, the rapid expansion of the flue gasses in the combustion chamber will exert less pressure on the combustion chamber. the lower pressure in the combustion chamber may be measurable by as much as .02" WC.

    Also, if less air and combustion byproducts are traveling through the heater they will move slower. If the hot gaseous byproducts of combustion move slower, they will spend more time in the heat exchanger and more heat can be absorbed into the water on the other side of the cast iron. If more heat is transferred into the water, then there will be less heat going up the flue. End result: Lower stack temperature, greater efficiency, lower fuel bill.

    this illustration is in a book by RW Beckett on page 3 of

    This is good reading for you to more thoroughly understand your oil heater

    Reducing the firing rate is fine as long as you don't go too low. Remember Furnaces and Boilers are rarely manufactured exactly to the size of your Heat Loss calculation. Any tweaking you can do to get as close as you can is a saving!
    Richtdowrick in Alaska
  • RichtdowRichtdow Member Posts: 22
    EdTheHeaterMan, as an engineer I appreciate your thorough explanation of the how and why. It's very helpful for troubleshooting to understand what's really going on. I think I'm going to grab a new nozzle and give your strategy a try ASAP. Thank you!
  • RichtdowRichtdow Member Posts: 22
    Also, what's the deal with the low firing rate baffle and when should I install it? Would the new nozzle be a good time to install it?
  • RichtdowRichtdow Member Posts: 22
    SOLVED I made a mistake when installing the new, out-of-the-box burner from Beckett and assumed that the specs were already set up or at least close. I was wrong. The pump pressure came from the factory set at 220 psi. I was concerned about possibly increasing it to 120 psi. No more. Knowing what it'll do at 220 psi has made me more comfortable with this aspect of the burner. Now that the burner is running at the spec'd 100 psi, it's quiet, the draft delta is tight, the flue draft is lower, and the temperature is down to 415F.

    Thank you for all of your help and my key takeaway here it to never trust that factory QC set the burner on the way out.
Sign In or Register to comment.


It looks like you're new here. If you want to get involved, click one of these buttons!