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6" Boiler Exhaust w/ 5" chimney liner. how bad is this?

DB77DB77 Member Posts: 2
Last year I had a chimney company inspect all three flues in my triple flue chimney.  They determined that the main flue which my oil boiler ties into had corroded and offset tiles.  They reccomended dropping a stainless steel liner down the flue.  I had them do the work and it ended uo costing $1600 for a 5' 40 foot ss liner.

The work was done last August.  I live in New England in a 2300 sq foot colonial built in 1999.  After the install I spent about $3500 on 950 gallons of oil to heat the home.  This seemed kind of exhorbitant to me so I started looking into having a pellet boiler installed to save some money.

Long story short I will be getting the pellet boiler installed but in the process of preparing for this I had a different chimeny company come out to put in a liner for that boiler and while they were there they looked at the other liner which is a 5" and told me that it should have been at least a 6" because the oil boiler exhaust pipe is a 6"

So my question is how much efficiency am I losing by going from 6" on the boiler to a 5" liner up a 40 foot stack?  Did I likely waste a lot of money this past winter on heating costs due to the chimeny company's not using the right size liner?  How would you go about addressing this issue?

Thanks guys


  • SWEISWEI Member Posts: 7,356
    Pellet boiler

    Sizing requirements and flue specifications will be in the boiler installation manual.  Solid fuel boilers may require a different type of flue.
  • Steve WhitbeckSteve Whitbeck Member Posts: 669
    edited June 2013
    liner capacity

    It is legal to bush down the vent pipe to fit the chimney liner IF the liner can handle the BTU input of the appliance.

    Measure the height and lateral of the liner and using the chart that came with the liner figure out if it can vent the total output of the appliances.

    I have done this before and all the inspector wanted was proof of it.

    Restricting the vent pipe will increase efficiency as long as the appliance is burning clean. Not the other way around.
  • unclejohnunclejohn Member Posts: 909
    Seen it once

    In DC. It was a 7" down to 6" and was reduced right at chimney. Had a passed sticker on it.
  • ChrisChris Member Posts: 3,056
    To Answer Your Question

    The liner should have been 6".
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • TonySTonyS Member Posts: 849
    Steve is correct

    Liner capacity is based on vertical length, diameter and horizontal distance . Also based on natural or induced draft. It is best to install the proper size liner and not just use what the manufacturer welded to the top of the boiler.

    You will notice manufacturers will use the same size flue outlet on several of the same series boilers before jumping to the next size.

    So I am not saying yours is correct but it very well could be.

    Give us the length of the chimney and the length of the horizontal flue and the input of the boiler and we will tell you if the 5 inch is large enough.
  • ChrisChris Member Posts: 3,056
    He's 40' Vertical

    Even if he is 5' horizontal and firing .75 on the nozzle the chimney needs to be 6"..
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • TonySTonyS Member Posts: 849
  • JStarJStar Member Posts: 2,668

    The very first question we need answered is What size is the boiler? A 3" liner might be good enough. A 10" liner might be good enough. We can't know until we know the size.
    - Joe Starosielec
    [email protected]
  • Bob HarperBob Harper Member Posts: 732

    The IRC requires the liner be the same size as the appliance collar or larger. That would start at 6". However, if this is a flexible liner, you would have to de-rate it 20% for corrugated plus 20% if it offsets. Having said that, you may get an AHJ to sign off on a downsized liner IF the mfr. says its ok. Same as whether or not to use a barometric damper. They are required for oil unless the mfr. specifically forbids them on oil.

    That chart was for CAT I gas and says do not use if the connector has the restriction of a vent damper or draft regulator.

    Oil liners must be stainless steel. They are not required to be insulated but mfrs. always recommend it. It is considered a good practice for exterior chimneys.

    The sizing charts in the NFPA 31 Annex are not part of the code and just FYI. Honestly a smaller liner usually does fine but you must test and just understand that if the liner gunks up with soot, it may backpuff and the ensuing litigation would hang you out to dry.

    A slightly undersized liner has nothing appreciable to do with efficiency. Unless the liner is blocked and the house sooted up, it really isn' t that much of a factor on efficiency.

  • ChrisChris Member Posts: 3,056
    It's An Oil Boiler

    At a minimum its a 3 section it isn't a 3-pass either with what he shared for oil cost. Should be a 6" flue.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • ChrisChris Member Posts: 3,056
    Post An Oil

    Chart and then we can talk.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Steve WhitbeckSteve Whitbeck Member Posts: 669
    edited June 2013

    BOB - air flow through a heating appliance directly effects the efficiency.

    Yes even with a draft control a bigger liner than is needed would cause less backpressure ( more draft ) and thus more airflow. Years ago when I was converting furnaces and boilers from oil to gas we had to use a oxygen analyzer to see how far we could block the flue outlet to hold the heat inside.

    You need to use the chart that came with the liner ( or get one from the manufacturer ) to see what size is needed for a given BTU.

    Here in MI you  can bush down the appliance outlet to fit the liner IF the liner can flow the total BTU input of the connected appliances.
  • TonySTonyS Member Posts: 849
    I cant find a chart

    that differentiates between the two. Could you please post your chart?
  • Bob HarperBob Harper Member Posts: 732

    A draft regulator such as a barometric damper is adjusted to maintain a certain measured draft pressure. Changing the size of the baro. does not in and of itself change draft pressure.

    Yes, efficiency is affected by airflow through the appliance but we're talking about the liner, which is not always the same thing. As long as you have a consistent draft pressure in the vent and it is not blocked physically or by a high draft induced "curtain effect" such as seen with some draft hoods, a rather consistent amt. of excess air will flow through the burner compartment and thus not change the combustion efficiency. The efficiency of the appliance itself, too remains essentially unchanged as long as their is a steady draft pressure prior to the draft regulator and there is positive flow up the chimney/ vent. Once you have the venting under control, you can look to firing and heat transfer to possibly improve efficiency. Personally, I'd be more concerned about the pump speed/ delta T than the venting and how the burner is set up. An underfired burner will cost you a lot more in oil than any minor change in chimney liner diameter.

    I have been advocating combustion analysis here for years. I didn't mention it I guess because I've grown weary of repeating advice that really shouldn't need to be stated over and over. You set up the appliance by full combustion analysis once you have been certified to do it. That includes more than just the O2 reading but yes, you read O2 and don't worry about CO2 as we were all lead to follow for years.

    If the liner is slightly oversized but still a thin-walled metallic liner, it should heat up quickly to establish a strong draft pressure. A grossly oversized flue or an high mass masonry flue that has cooled down since the last firing, will rob BTUs as it tries to dry out and warm up as well as trying to push that push-pop slug of cold dense air up out of the flue to establish positive flow. Notice I didn't say "draft" which is a mis-used term.

    I find it interesting in the land of regulation that Mass. would allow a bending of the national codes on this. How do they determine the BTU capacity of an oil liner? If by charts, which? If by performance, by which method to what standard? Just curious.
  • ChrisChris Member Posts: 3,056
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • TonySTonyS Member Posts: 849
    Chart is incomplete

    According to that chart if I were to install a Buderus 115 ws, I would need to increase the 5 inch factory collar to 6 inch.

    Obviously Buderus feels comfortable firing 1.10 gph into a 5 inch outlet.
  • jonny88jonny88 Member Posts: 1,139

    I came across an es2 burnham a little while ago.It was 5" exhaust with a six inch liner.The draft on the boiler was tremendous.On the burners you could only see a orange flame.Called local burnham rep and explained.He explained how important it is to size your liner and that the chimney in this case was oversized.Homeowner had liner changed to 5" and burner ran with a perfect flame.It was a good lesson for me in the future.
  • Jamie HallJamie Hall Member Posts: 6,708
    Strictly from a non-professional

    point of view, I might add -- based on experience -- that if one has a big enough, tall enough stack which is nice and warm, unless you have both a barometric damper and a draft regulator on the boiler (talking oil here) the draught up the stack may be enough to keep the burner from lighting off at all at times -- just pulls the poor little flame right off the burner.  Fun to watch, but a bit annoying... (stack in question is a 9 by 12 tile in double brick masonry chimney, som4 40 feet tall... originally sized for a 5.5 gph Quiet May burner).

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • JackJack Member Posts: 941
    Appendix E of NFPA 31

    Bob, you are correct that the Appendix is "not a part of the code". That said, over the years I've talked to many people and been involved in many installs where the sizing in Appendix E worked perfectly. Many of those used 4" liners or pellet vent.

    These tables were initially put together by Rich Krajewski and John Strasser at Brookhaven. I think they should have been adopted in '93 or '94 when first presented, and when I was on 31 I voted for them. Instead, 20 yrs later we still do not have lining standards for oil. The code says it must be the same size as the breaching. Not much science that I can see behind that rote statement. It's just the way it has always been.
  • Steve WhitbeckSteve Whitbeck Member Posts: 669

    Too large of a flue also causes a higher loss of heated building air. Having to adjust the draft diverter to a more open setting because the liner is TOO large and creates too much draft just puts more heated room air up the chimney. A properly sized liner doesn't waste heated room air. It is kind of like installing too big of a pump and then controlling the flow with a circuit setter. Waste of energy.

    A properly sized liner will make it easier to adjust the burner and draft.

    Too large of a liner WILL effect efficiency
  • Charlie from wmassCharlie from wmass Member Posts: 3,925
    Mass code says if it will handle the btu

    load you can go down one size. If it is 15 feet tall it may not work but if it is 30 feet tall it may be fine. The charts are in the code books there is no guessing.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • TonySTonyS Member Posts: 849
    Does that go for Gas and oil?

    Is there a difference in btu capacity between gas and oil?
  • Charlie from wmassCharlie from wmass Member Posts: 3,925
    edited June 2013
    It goes for oil and for gas

    they each have their own code book though. a BTU is a BTU. The size of the liner is different for power burners of a given BTU versus an atmospheric vented burner.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • TonySTonyS Member Posts: 849
    Thats what I had assumed

    Thank you for the info Charlie
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