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Can chimney liner be sized based on combustion level only?

weekendguyweekendguy Posts: 181Member
Some background - the boiler is a Weil GO-6 fired at the level of a GO-5 using a Carlin EZ-gas propane. Even the current output is too high so I am now dropping the input from 203K to 150K. Partly this is to test "how low can you go" since I am replacing this with a Burnham ES2 (5 or 6) after this heating season. I'd like to get the chimney work done soon so that's not a problem if the boiler suddenly needs to be replaced this winter.

Stack is currently 7" but that was for an input of 210K (oil). The new boiler will have a 6" stack. NFPA says that the liner can therefore be 5" and according to the liner manufacturer, that will handle at least 200K. Chimney is over 40 feet.

Chimney guy prefers to use the 5" since he'll have to navigate some offsets and mortar lumps. I have no question that the 5" will be fine for the new boiler with input between 140K and 175K.

In the interim this new chimney needs to work with the current boiler for a few months. Am I right to think that the liner needs to handle the actual combustion level and since I have dropped this boiler to the size of the smaller future unit the smaller liner will suffice?

The EZ-gas is fan driven (like an oil burner) if that matters.


  • captaincocaptainco Posts: 424Member
    I think your liner guy is pulling your leg and just making the job easier on himself. Because liners are more restrictive than standard flues they actually should be larger than normal. If he puts in a 5" liner you will probably be removing shortly after you find out it doesn't work. The fact that it will have offsets will make it even more restrictive. Don't go smaller than 6". The smaller liner will cause the flue gases to cool faster and also cause more condensation problems.

    Taller flues normally have higher draft but only if there is room in them for the gases to flow. A flue is no different than a piece of ductwork. The longer the duct the more resistance to flow.
  • weekendguyweekendguy Posts: 181Member
    @Hatterasguy Thank you. That's what I had reasoned but obviously I don't know all the factors involved.
  • ChrisJChrisJ Posts: 9,809Member
    edited December 2016
    When we were looking to line my chimney before it was torn down, I was told you must go by the liner manufacturer's specifications.

    They will give input btu ratings based on amount of horizontal offset and height for each size.

    For example :

    According to that their single wall 5" liner with a 2' offset at 45' is good for 160 btu/h input with a single natural draft appliance. 247 btu/h with fan assist. Not sure if that means a power burner, but I doubt it.

    If you have any concerns, I'd recommend calling the manufacturer of the liner they are going to use. They will likely tell you exactly what you need to know.

    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Mark EathertonMark Eatherton Posts: 5,843Member
    I believe you are required by code to install a liner to the maximum allowable input to the appliance, not its derated input. If someone else buys your home and decides to run the burner up to handle additional loads, it will be undersized.

    Here in Denver, we have to deal with a 20% altitudinal deration due to altitude, but are not allowed to take that deration into consideration.

    If in doubt, check with the authority having jurisdiction and as Chris said, the liner manufacturer. The AHJ may defer to the manufacturer's requirements.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Tim McElwainTim McElwain Posts: 4,239Member
    I'm quite confident that you'll be fine with 5" when using forced draft. There might be a concern under certain conditions with atmospheric draft. The 40' height would seriously mitigate any issues with that, however.

    Hatterasguy this is a power gas conversion burner which is a premix burner it does not in any way power the vent. It is therefore an atmospheric application.

    I would not reduce the flue for reasons already mentioned by others.
  • ChrisJChrisJ Posts: 9,809Member
    @Tim McElwain

    Here's my concerns on this specific topic.

    The chimney guy said he wanted to use a 5" because he needs to get it around bends in the offsets and mortar in spots.

    If he uses a 6", he may end up deforming, bending or damaging the liner in those areas and not tell anyone resulting in a far more dangerous, and impossible to clean product vs the 5" liner which would fit nicely..

    A 5.5" liner may also an option, I don't know.

    This was why my chimney was so dangerous. Someone jammed 4 sections of 6" liner into a small rectangular flue which also had an offset and it was impossible to clean and drafted very poorly. It would've been far safer without any liner at all.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Tim McElwainTim McElwain Posts: 4,239Member
    What is the internal dimension of the chimney without the liner? Is it brick and mortar or did it have a clay tile liner? I would say try firing it without the liner except you are already planning to reduce boiler input so it really will be a condensate issue with the chimney. I do not think you are going to improve draft no matter what you do. You might have to install a draft inducer.
  • captaincocaptainco Posts: 424Member
    Chrisj is pretty close. According to the National Fuel Gas Code you de-rate the B-Vent chart by 20% which includes one 90. Then it says de-rate 5% for each 45 degree(offset) X2 or another 10%. This is assuming there are no other kinks or deformations in the liner (fat chance).

    What these charts don't take into consideration is whether the mechanical room is positive or negative pressure but they assume positive pressure, which is rare if you follow Code. They also assume the building is fairly tight but not too tight?

    If you go with 5" liner you better plan on a smaller boiler.
  • weekendguyweekendguy Posts: 181Member
    Thank you all for your comments. I'll do some more research!
  • Bob HarperBob Harper Posts: 813Member
    If this vents under positive vent pressure you cannot vent into a liner listed to UL 1777. You must use a liner listed to UL1738. The manual on this boiler is all over the place on venting. They want it to be all things. At 85% AFUE, this cannot vent under atmospheric pressure. Treat it as a CAT III appliance and use AL29-4C venting and be done with it. Jim is correct on derating if it is a corrugated liner. There are smoothwall ss liners available for CAT I gas and oil in 316Ti, which is not suitable for a CAT III or IV appliance.
  • weekendguyweekendguy Posts: 181Member
    @Bob Harper That you for your comments. I'm not following your statements, unfortunately and could use some help understanding. Update on the plans - the new boiler will be a (smaller) Burnham ES24 with a 105mbh input. Both the chimney installers I had quote stated that the standard 5" SS liner is very adequate for this boiler, especially with almost 50 ft of chimney. The boiler also has a 5" vent. Does this plan sound ok for the new boiler?

    If this sounds right to you, then the remaining question is how the liner will work with the current boiler for a few weeks, which is currently venting right into the clay tile chimney like the oil burner did. It was a 7" vent but I have dropped its firing rate to 150mbh input which corresponds to a burner having a 6" vent. The chimney installers again look at the charts and see that 150mbh is fine with a 5" liner and the size fits within the NFPA guideline of 1" reduction.

    I'm not sure about positive vent pressure, actually. It's a Weil oil boiler with a Carlin propane conversion. It appears to move combustion air much in the same manner as the oil burner did. There is no separate induced draft device. Intuitively it seems like a chimney 50' tall would have plenty of negative draft (burner wants - .01 to .02). If this is unlikely to work, I can try to plan the install so that the liner goes in immediately followed by the boiler. I was leaving some wiggle room (time) in case the chimney installer has trouble fishing the liner down past the fireplace and I had to go a completely different route. I hear stories of unforeseen problems and I'm trying to be cautious. I also don't want to jerk around any installers and waste their time.

    I appreciate your comments and guidance. WEG
  • captaincocaptainco Posts: 424Member
    weekend guy this is definitely a negative pressure flue not positive. Doubtful the liner will work with existing boiler.

    Understand that most flue sizing is based on mathematical formulas versus actual measurements and then based on a fixed environment. If the math used to size flues is anywhere close to how they size duct systems there is going to be a problem. Theoretically a 50' tall flue would have good draft if what is vented into has plenty of room to flow. Hopefully the new boiler will be induced draft and not a drafthood type.

    In the mean time: Merry Christmas!!
  • weekendguyweekendguy Posts: 181Member
    @captainco Thanks for your comments. I am really confused about this. The duravent manual says that 50 ft of 5" liner shows maximum inputs around 200mbh. The current boiler is 150mbh. The combustion air is 4" vent pipe and set at 20% open. When you say "doubtful the liner will work with existing boiler", do you mean the chimney will not be able to create a draft of -.01? Why would it not?
  • captaincocaptainco Posts: 424Member
    Draft is created by pressure and temperature difference of indoors to outdoors. Under one set of perfect conditions I am sure the 50' 5" flue will work. It is all the other conditions that I fear it won't. The venting tables are based on a set of fixed conditions, not variable. The actual standard venting tables originally had a disclaimer that stated if your house is tight or loose, you have windy conditions, if you have any other exhausting appliances in the building or any other the conditions the vent designer must take this into consideration and make necessary adjustments Unfortunately most are unaware of this.

    50' of flue can create a great deal of displacement, cold air displacing hot air which is how venting works. However it also create a lot of resistance or static to flow just like ductwork. There still has to be room in the flue for the gases to move.
    I teach in class the 100% of rusting of equipment is caused by improper venting. I rarely have seen a piece of equipment that didn't have rust on it which means over 90% of equipment in the field today isn't venting correctly. Everyone follows the same venting tables most of the time. What does this say about the venting tables?

    The next problem that will occur is condensation. Liners tend to absorb heat from the flue gases extremely well. This will cool the gases and cause them to condense. The more contact the gases have with the surface of the flue the more they exchange heat and therefore the smaller the flue the more contact especially if it has ribs. I have never in over thirty years working with contractors in the field recommended making the flue as small as possible but rather always go slightly bigger.

    105,000 btus has a good chance of venting into a 5" liner. The only question is will the flue gases escape before cooling down and condensing?
  • weekendguyweekendguy Posts: 181Member
    Thank you for your comments.
  • j a_2j a_2 Posts: 1,796Member
    You must follow the manufactures installation recommendations...and any local codes....It is not up to the chimney guy, to decide anything, it's up to the gas installer and permit's his call and his liability....God forbid something happens....Venting is very very important, as you know,otherwise you would not be asking...Is he proposing a s/s liner ,a aluminum liner,or a smooth liner...or what...Had some squirles feast on an aluminum liner to the point it It became disconnected from the cap....liner dropped down....could of been real ugly...The fire department when they returned to the station were savvy enough to check to see if a permit had been filed on the job....Thank God I filed one and no one was hurt...
  • weekendguyweekendguy Posts: 181Member
    Thanks for all the comments. I have read enough reports of problems with "permitted" alum liners that I'm going to use SS for sure. There is no permit required for a liner install here. I will have a permit for the new boiler. By "gas installer" - the boiler installer or the propane delivery company? At this point I'm going to forget about trying to run the old boiler on the new liner and just plan to have them put in back to back. I'll wait until I am sure the liner can fit around the smoke shelf and then we'll start the boiler install. Probably be cold for a few days here :-).
  • j a_2j a_2 Posts: 1,796Member

    Thanks for all the comments. I have read enough reports of problems with "permitted" alum liners that I'm going to use SS for sure. There is no permit required for a liner install here. I will have a permit for the new boiler. By "gas installer" - the boiler installer or the propane delivery company? At this point I'm going to forget about trying to run the old boiler on the new liner and just plan to have them put in back to back. I'll wait until I am sure the liner can fit around the smoke shelf and then we'll start the boiler install. Probably be cold for a few days here :-).

    Can we ask what area code are you in? And what gas code does your area go by?
  • weekendguyweekendguy Posts: 181Member
    @j a

    Thank you for asking. I'm in area code 215. Philadelphia exurbs. The local authority does not specify a fuel gas code. I'll check with the code officer to clarify who has ultimate responsibility for correct chimney venting. I want to get this straightened out before any more steps are taken.
  • j a_2j a_2 Posts: 1,796Member
    Yea, no problem....Different regulations, everywhere..I would just think if a gas permit is required it has to include the venting....But not my area of country...Someone down there will no better.....I am sure it's available on line
  • Bob HarperBob Harper Posts: 813Member
    Sorry I'm just getting back on this. If the chimney contractor is worth his salt he can break out tiles at any major restrictions. From all that has been said, if you haven't already done the liner I'd recommend a smoothwall 5.5" 316Ti ss liner. This has about the same venting capacity as a 6" corrugated liner and goes down much easier as its outer wall is slick. It is heavy as its 4 plies but smooth inside for very little flow resistance. Aluminum liners are junk and should not be used. They only have to pass a 100lb dead load test. However, it takes stretching out, which often exeeds that load easily. They cannot take offsets as they crush easily and rip at the slightest snag. To combat condensation, put a post purge timer on the burner set for at least 4 minutes. The addition of a double acting barometric damper will add dilution air. You can pull the liner through at the base, which has less flow resistance than a tee.
  • weekendguyweekendguy Posts: 181Member
    Thank you all for your help with this. Yesterday the liner installer arrived and after setting up 6 levels of scaffolding was able to drop a test piece of 5.5" pipe all the way (I could see it at the cleanout). This size is sufficient for the boiler size. I asked about smooth wall but he discouraged that saying they dent easily. I may revisit that but the 5.5" does meet the BTU requirements with a good margin so I'm thinking this is ok. I'll look into the post-purge timer. I currently have a DA damper and I see what you mean - it opens somewhat to help control the draft (I never realized this was mixing in drier air until now) but the new boiler has no specs for a damper and must be installed to the mfr instructions to pass the code inspection.

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