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venting in a retrofit situation

deek Member Posts: 21
Hi Folks

I have a viessmann ecd 140 boiler with a 7 inch draft hood outlet.  It is an 84

% efficient non condensing boiler after therefore a category 1 gas appliance.  The problem is my chimney is 6 inch B vent.  Is there any code compliant way to use my existing chimney with my boiler?



I should add a few details...

116000 btu DOE. 2 foot horizontal run. 14 foot vertical to the roof top. Two 45 degree bends.



  • Tim McElwain
    Tim McElwain Member Posts: 4,588
    The input of 140,000 BTU's

    for that unit is well within the ability of the 6" Double Wall "B" vent. A 6" vent will handle up to 225,000 BTU's at 15 feet height and 2 foot lateral. Code also allows for reduction under NFPA 54/ANSI Z223.1 section

    13.1.2 Vent Downsizing.

    Where the vent size determined from the tables is smaller than the appliance draft hood out­let or flue collar, the use of the smaller size shall be permitted, provided that the installation complies with all of the following requirements:

    (1) The total vent height (H) is at least 10 ft (3 m).

    (2) Vents for appliance draft hood outlets or flue collars 12 in. (300 mm) in diameter or smaller are not reduced more than one table size.

    (3) Vents for appliance draft hood outlets or flue collars larger than 12 in. (300 mm) in di­ameter are not reduced more than two table sizes.

    (4) The maximum capacity listed in the tables for a fan-assisted appliance is reduced by 10 percent (0.90 X maximum table capacity).

    (5) The draft hood outlet is greater than 4 in. (100 mm) in diameter. A 3 in. (80 rom) diam­eter vent shall not be connected to a 4 in. (100 rom) diameter draft hood outlet. This pro­vision shall not apply to fan-assisted appliances.


    Is it permissible to use a vent smaller than the flue collar or draft hood outlet of the appliance?


    There is an economic incentive to using smaller vents wherever possible. If the vent is smaller than the draft hood or flue collar, venting problems can occur. The restrictions in 13.1.2 rec­ognize and avoid these venting problems. Limits are placed on downsizing. In particular, note that a 4 in. (l00 mm) draft hood outlet may not be reduced to 3 in. (80 mm).


    So go ahead as you meet all the requirements. I would alert code officials however just in case some local AHJ has a different rule.
  • deek
    deek Member Posts: 21
    edited June 2012
    What a relief to hear...

    Wow...Thanks Tim.  That was a much better answer than I ever expected.  I am really blown away by the knowledge and the willingness to help on this forum! Thanks so much.

    I have one other question.  Do manufacturer instructions overrule NFPA rules?  The Viessmann manual states that the intermediate piping from the vent collar to the chimney must be of the same diameter as the vent collar.  Should I then place the reducer at the beginning of the vertical b vent?

    Of course I will call my inspector to verify but in my experience my local inspector only knows typical installations so its good to have as much proof as possible before we talk...


  • Tim McElwain
    Tim McElwain Member Posts: 4,588
    Yes manufacturers instructions

    typically prevail. In this case it has been my practice to make the reduction at the flue collar. I would use 6" "B" vent also on the horizontal run to the vertical 6" "B" vent. There is a single wall to "B" vent adapter your local supply house should have. I would use a 7" to 6" reducer then the adapter to "B" vent. Good idea to talk to the local inspector and give him the reference from NFPA 54 on the reduction.
  • VictoriaEnergy
    VictoriaEnergy Member Posts: 126
    edited June 2012
    Vent could have issues

    I'm thinking that the limit for appliance efficiency allowable under Cat I venting is 83%, so you might be a Cat II, and if not, then certainly close to it. 

    You have to consider this vent may run long wet times since there will be much less dilution air drawn through the draft hood with the smaller vent.  Long wet times = short vent life.
    Home Owners Please Note:

    You are receiving advice from some very skilled pros completely free of charge. One of the reasons I participate is to sharpen my own troubleshooting skills. So; did we get it right? I would be grateful if you extend this courtesy back by posting the final outcome of the issue you are inquiring about. Thanks
  • Bob Harper
    Bob Harper Member Posts: 977

    The mfr requires you inspect the B-vent and replace it if ANY deterioration is present as they deem deterioration an unsafe condition. The specify inspect it inside and out. That means an NFPA 211, Ch 14. level II inspection. When you look at B-vent warranties, you will quickly realize an appliance rated at 83.7%AFUE will rot this vent much quicker than a low efficiency typical CAT I B-vented appliance. Now, the mfr also states this appliance cannot EVER be under negative pressure. That means you would have to install a powered makeup air system interlocked to the gas control in order to meet this requirement. Failure to do so with such a higher efficiency CAT I appliance will likely result in flue gas spillage and backdrafting regardless of vent sizing or materials. The section of code Timmie quoted allows vent downsizing on a restricted basis. One of those restrictions is that fan assited appliances cannot be downsized. Not sure how this unit was listed without a fan assist at that AFUE but that doesn't make it a good idea to sell or install. The code also requires the venting work. The only way to determine this is testing. It is not enough to verify it meets the written requirements by the code and mfrs listed instructions.
  • Tim McElwain
    Tim McElwain Member Posts: 4,588
    It has been lately

    that manufacturers have specified Category I appliances at 83.5% to 83.9% (Just under 84%) in order to stay within Category I venting options. I realize this is a marginal area but inspectors and manufacturers are allowing it so there is really no official basis for saying you can't do it. What I try to recommend is to go to double wall all the way and attempt to get some RISE at the appliance for some self venting. In reading and to the best of may ability to decipher the engineering data the Supplement 1 "Development of Revised Venting Guidelines" in the 2009 National Fuel Gas handbook it allows some margin for error the choice has to be made by the installer and he will have to live with the marginal stuff.

    The truth is venting has always been an issue and with higher efficiencies on new what is classified Natural Draft or Fan Assisted equipment even though the code allows something I always tell my students bottom line it has to work.
  • Tim McElwain
    Tim McElwain Member Posts: 4,588
    edited July 2012
    Here is the report from

    Viessmann on their product and its venting:

    Hi Timmie,

    The ECD boiler is indeed listed and approved as a Category I appliance (Negative Pressure / Non-Condensing). The AFUE efficiency of the ECD140 is actually 83.5%. In the 15 years that I've been selling this particular boiler, I cant recall any chimney condensation issues. 

    I have included the underlined venting spec for the ECD boiler below. You'll notice a section that speaks to insulating the flue sections between the boiler and the chimney. 

    Let me know if you need anything else. 


    For proper operation of the Vitogas boiler, all products of combustion must be safely vented to the outdoors, while ensuring that flue gases do not cool prematurely.

    It is critical that the chimney system be properly designed and sized to handle the relatively cool flue gas temperatures which the Vitogas boiler produces.

    Flue gases which cool too quickly and produce condensation lead to damages if the chimney diameter is too large and the chimney system is not well insulated.

    If a calculated chimney diameter lies between two values, the larger diameter should be selected.

    Intermediate vent section

    The intermediate (vertical and horizontal) section of venting between the boiler vent pipe collar and the chimney must be of the identical diameter as the vent connection of the boiler. Use the shortest possible path between the boiler and the chimney. A maximum of two elbows may be installed in the intermediate section. Avoid the use of two level 90° elbows.

    The intermediate section must be sealed pressure tight at the boiler vent pipe collar and at the chimney connection. Ensure any test port for combustion values is sealed as well.

    The chimney connection length between the boiler vent pipe collar and the chimney must be installed with insulation. We recommend consulting a reputable chimney installer for advice in project-specific circumstances. 

    Kevin Flynn

    Viessmann Mfg.

    45 Access Road

    Warwick, RI. 02886

    Phone (401) 732-0667
  • deek
    deek Member Posts: 21
    OK so...

    Thanks everybody for your input. I can see why condensing boilers are so appealing these days.  High efficiency non condensing appliances seem tricky.I am well aware that "I read it on the internet" doesn't stand up in a liable case so rest assured I am working with an installer and my local codes officer to make sure its done right. But knowledge is power and even my installer admits that hes no expert when it comes to category 1 appliances that operate at this efficiency.  Around here forced air is king and nearly everyone is going high efficiency with PVC venting.

    I read through the NFPA 54 and agree that it clearly states that downsizing is permitted in this case.  Of course that doesn't guarantee trouble free long term performance but it does give some assurance.  There is quite substantial difference in area and flow when going from 7 inch to 6 inch venting.  The area is 39 square inches versus 29 square inches or a 27% reduction. I cant help but to think that viessmann engineers put a 7 inch draft hood on for a reason.  Perhaps to allow for longer horizontal runs?

    The Technical data manual for the boiler states that "The intermediate (vertical and horizontal) section of venting between the boiler vent pipe collar and the chimney must be of the identical diameter as the vent connection to the boiler." Is this just so everyone's butt is covered?

    Also, I have never seen insulation installed on B vent. Is this common in the industry?  What type of insulation is suitable?

    I'll be sure to let you guys know how it all works out.  The job should go ahead in a few weeks.

    Cheers and thanks again.

  • Bob Harper
    Bob Harper Member Posts: 977
    venting per mfr

    According to the information presented, you must maintain the same diameter of the vent connector all the way to the chimney and only there can it be downsized one size. The mfr. requires this connector be 'insulated' yet this goes against NFPA 211 with one exception--listed connector. You can use a type 'B' or 'L' vent connector and meet this requirement legally. This will tend to reduce condensation, reduce clearances to combustibles yet meet NFPA 211, the listed instructions and the code. You can NOT wrap insulation over unlisted single walled or listed connector regardless of what a mfr says. This hides the connector from inspection which leads to failures going undetected. Wrapping a listed connector voids the warranty and listing as it invades the stated clearance to combustibles. Clearances are air spaces. Sure sure to support this connector, too. Use a 'draft hood connector' where it joins with the appliance flue collar. Seal it with a sealant rated for this duty rating, which should be in excess of 600F, which takes red RTV out of it. For a high temp. seal that is removeable unlike most furnace cements, try Mil-Pac refractory sealant. It is used a lot in the hearth industry and works great.
  • deek
    deek Member Posts: 21

    Hi all

    I just wanted to let you all know how it worked out.  The system is up and running and it is great!  Endless hot water and good even heat through the whole home.  I ended up contacting viessmann via email and they just referenced the same NFPA code that Tim mentioned.  So I followed his advice and downsized right at the vent damper and then switched to b vent.  My inspector had no issues with that.  I live in Canada so I have to follow CSA code but as it turns out it reads word for word the same as NFPA rules regarding down sizing.  One nice thing is the NFPA code is available to all for free via the internet but the CSA code costs nearly 300 dollars!  I believe the code should be available for free to all...contractors and the home owners. It is the rules that gas fitters and installers work to and the rules the home owner should expect a contractor to work to. Everybody should have access to the requirements.  Good on the NFPA and shame on the CSA...

    Thanks to all of you for the advice.  This website really is a great resource.


    Derek Rowney

    Calgary, Alberta
This discussion has been closed.