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Community and Q&A

How exterior air seal at chimney to wall interface? Chimney spalling.

Mike_McL_cz_4a | Posted in Energy Efficiency and Durability on


Using a FLIR One, I’ve found a major source of air leakage into my house from around the fireplace mantel. Using a homemade blower door and a smoke pen I’ve discovered that the air is in fact entering from the exterior chimney to sheathing interface. While I’ve been able to find extensive discussion and documentation on this site how to handle through attic air gaps for a chimney, it’s been less clear to me how to handle this detail on the exterior wall to chimney brick intersection.

Appreciate any insight or examples others have undertaken to tackle this problem. While I’m open to the simple solution at the interior by just caulking the mantle to drywall joint, I’m most interested in the exterior detail to really solve this problem.

Also, secondary, is the spalling at the chimney brick face. I’ve had the top of the chimney repointed and capped with a rain cover, but spalling continues during freeze/thaw cycles. I’m concerned it may not be a bulk water intrusion issue but instead from flue gas condensation. Not sure how to isolate the problem.

I’ve attached a series of photos to support this question. Appreciate, in advance, responder’s help and advice.


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  1. GBA Editor
    Martin Holladay | | #1

    You might want to read my article, When a Flue is Too Big.

  2. Mike_McL_cz_4a | | #2

    Martin, thanks for the prompt reply. Read it, makes sense. Previous owners upgraded to a high efficiency furnace, so between the shorter cycling times and my own energy saving improvements you must be right. I looked up the furnace manual and found these words:

    "The new designs of high efficiency oil furnaces and boilers in conjunction with flame retention oil burners
    are more efficient. One result of increased efficiency is lower flue gas temperatures. As flue gases rise in the chimney, they will cool and condense when they reach the dew point. The condensation will mix with the sulphur in the flue gases creating sulphuric acid. The acid will attack the chimney mortar, brick and clay liners causing corrosion, deterioration and blockage of the chimney. Eventually the blockage could prevent exhausting the flue gases. Instead, the flue gases could vent out the barometric damper into the living space.
    Therefore, it is strongly recommended that an approved insulated stainless steel liner be installed."

    So frustrating to be left with this mess rather than doing it right the first time. Also, why doesn't either my furnace servicing company or the chimney sweep know to advise me on this? Rhetorical question, just a bit of frustration.

  3. walta100 | | #3

    Looking at your photos I think you have 2 problems.

    I do think you need a SS liner. The reason you do not have is because the manufacture of your furnace only recommended a liner instead of requiring one. My guess is you will find this is a $3000 upgrade.
    Just for fun make sure to get a bid from the contractor that installed the furnace, once they see the mess they made, they may stand behind there fix it.

    I think it is unlikely you can seal the air leaks on the exterior. I think you should remove the mantel and trim and seal the gaps with fire rated calk. Once it passes your blower door test reinstall your trim.


  4. GBA Editor
    Martin Holladay | | #4

    Walter is right. The standard solution is to line your flue with a stainless-steel liner.

    When it comes to air sealing, it isn't rocket science -- just ordinary crack-sealing. Buy some caulk. Either work from the interior, as Walter suggests, or temporarily remove the vinyl siding and J-channel and work from the exterior. Once the leaks are caulked, re-install the housewrap -- I hope your walls have housewrap -- and re-install the vinyl siding.

  5. Mike_McL_cz_4a | | #5

    #3 Walter: I had the same though re: approaching the installer (who happens to also have my oil and service contract last few years). I think your point about interior vs exterior is good, my wife hates the mantel anyway so might be the motivation to remove, fix the gaps, and replace.

    #4: Martin: I overthink all this stuff. Glad to have the grounding that the air sealing is actually simple and doesn't involve some complex solution. The housewrap....yeah hit and miss. As we remodel we are finding areas that never got it installed. We fix it as I come across it.

    We have the standard open hearth and know that it is a thermal nightmare. When installing the SS liner for the furnace it seems to make sense to go ahead and install a liner and Y-connection to the hearth for a high efficiency fireplace insert to go along with a new mantel.

    I'll revisit this post with updated thermal pictures once I tackle the exterior air sealing. Thanks for your thoughtful responses.


  6. Expert Member
    Dana Dorsett | | #6

    A couple of comments:

    Installing an oil-fired furnace or boiler without a properly sized stainless flue liner is a code violation in some states. It's more than just a good idea- it's the law (of physics, if not the local code.)

    A boiler is not a furnace. In most US -'merican dialects "furnace" in the context of HVAC is always hot-air delivery. You have a boiler. And...

    ... a 126,000 BTU/hr boiler (as indicated in the caption of the third picture) is LUDICROUSLY oversized for the actual heat loads of 19 out of 20 homes in the US, and may even be oversized for your radiation (particularly if there is more than one zone.)

    For a sanity check on your oversizing factor, first run a fuel use heat load calculating using weather data for the periods between fill-ups (mid to late winter fill-ups only, for best accuracy) using these methods: (If you're on a regular fill-up service that stamps a "K-factor" on the slips, the K-factors have enough information, since K= HDD65 / gallon.)

    To see what the oversize factor does to your as-used efficiency, see Table 3 in this document, which shows the tested seasonal efficiency of a number of different boilers at both 2x and 3x oversizing factors:

    Then, zone by zone, add up both the zone and total radiation. If fin-tube baseboard, multiply feet x 500 BTU/hr (roughly the output of the baseboard at an entering water temp of 180F). Compare both the total and zone ratios to the DOE (not IBR-net water) output of the boiler, and report back. If it's high volume radiators, estimate the square feet EDR (equivalent direct radiation) using this guide:

    Zone by zone, multiply EDR x 170BTU/hr (the approximate output at 180F EWT.)

    If your boiler is 3x oversized for the whole house load, installing a retrofit heat purging boiler control such as the Intellicon HW+ (a DIY-able, decent, not too expensive unit) will pay for itself in less than two heating seasons. If your boiler is more than 3x oversized for any individual zone radiation it'll pay for itself in less than one heating season. With heat purge controls the hit in efficiency comes closer to that system #3, which is a significant improvement for most of the boilers tested.

    Run the numbers. Sharing therm here may prompt more more detailed feedback & recommendations on those issues.

    Any near-boiler or heat distribution plumbing would be required to be insulated to at least R3 under IRC 2015. If you see a lot of gleaming copper it's worth installing at least 1" wall fiberglass pipe insulation on everything that is accessible, not the crummy half-inch stuff (R1.5-R2-ish) stuff found at box stores. (Plumbing supply houses catering to the trades, or online supply houses have the right stuff.) Temperatures are going to be too high to use cheap foamed polyethylene more suitable for potable hot water plumbing (also required under IRC 2015 for 3/4" or fatter plumbing, or any recirculation loops- both supply & return, and any size pipe between the water heater and distribution manifold etc.)

    An oversized boiler giving up it's standby heat to an uninsulated basement would also be an issue.

    Not all boilers are created equal- got a model number for that Peerless?

  7. Mike_McL_cz_4a | | #7

    Dana, there is a lot to take in here so I'll need a day or two to get this all calculated and read through the references. I'm at the point tonight that I've got historical data from 2014 when I tried to do something similar. @65 degree HD I'm at ~24K BTU/HR heat load (95 gallon oil=13.4MBTU; *.85 eff=11.4MBTU; 1104 HDD, 9 degree 99% [oil burn rate is from January]). I'll need more time to do the measurements and calculations for the zones and base board fin heat.

    At some risk to expose what might be poor workmanship on my part, I'll share a picture of my DIY pipe wrapping and insulation around the furnace to address the heat loss. It was bare copper throughout the basement when we moved in (and no basement door to outside! Now fixed). I work on submarines ('merica!) and know what good pipe insulation and lagging is supposed to look like...this isn't it.

    Also, posting photo of boiler plate for model (WBV-03-WPCTL), I have an "F4" head on my Beckett burner for my boiler (not furnace, I stand corrected) so gallon per hour is somewhere .85 to 1.35. I will point out that I had the outside air kit installed to ensure I maintained combustion safety as I air seal my house.

    Nice exercise to work my way through these calculations, I'll report back soon.

  8. Expert Member
    Dana Dorsett | | #8

    Seems you're BTU/gallon estimate is on the high side for #2 oil at 141,000 BTU/gallon. It could be as low as 137,000 or as high as 142,000. The EIA uses 138,500 BTU/gallon for the general estimate, but I usually use 138K. Whatever source fuel BTU numbers are use, they're, within a few percent of each other.

    Using 138,000 BTU/gallon and 85% efficiency I'm coming up with 421 BTU per degree-hour. So at even at 0F outdoors you would have (base 65F -0F= ) 65F heating degrees x 421 BTU/F-hr = 27,365 BTU/hr.

    At +9F you'd have 56F heating degrees x 421BTU/F-hr = 23,576 BTU/hr.

    Even jetted down to a 0.85 gph firing rate the output would be 138,000 x 0.85 gph x 0.85 efficiency= 99,709 BTU/hr, which would be ~4x oversized for the 99% load, and at least 3x oversized even for the Polar Vortex extremes.

    Theoretically you have enough burner to keep the place cozy when it's an uncomforatably cool -160F outside, eh? ;-)

    For the radiation to balance with the ~100K output without ANY cycling takes about 200' of fin tube baseboard, or 600 square feet EDR of radiator.

    It seems you have an embedded tankless coil for domestic hot water, which is probably the rationale for the gross-oversizing factor. To get reasonable performance out of the tankless coil required a higher standby temperature, which necessarily comes with a higher standby loss. Without the coil you'd be able to set the low temp limit for a heat purge or other control to 140F (after the stainless liner is installed) to lower the standby loss. The higher standby temperature also limits the amount of heat-purging temperature swing window you'd be able to get out of it with a heat purging boiler control.

    Given the oversizing factor, after installing the necessary flue liner, seriously consider insulating the foundation to IRC code min (probably R15 continuous insulation, for any location with a 99% outside design temp of +9F.) With an uninsulated basement a large fraction of the standby loss of the oversized boiler is truly lost through the ~R1-R1.5 above-grade exposure on the foundation (as well as to the below grade soil.) With insulated walls the bulk of that standby loss will stay inside the house, lowering the total heat load and fuel use.

    Starting on p.11 of the manual for that boiler ( ) shows wiring options for two different smart-controls the Beckett AquaSmart 7600, and the Hydrolevel Hydrostat 3250:

    Both of these economizers have more programming bells & whistles than the Intellicon HW+, ( ) but any one of them are going to be worth it, at your oversizing factor, even if continuing to heat the domestic hot water with the tankless coil. They typically run ~$150 at internet store pricing.

    BTW: You can keep calling it a "furnace" if you like, but it's still a boiler, and not a furnace. :-)

  9. Mike_McL_cz_4a | | #9

    Dana, my mind is blown by your method and the result for my house. I had no idea, although in retrospect my wife has always said that the *boiler* sounds like a jet engine when it lights off and I have been in awe how quickly the digital water display for jacket temperature rises during a cycle. So maybe I shouldn't have been so surprised. I'll take a look at your suggested smart controls. Ultimately, with this knowledge, I'm more convinced than ever to add mini-splits and isolate hot water production from the boiler.

    I've been reading up on basement insulation effects on energy usage. Nice paper on modeling efforts here:

    I've got a post up listing some lessons with my experience installing exterior insulation here. I need to update with final pictures and I'm sold on the need to do this on the remaining perimeter of the house.

    Off to update that post....

  10. Expert Member
    Dana Dorsett | | #10

    I jnoticed in your second picture, you DO have one of the heat purging controls already installed! (which is good!) It looks like the HydroStat 3250+, but I can't read it at the resolution posted:

    For yuks, over the weekend time and count the burns for a few hours when it's cold out.. If the minimum burn-time is <5 minutes it's slipping over the knee in regression curves indpendently of duty cycle, but cycle matters too.

    A duty cycle of less than 10% means the as-used AFUE is closer to the summertime hot-water-only efficiency than it is to the steady state combustion efficiency in that Brookhaven National Laboratory test. See Table 2 and Figure A1-2 in Appendix 1, but also scan through Appendix 1 & Appendix 3 (with heat purge control applied.) You can see in Figure A3-2 ia lot better than Figure A1-2 when operated at low duty cycles. If you're lucky the Hydrostat can be tweaked to deliver the longer burn times, though nothing can be done about duty cycle. Read the manual on the Hydrostat (I haven't fully read it) and write down what the current parameter settings are. Your system is somewhere between System 1 & 3, since you have (at least the potential for) heat purge, but a higher idling temperature, and don't have the thermal mass of an indirect water heater full of water to work with. If you can still get reasonably domestic hot water service out of it with a 140F minumum temperature, that's where it should be operated AFTER installing the stainless flue liner. (Running that cool will increase mortar-destroying flue condensation in an oversized clay-lined chimney.)

  11. Mike_McL_cz_4a | | #11


    Searched all over for a model number unsuccessfully, but from Hydrostat website photos this must be the 3250-Plus. Setting are "Economy=1"; "Lo Temp=180'; "Hi Temp=200". I have a two zone system, pretty sure I ought to be set to "economy=2". Upstairs and downstairs zones. From post #6, I added up floor board fin length. 55 feet and 2 under cabinet electrical driven blowers (hydronic heat source); upstairs I have ~65'. From oil company I got data from last three years of delivery, working to isolate HDD for each delivery (I'm at ~34KBTU/HR average). Fin baseboard BTU to oil BTU comparison seems way off, but I haven't fully followed through on your methods above. I think I'm at the point where I know I'm >3x oversided, my hydrostat is probably not programmed/set right, and I still need a SS liner.

  12. Expert Member
    Dana Dorsett | | #12

    Only the wintertime fill-ups are relevant for use on heat load calculations. That's when the duty cycle is highest (= fraction of standby loss lowest), and when the solar gains are lower, and when the domestic hot water use is a lower fraction of the energy use. Fill-ups after March or before December are prone to much higher error from those factors.

    If the oil company lists a "K-factor" with each delivery you don't need to look up the degree-day-data for the time period. The K-factor is HDD65 per gallon, so the information is already there. The highest K-factors will usually be during winter periods, when the heat loads were high, and the errors from other factors were low, with solar gain error canceling some of the hot water use error and conversely.

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