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Community and Q&A

Stack damper for residential steam boiler

kjmass1 | Posted in General Questions on

I’m working on getting my basement insulated and airsealed, but on top of that I was wondering if I should consider an automatic stack damper for my 1970s era oversized 200k natural gas steam boiler. Seems like their reviews are mixed- either only a 1-2% savings or don’t last long enough to recoup their cost. Any insight here?


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  1. Expert Member
    Peter Yost | | #1

    Hi Kevin -

    Looks as though the cost of an automatic vent damper is $100 - $400. Seems sort of pricey for minimal gain.

    How close are you to the end of this unit's service life?


  2. kjmass1 | | #2

    It’s a 1970s boiler but I’m trying to keep it as long as I can- hasn’t failed me yet and steam boilers are a very basic as far as tech goes.

    I have a hard time believe only a 1-2% savings with a damper so that’s why I mentioned it. A tremendous amount of heat goes wasted so I figured trapping it in the basement and keeping the boiler warmer, longer would make sense.

    I’m putting the money towards a replacement towards closed cell on my basement walls and sills, so hoping that is a better strategy at this point. Also finishing blownin on second floor walls.

    Hard to justify $7k-10k on a boiler that will never last as long as this one.

    1. Expert Member
      Dana Dorsett | | #3

      >"Hard to justify $7k-10k on a boiler that will never last as long as this one."

      A new steam boiler would last as long as that one, but you probably won't want to continue the low-efficiency fossil burner legacy going forward. Putting the $7-10K toward something more efficient than steam, or with greener energy sources might be a better bet.

      It's a long shot, but in the unlikely event that your system is 2-pipe steam, those can usually be converted to pumped hot water hydronic heating without ripping it all out and starting over. With a condensing boiler the efficiency could be running in the 90s rather than the 70s. At the fewer bells & whistles end of what's out there, a right-sized mod-con boiler is less than half the cost of a typical steam boiler.

      1. kjmass1 | | #4

        Unfortunately it is 1 pipe steam so converting to HW isn't a feasible option.

        My non-hyperheat multi splits probably have enough to cover the load once I finish insulating, with some head rearranging, but with a lousy 9.5 HSPF and high electric costs in MA on top of not being the low temp models it might be a stretch with my current set up. I use about 1100-1200 therms per year during the heating months.

  3. Expert Member
    Dana Dorsett | | #5

    At a generous 80% efficiency 1200 therms/year means the boiler is delivering at best 96 million BTU/year, in a ~6000 HDD65 climate comes to 16,000 BTU/HDD or 667 BTU per degree hour. Assuming a design temp of +5F and a balance point of 65F you have 60F heating degrees, and a design heat load of about 40,000 BTU/hr.

    A 3 ton cold climate multi-split can deliver that much with margin:

    In competitive bidding either of those would come in under $8K during seasons when the installers are hungry, maybe $12K during periods when it's a seller's market.

    After insulating the basement you'd be looking at a 2 or 2.5 tonner, which should be do-able for well under $10K, and at an as-used HSPF of 10 and 20 cent electricity (10,000 BTU for 20 cents, 80,000 BTU for $1.60) would have a comparable operating cost to 80% efficiency gas at $1.60, neither of which is not too far off of average MA pricing.

    With tempertures better controlled by zoning and fewer parasitic losses it's pretty much a wash on operating cost, and might even favor the multi-split even at 22 cent electricity and buck-fifty gas.

    The cost of rooftop PV after all incentives are applied is well under 20 cents/kwh in MA, which would ultimately be cheaper operating cost than a shiny new best-in-class gas fired residential steam boiler.

    There is not a strong financial incentive to make that pretty much sideways move just yet, but keep the cold climate multi-split (or multiple cold climate mini-splits, which is usually cheaper and more efficient) in mind when the boiler is finally at end of life, or if/when natural gas hits two bucks.

    1. kjmass1 | | #6

      That's pretty spot on but with 5500HDD outside of Boston. I did a room by room Man J and came up with 44K+infiltration @ 9F/70F. Getting my basement walls to R15 + second floor walls to R15 should get that close to 20K like you said.

      I have (2!) AOU24RLXFZ's. Poor choice I now realize. Will those still hit the advertised 9.5HSPF with temps near zero like we had this year? If I'm reading the specs right the pair will still be good for ~34K at 5F. I only have 1 head on the first floor/1100sf so not sure that would cut it keeping all the rooms warm.

      Regardless, not sure what happened with the boiler this past billing period as it shot up to 900BTU/degree hour from a seasonal average of 727BTU/degree hour in winter '16-17. Yearly serviceman mentioned he tweaked the firing, but doesn't seem like for the better. Probably mad I skipped a year.

      1. tommay | | #7

        How does one "tweak" the firing of a gas burner? Check to see if he adjusted the pressure switch. It should be set at 2 lbs max or less. If he adjusted it higher, the boiler will run longer and harder before shutting down, cooling and allowing the pressure to dissipate through the system. If heat is still calling, once the pressure drops, the boiler will kick back on.
        Did he check all the air vents on each radiator? If not working properly, opening and closing, heat may not be reaching the radiators properly, or to those in proximity of the thermostat. When was the last time he removed and checked the pigtail pressure tube for blockage? If even partially blocked, boiler pressure will raise higher to satisfy the pressure switch and if it is totally blocked, pressures may reach relief valve settings and not shut off, until boiler water level drops. ( Hopefully no automatic water feed)
        If it has a standing pilot, an automaitic damper has to have a hole in it to vent the pilot exhaust. If your boiler did not come with an automatic damper then controls would have to be switched out, if possible, otherwise the manual weighted damper should suffice. Just make sure it operates correctly and closes when the boiler stops and opens when exhaust gas is present.

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