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Does cast iron boiler efficiency decrease over time?

Shahrdad
Shahrdad Member Posts: 100
I have an almost 50 year old cast iron WM boiler which is labelled as having an input of 245,000 output of 196,000. This boiler has run all these years with return temperatures between 90s and low 120s degrees, and it has never had a loop installed to keep the return temperatures above condensation.

I am wondering if over time with the aging and rusting of the boiler, the efficiency of the boiler decreases, or is the boiler still able to put out something close to the original claimed heat output?

Thank you all, and Happy Valentine's Day!

S.

Comments

  • WMno57
    WMno57 Member Posts: 194
    New efficient boilers get some of their higher efficiency by having thinner castings.

    I figure my Weil-Mclain number 57 has lost so much to rust over the last 70 years that it is now 120% efficient. (If the Heat Pump and eMPG people can make up numbers, then I can to).

    Old boilers never die, they just rust away.
    JohnNYShahrdadbucksnort
  • hot_rod
    hot_rod Member Posts: 14,786
    edited February 14
    hard water deposits on the inside, soot on the outside would certainly lower the heat transfer from fire to water. Do a combustion analysis, note flue temperature, to get some idea.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Shahrdad
  • pecmsg
    pecmsg Member Posts: 1,989
    Clean boiler in relatively good shape. A small %. 

    Bigger Question is how oversized it is?
    Shahrdad
  • JohnNY
    JohnNY Member Posts: 2,659
    Surface rust/carbon buildup on the sections, sludge and sediment buildup inside the heat exchanger, dirty/rusty burner tubes, old gas valve, etc will certainly contribute to a measureable decrease in efficiency.
    Contact John "JohnNY" Cataneo, Master Plumber
    in New York
    in New Jersey
    for Consulting Work
    or take his class.
    Shahrdad
  • Shahrdad
    Shahrdad Member Posts: 100
    edited February 16
    pecmsg said:

    Clean boiler in relatively good shape. A small %. 


    Bigger Question is how oversized it is?
    I don't think it's oversized. It's a 1970s Weil-McLain with an input of 245K, output of 196K, and Net IBR of 170.4. It's pumped with the original 1970s Taco 110. It's connected to the home's original 1897 gravity system with cast iron radiators. The house is a 2 1/2 story brick house somewhere between 5,000 and 5,500 square feet. The attic has been insulated and there are storm windows. For an old house, it's not too leaky either.

    With temps at zero degrees or just a bit below, the boiler runs continuously, producing a supply temp of 132 and a return of 119-120 degrees. When the outside temp has warmed up to 6 or 7 degrees, the boiler starts to cycle off and on. The house has stayed at a comfortable 70 degrees.

    The reason I was asking the question is that I am considering replacing the almost 50 year old boiler with a Viessmann Vitocrossal CU3A rated at a max input of 199 and Net AHRI rating of 161. Watching my small cast iron boiler run non-stop at these temperatures, I'm wondering whether the Vitocrossal is going to have enough heating capacity for temps such as this.

    If my old boiler is still giving me the full 196K output, but has to run continuously at these temps, I'm starting to worry that the Viessmann might not be big enough for the house. If the old boiler is actually producing something lower than what it was originally rated at, the Viessmann might be adequate.
  • pecmsg
    pecmsg Member Posts: 1,989
    edited February 16
    Shahrdad said:

    pecmsg said:

    Clean boiler in relatively good shape. A small %. 


    Bigger Question is how oversized it is?
    With temps at zero degrees or just a bit below, the boiler runs continuously, producing a supply temp of 132 and a return of 119-120 degrees. When the outside temp has warmed up to 6 or 7 degrees, the boiler starts to cycle off and on. The house has stayed at a comfortable 70 degrees.
    that boiler shouldn't see return water below 140°f.

    What are the emitters, baseboard or cast iron?

    Have a accurate heat load loss performed, with those operating temperature Yes it is oversized. If it heats OK consider a condensing boiler.
  • Shahrdad
    Shahrdad Member Posts: 100
    pecmsg said:



    that boiler shouldn't see return water below 140°f.

    What are the emitters, baseboard or cast iron?

    Have a accurate heat load loss performed, with those operating temperature Yes it is oversized. If it heats OK consider a condensing boiler.

    The way the boiler was piped originally (no bypass loop or anything) it's been seeing return temperatures of below 130 for almost 50 years. Most of the winter, the supply is in the upper 90s and return in the 80s. The emitters are ornate cast iron radiators. I think it's a matter of time before the boiler springs a leak, as there is always a rust powder on the bottom of the boiler and in the burner tubes.

    The previous owner had a heat loss done before this boiler was chosen in the early 1970s. I should add that all my neighbors, many with houses half the size of mine, have boilers that are 1 1/2 times bigger than mine, and most of them are recent installations!

    The Viessmann Vitocrossal I'm considering is a condensing boiler. But how can my current boiler be oversized if it has to run continuously at design temperatures of zero degrees to keep the house at 70 degrees? It's been heating the house just fine, even with these low temps. However, with outdoor temps just below zero, I have never seen a supply temp higher than 132 on my boiler.

    Thanks!!

  • Shahrdad
    Shahrdad Member Posts: 100

    @pecmsg we've already had this conversation in a different post. 

    I should have continued it on that post. I was pretty much set on the Viessmann that is just slightly smaller than what I have now, but seeing my boiler operate nonstop at 0 degrees is giving me second thoughts.

  • AdmiralYoda
    AdmiralYoda Member Posts: 319
    If zero degrees is your design temp, it seems like it's doing what it is supposed to do.

    It works its butt off at zero degrees, anything above makes it work less....anything colder and it still works its butt off but can't keep the house at 70 degrees.
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