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Modulating condensing or traditional cast iron boiler for hot water cast iron radiator system

mariusz6100 Member Posts: 4
edited April 7 in Gas Heating
I need to replace my old AO Smith copper tube 128K boiler that heats a gravity conversion cast iron radiator system and am in between whether I should go with a new cast iron boiler (like a Burnham 2 series) or a modulating condensing boiler (Nortitz NRC or NTI TNT).

I noticed that my current boiler has somewhat of a big temperature differential. The boiler temp is 130 and the water was 110 after the air separator (a big old Purgetrol) and returns at 96. I am concerned about the condensation risk going forward and I want to eliminate it.

If I go with a non-condensing cast iron boiler, I am concerned about condensation, but that risk can be mitigated with a primary/secondary loop with a mixing valve and maybe running the boiler at a higher temperature than 130 (I wonder if there is a reason it is set at 130 and not higher). I could also go with a radiant mixing block, but not sure if this is an overkill. The main benefits would be longevity (20-30+ years) and less maintenance. And I can keep venting through the chimney, which I am getting inspected to make sure it is fine and doesn't have any condensation damage. This is probably a cheaper option.

If I go with a modulating boiler, I eliminate condensation risk, I can get some space back. I would probably save money on gas, but the average life is shorter (12-15 years). I would vent outside and drain the condensate to the sump pump. There is more maintenance. This is probably a more expensive upfront investment, especially if I need to also replace the hot water heater with an indirect tank.

The system is a single line consisting of 10 hot water cast iron radiators. I would love to get your opinions on the performance of modulating condensation boilers on a converted gravity system or if it makes sense to go with a traditional cast iron boiler.

Thank you in advance for your help!


    EBEBRATT-Ed Member Posts: 8,840
    Sounds like you have the ups and downs and pros and cons figured out already. On this forum many strongly would recommend condensing boilers and probably an equal # would stay stick with cast Iron.

    I would look at how long you expect to live on the property, will you have enough time to get payback on a mod con? Parts for mod cons can be difficult to get supply houses don't stock much and the parts are $$$$. Cast Iron has more standardized parts easier to service.

    But the choice is yours. With Cast Iron radiators and if you can heat with low temperature water a mod con may be the way to go.

    Condensing with a CI boiler isn't an issue if installed right.

    Everyone will tell you the installer is more important than the cheapest price. Pick wisely

    Check "find a contractor" on this site
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