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Options to consider when replacing Weil-McLain CG-6 Series 7 Natural Gas Boiler from 1981

tdi Member Posts: 4
edited January 18 in THE MAIN WALL
Good day, I have a 132,000 BTU/hr Weil-McLain CG-6 Series 7 Natural Gas Boiler. Serves a home from 1924 with cast iron radiators in zone that serves most of the hose and zone 2 serves baseboard radiators and a toe kick heater in an extension at kitchen and bonus room from 2005. The furnace was originally installed in 1981. One circulator on supply to boiler.

I would like to begin planning to replace the boiler and incorporate current day standards such as backflow preventer, air extractor (or scope), etc. I can provide photos of the current supply and return piping, most of which is likely 3 inch in diamater that go to and from various radiators. Nothing is served from a manifold and I'm not certain that would be possible with the piping.

I have been viewer at this site for years and it has helped me along the way. What a great amount of expertise.

With that said, are there recommendations as to what boiler system I should begin to consider as a replacement?

Thank you in advance for your feedback.


  • hot_rod
    hot_rod Member Posts: 21,862
    My newest first question.
    "Are you interested in or comfortable with higher efficiency computer controlled equipment with a 20 year life expectancy and some occasional - required service costs"?

    Regardless of a yes or know I would next recommend a heat loss calculation, maybe even an energy audit including a blower door test, Infrared scan of the exterior. In many areas there are companies that specialize in energy audits. Some utilities offer a basic audit service.

    www.dsireusa.org may have some info on programs available in your area.

    Find and fix any heat leaks. This is a gift that keeps on giving, a lower heat load.

    Next we can look at radiators and determine some pros and cons of a mod con vs another cast iron boiler option.

    All this involves some pencil pushing, either by you or hire a competent hydronics person to at least do a load calc.

    I have had positive experiences with many mod cons installs. Others have not. But missing from the "have nots" is the original design, installation and commissioning of a high efficiency boiler. Expect poor results from a botched installation :wink: We may never know that %.

    In many, most cases older homes have over-sized heat sources, boiler or furnace. Regardless of which boiler option, getting it accurately sized is a big part of a successful, long lived, efficient heating system.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • tdi
    tdi Member Posts: 4
    Thank you for the response. You asked "Are you interested in or comfortable with higher efficiency computer controlled equipment with a 20 year life expectancy and some occasional - required service costs?"

    My response: YES.

    For example would be very interested in an appropriately sized system such as "Weil-McLain Ultra 155 - 139K BTU - 94% AFUE - Hot Water Gas Boiler" as long as it would work well with our current cast iron radiators. I do not want to get into a whole home renovation as existing hardwood flooring and lathe and plaster walls would be affected.

    What is the best way to perform "heat loss calculation?"

    Thanks again!

  • pecmsg
    pecmsg Member Posts: 4,695
    Get a heat load loss done on the building for proper sizing.

    Get a blower door test performed to determine air infiltration rates and where it can be improved. (That lowers the size of the boiler)

    Tightening the envelope has a high rate of return of investment.
    Just installing a high efficiency boiler rarely does.
  • hot_rod
    hot_rod Member Posts: 21,862
    If you want to give it a go, I like the downloadable load calc shown here. I think you can try a demo version, at the hydronic pros website.

    The Uponor design guide has a long hand form version and walks you through an example.

    A tape measure is about all you need.


    If the size boiler you have keeps up on the coldest days that is a good start. Possibly you could drop one size. See what the load calc tells you

    You will need to make some assumptions for wall insulation, areas where you cannot determine the insulation r value or installation.

    The next step is to count up all the radiators determine their output. Post some pics of them, usually someone here will recognize the brand.

    What you want to know is how low of a temperature you could run the boiler with the radiators you have. The lower the boiler operating temperature the more efficiently it will run.

    A pic of the current boiler and piping.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream