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Can i replace 2.3" galvanized boiler pipes in my hot water radiator system with something smaller.

dgordon201 Member Posts: 1
I have an old 3 story house built in 1890 with original radiators and a new Slant Fun gas boiler, which replaced an oil fired Thatcher brand boiler.

The main trunk supply and return pipes are 2.3" galvanized pipes and each radiator has 3 /4 " galvanized supply and return pipes attached to these large pipes. A couple of the radiators have 3/4" copper supply and return pipes.

I want to replace or move the big pipes so that I quit bumping my head on them.

Is there a plastic pipe that I can replace them with?

The plumber who replaced the Thatcher monster boiler also replaced a large red circulation pump with a small Taco circulator.

The system works great except for the third floor, which never gets very hot, but it's not much of a problem since the room is generally warm being on the top floor.

I would like to diy this project if possible, but I am obviously a raw rookie when it comes to hydronic heat.


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    edited May 2020
    Slant Fun...lol
    It really depends on what type of hydronic system, how it’s piped. You may be able to homerun every thing with smaller piping but it might affect circulator sizing.
    The 3rd floor issue may be from not pumping away from the expansion combined with not enough static pressure to lift the water to the top of the system, and/or air in the top of the system and/or under pumped.
    Would need to see some pictures, then you probably need a hydronics person onsite to do some math.

    There was an error rendering this rich post.

  • Ironman
    Ironman Member Posts: 7,376
    It's probably not a DIY project since it not only involve piping skills and tools, but would also involve a high level of hydronic knowledge and possibly the addition of more components. In other words, it's not something for your average plumber or HVAC guy to try and tackle either.

    The pipes are probably 2" black iron and the system was only gravity flow before a circulator was added converting it to forced flow. These old system were carefully designed and balanced (usually) and attempting to alter part them can throw the system out of balance.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    Have any pictures? We like pictures....
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • dopey27177
    dopey27177 Member Posts: 887
    Possible correction to the third floor heating problem.

    I am making an assumption as to the height of the building.

    Assume each story is 8 feet high three stories would be 24 feet.
    add the basement height of 8 feet that now increases the height to 32 feet.
    System water pressure needs to be 32 X .434 =13.9 psig.
    add to that 2psi to prevent possible steaming when boiler water may reach 212 on a very cold day.

    System water pressure cold fill should be 16 PSIG. That assures water will be present in the entire heating system. At that point all the circ pump does is move water around the system.

    As far as the new cir pump goes it may be correct in its sizing.

    One problem with altering the piping in a gravity system is maintaining proper pitch. Since you have had the system altered to a pumped system you have a little latitude with changing or relocating some of the pipe.

    Before you do this you will want to install the pumping away method of circulation.

    Try increasing the water pressure in your system first to see if the third floor heats better.

    See attachment

  • Ironman
    Ironman Member Posts: 7,376
    The old times who installed those gravity systems put orifice plates in the unions of the top floor radiators to prevent them from overheating those rooms. That was the path of least resistance for the water in those systems.

    When the system was converted to forced flow, the closest rad's on the first floor became the path of least resistance. The plates should have been moved to the first floor rad's at that time.

    Most plumbers and HVAC guys don't know about this and the plates get left in the top floor rad's.

    Here's a link to an article by Dan about this:


    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.