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Gravity conversion

joe amadio
joe amadio Member Posts: 41
Looked at an old closed gravity system were the customer wants to change the boiler and raise about 20 feet of the mains so they finish a portion of the basement. The current boiler is an old Hydrotherm. I'm guessing about 50 or 60 yrs old. The supply comes off the top of the boiler in 2'' steel pipe runs upward toward the ceiling and splits. One main heads off in one direction and the other in the opposite. Both are 2''. Each radiator is fed off a 2x1 tee. The return mains mains do the exact opposite. Each radiator comes back into a 2x1 tee. The return runs parallel to the supply back to the boiler were it tees together with the return from the other side of the house then drops down into the 2'' return tapping at the bottom of the boiler. The customer wants to raise the mains 20 feet furthest from the boiler into the ceiling. There are 4 rads being serviced off the mains they want to raise. They also want to add an indirect water heater off the new boiler. They would like to use a Weil-McLain CGi gas boiler. The house has a total of about 75' of mains going out and about 75' of mains coming back. There are about 15 radiators in the whole house. What is going to be the best way to economically thermally protect the boiler? P/S? Bypass? Or is there another method out there being used? Secondly, when we introduce a circulator to the system can we decrease the main size of the section to be raised?


  • icesailor
    icesailor Member Posts: 7,265
    Sticking my neck ut:

    I'll suggest this.

    My personal feeling is that if you try to run it as a gravity system and downsize parts of the system, you will have a giant problem where you reduce the system. When you add a circulator to the system like they did when they converted gravity systems to pumped systems, you were really mimicking the flow by gravity with a circulator. So, you could use smaller pipes to pump water, as you needed larger pipes for the water to flow the same amount of water. If you understand that.

    What I have observed and what I think might answer your problems is to install the boiler you want, and install a 4-way mixer like the Taco "I": series that makes a primary side for the DHW/Indirect and a secondary mixed/pumped side. By having ODR, the system will run at whatever it needs to run at but won't be running at high limit and overheating the first radiators. Because the circulator will be allowed to run for very long runs at a low temperature, the whole system will balance out evenly. Consider it a form of constant circulation. I've seen gravity systems with added on circuits that would never balance. Or circuits that wouldn't balance. But if you pumped them with a properly sized pump, and properly regulated water temperatures,  it would overcome the restrictions. The key is in the pumping. If you overcome the resistance, with the properly designed water temperature, you can get balance. There are situations that it won't work as well, but you have to experience them to figure it out and understand it.

    If someone adamantly disagrees with me, fine. I just think that you could do it by doing it with a 4-way mixer with ODR/system temperature control.

    If they're willing to spend the money, I personally think that there is no end to the comfort they can have with an old system in an old house.

    As long as you keep the moved pipes proper to good hydraulic pumping practices, it should work.

  • jumper
    jumper Member Posts: 2,333
    yes to both questions

    Modern boilers on old systems. You can use a low limit control to prevent too cold water or you can install a shock proof boiler.
  • JStar
    JStar Member Posts: 2,752

    Mod/cons and converted gravity systems go together like peanut butter and chocolate. Why bother installing a cast iron boiler, only to install P/S piping or a bypass to purposely make it less efficient? The whole concept is backwards to me.
  • SWEI
    SWEI Member Posts: 7,356
    I'm with Joe

    A properly sized fire-tube mod/con, one pump and pay attention to fluid quality.
  • icesailor
    icesailor Member Posts: 7,265
    Original Question:

    Joe said that the owner wanted to use a Weil-McLain CGI gas boiler. There was no mention of any fire tube Mod-Con in the posting. I stick to the questions asked. Maybe the owners have done their homework. Maybe if Joe gave a price ONLY on a Mod-Con and not a CGI, and the owner got a price from someone else that included a CGI and a Mod-Con, the CGI would be significantly cheaper than the Mod Con, and Joe looses the job to the CGI. The more choices you give someone to decide on, the happier they will be. People like to have choices.

    I personally think that with  I Series 4-way, the 4-way has an easier time of controlling the gravity part that may have "issues" than a Mod Con will have with all the other issues you dump on it. Never mind the extra cost.