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Repipe gravity system w/ monoflow, etc.

ALH_4 Member Posts: 1,790
I would skip the monoflo tees and install a two-pipe system to lower the return temp and give every radiator the same supply temp using pex to keep cost down. If you're using a condensing boiler, then go with constant circulation and Thermostatic Radiator Valves. If you are using a cast iron boiler, you might zone using two 3-speed system pumps (like Grundfos 15-58) with internal check valves. The thermostatic radiator valves would let you zone each radiator separately for very little additional cost and give you indoor temperature feedback. I would also install a strainer to catch any debris that might be in those radiators.


  • Russ_9
    Russ_9 Member Posts: 10

    Several family members and I will be redoing my gravity hydronic system with a new boiler, and possibly all new piping. It's a one story house, and we'll probably add a radiator or two to the basement.

    Our plan is to knock out all the gravity system piping (3" pipes) and replace with all new copper, monoflow tees, and add a couple radiators. My family has also done other installs, so that's not an issue.

    Like I said, the labor's free, or near free, so the only cost is materials. We decided that it would be much more efficient this way since many of the radiator feeds are currently 3 or more feet horizontally from the feed pipes, and if the loop were not redone it would essentially still be a gravity system from the loop to the rads, with a loop circulator. We will also probably do two zones while we're at it.

    Is it worth doing the extra work? It seems to me that it is, but maybe I'm missing something.
  • Dave_4
    Dave_4 Member Posts: 1,405
    Twisting Pipe time!

    I'd twist up a manifold and twist to all the emitters. A neatly spaced row of red round handled valves would sit atop some synchronized slightly over-sized black iron threaded pipes with a glossy coat of any coloured paint. It adds a certain mass to your home and on continuous circ... black iron emits far far less heat per degree er surface area than copper, yet it has far far more mass. With the price of copper the materials are close enough in cost (even at one size up in the black iron). If you guys want to do the labour you'd have your best system. It'll last as long as the fittings which still could be the life of the building, and help buffer and moderate the heat the entire time. And it looks good to a certain section of this viewing audience from here to the North Pole. I'd even pipe to where you actually need any PEX connections or a Brass manifold for the PEX loops.

    How many times would anyone guess this would take between doing it in copper or doing it in one size larger black iron?

    (I think one size up makes up for copper being smoother inside).

    TRVs on emitters, continuous circ, central manifold, differential controlled.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Unless you feel you utterly must get rid of the big piping, I'll submit that adding TRVs to the existing (and new) radiators will take much less time, cost significantlly less (especially considering the insane price of copper currently) and provide better comfort and versatility. Each room will [seem] like it is individually zoned, but the boiler itself will only see a single zone.

    Nothing at all wrong with well-designed monoflow systems, but do be aware that two-pipe systems are generally preferred for "new" systems with a condensing boiler.
  • Steamhead (in transit)
    Russ, don't forget

    that a gravity system can run without electricity if the proper boiler and safety devices are installed. This might be a consideration in the future if electricity gets unreliable. That's why I still have my gravity piping, and plan to keep it.

    The basement zone could be tied into the boiler in the normal manner.

    If the pipes are insulated and TRVs are installed on the rads, your existing system piping will work very well and not show much heat loss from the boiler to the rads- ask me how I know that. Be sure to size your circulator so as not to over-pump the system- go here for more:


    I really don't think a total repipe is worth the effort and expense.

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