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How many cast iron rads in series?

NYplumber Member Posts: 503
I was once taught that the only stupid question is the one not asked.

How can i figure out how many radiators can be placed in series before the btu train runs out of passengers and can't heat the rads at the end of the loop?

Optimum install would be to home run each rad to the boiler room for trv or valve balancing, but the customer asked for a single loop.

With the above question I am eager to know if anyone has thoughts on installing a monoflow loop to cast rads by the use of pex vs copper. Pondering about the idea of pex monoflow loop makes me think of it as a handyman style job, however the idea just crossed my mind when trying to calculate how many rads I can put in series.


  • Greg Maxwell
    Greg Maxwell Member Posts: 212
    Rads in series

    Instead of piping them in series, pipe them in a monoflo loop. Figure the output of the rads, and put no more than 40,000btu worh of rads in that one loop. Place your regular tee on the supplu of the rad, and your monoflo tee, or venturi tee on the return. That way, you will keep your supply water temp the same across the whole loop.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Don't let the customer tell YOU how to pipe YOUR system...

    They brought you in as an expert. They are expecting comfort. Unless they are willing to sign a liability waiver as it pertains to delivering comfort, I would NOT consider doing it that way. Series radiators are serious trouble.

    I've had numerous jobs where an unknowing person piped them in series, and it never works right. I've had to retrofit them with a 4 way flow reversing valve, and it worked better, but it still isn't right.

    One pipe mono flow would be better, but you will STILL have a significantly lower entering water temperature for the last group of radiators.

    Parallel reverse return is the best way to pipe them. Home running them will take more material and labor. Calculate the connected loads (not the EDR), and pipe the system accordingly. You will be amazed at how small the piping system really needs to be.

    As I've stated here no less than a million times before, there really is no right way to do things wrong....

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • NYplumber
    NYplumber Member Posts: 503
    Parallel reverse return

    Mark you are correct, parallel reverse return  is what I was missing. Sometimes we take the basics (one loop), then try to think of the most comfort/efficient (home run to the boiler room) and forget the way with the least hassle.

    I know the limitations of piping in series, which is why I asked the question, and now remember an old post of yours stating the story with the four way mixing valve.

    Just some food for thought, I was contemplating splitting the load for that loop in two parts, supplying water from both ends, and having a common "t" return in the center (center of the load divided in two parts, not in the center of the number of rads).

    While on the subject, is PEX monoflow a possibility?
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    PEX mono flow...

    You would have to make a transition from PEX to copper and back to PEX again, but yes, it is doable.

    NIBCO makes a conical insert in 1/2", 3/4" and 1" that can be dropped into a tee, and will create a venturi tee that works great.

    If the main is below the emitter, then the venturi goes in the outlet return flow tee. If the emitter is below the main, or is a high pressure drop unit, like a fan coil, then you'd use one in the supply and one in the return.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Big Ed_4
    Big Ed_4 Member Posts: 2,783

    You can pipe radiators in series if you do the math , they just need to get bigger as you go along per = output ... But Who's paying for radiators ? A diverter or mono flow would be a better way to go in the real world .. Home run piping with TRV valves would be even better... ...
    I have enough experience to know , that I dont know it all
  • meplumber
    meplumber Member Posts: 678
    I do a lot of monoflow.

    When doing remodels or retrofits in these old New England houses, we do monoflow quite a bit.  40,000 is the upper limit of an effective loop, but you can normally divide it into two loops if necessary.  We use the loop with TRV's on the radiators and a Delta P circ on the loop.

    Good Luck.  Let us know how you make out.
  • Jason_13
    Jason_13 Member Posts: 304
    Pex & Monoflo????

    You are right it would work but my concern would be a possible headache with expansion of pex creating up and downs causing problems until all the air comes out. All pipes need to run uphill all the way on both supply and returns. On radiation below the main would not be a problem as the air will not go down. Just another point if radiation is below the main do not use less than 3/4" even if it is too large for the radiation.
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