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Two Pipe Reverse Return Question

knotgrumpyknotgrumpy Member Posts: 207
I am replacing my Monoflow piping with reverse return piping done in Fostapex.  I have a question about flow and tubing size.

One of the loops is right around 39,000 BTUH or ~ 4 gpm which is within the capacity of the 3/4" Fostapex (max gpm 4.6).  I had intended to use 1/2" Fostapex on the return line from the first radiator and then increase to the 3/4" on the rest.

Similarly, I had intended to use 1/2" to feed the last radiator.

If I do that and have the loop set up to flow at 3.9~4 GPM, won't the 1/2" legs be flowing too fast?  I'm sort of guessing there needs to be some balancing device on each radiator, but I dunno.  It's sort of like a 'distributed' manifold, isn't it?

Oh, radiators will have Danfoss TRV's on them. Maybe I should have picked the Honeywell valves that you can adjust the flow on?

2 other loops will be about the same size.

Thanks for any help.



  • Steve WhitbeckSteve Whitbeck Member Posts: 669

    If you are going to use TRV's then balanced flow won't be that important.

    The only time it might be an issue is when it is very cold out and all of them are open.

    With the TRV's you will only get the flow that each rad needs to heat the room.

    Why are you getting rid of the mono flow system. Just install the TRV's on the mono flow system and save the cost and time of repiping it.
  • knotgrumpyknotgrumpy Member Posts: 207

    Thanks Steve.

    I have read conflicting opinions on if TRV's would work on a monoflow system.

    The biggest reason that I want to change though, is the shear water volume in the 1 1/4" iron pipes. I can't see heating the twenty or so gallons of water in the pipes just to take the chill out of a room during shoulder seasons. My thinking might be flawed, I'm open to correction and direction.
  • SWEISWEI Member Posts: 7,356
    Heating the twenty or so gallons of water in the pipes

    is not a problem for a mod/con running ODR -- in fact, it's a good thing.
  • Steve WhitbeckSteve Whitbeck Member Posts: 669

    The volume of water in the pipes is small compared to what is in the rad's.

    The TRV's will cause a restriction to flow - but if you install a system pump with slightly higher head it will over come the resistance.

    I usually install a stronger pump on monoflow systems anyways.

    Just make sure that You install at least a 100 mesh strainer in the secondary loop return line going to the boiler. ( not in the rad loop )

    I would only replace the piping IF it is at least 80 years old or leaking. OR to gain head height in the basement.
  • knotgrumpyknotgrumpy Member Posts: 207
    Hot Water

    Thanks Steve.

    20 gallons of heated water just sitting in those pipes is my issue.  Once the radiators are up to temp I  would rather the 20 gallons of hot water be in an indirect for a nice warm shower than just dissipating when the sun comes out and it's a 65 degree day.  I see the Europeans using almost incredibly small diameter tubing to feed their radiators and it just seemed to make sense.  Don't know.

    I have the time, the skill to do the re-pipe, but I do have to rely on you guys that do this for a living to provide direction.  (Oh, and I have miles of Fostapex and a lot of Viega fittings as well.) More to think about.

    My initial intention was to zone everything, but the TRV's seem to offer a better solution.  I can have what amounts to 14 separate zones if I want.  That is why I'd like to redo this.  Greater comfort and I hope slightly better e'fish'ency.

    Oh, SWEI, I don't have a modcon, just a WM CGi with a Tekmar ODR set for a min boiler temp of 140F.
  • Steve WhitbeckSteve Whitbeck Member Posts: 669

    You might want to think about an Alpha pump with fixed head pressure. That way the pump is not dead heading when most of the TRV's are closed.
  • knotgrumpyknotgrumpy Member Posts: 207
    I will

    Thanks again, Steve.  I'm going to try one of the new B&G / Laing Ecocirc Autos.

    Does any of my reasoning about the lower supply water volumes make sense or am I way off base?

    Reason I ask is I was going to do some work today and don't want to make a bad decision.  Just because I have the mechanical ability to make the change doesn't mean it might be wise to do so.
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