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primary/secondary piping

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bkoz
bkoz Member Posts: 3
in Radiant Heating
Can the secondary pump be piped with the flow away from the common piping,but the common pipe tee that enters the secondary pump is on the leaving side of the loop.The length of the common pipe is less then 2ft.If not,can the pump be reversed to pump into the common pipe circuit tee,going with the flow of the primary pumps circuit.This zone has always been a problem heating up since the zone was installed by another contractor 8yrs. ago.All the air has been purged.

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  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
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    Hard for me to picture what you are asking.

    What do you mean by the common piping? Do you mean the piping between the two closely-spaced Ts? When you refer to the common-pipe T, you confuse me because there are two. If so, you would surely not want the circulator between them, because the idea is to have no pressure drop between them, and a circulator would create a pressure drop for sure. Also, it would be unusual to have them separated by as much as two feet. The spacing could be a bit more if you used a low-loss header instead of the closely-spaced Ts, but two feet would still be a lot unless you were using a really large header.



    Or do you mean the piping including the closely-spaced Ts? In that case, if the flow in the secondary circuit (the one going to the loads) is from left to right and the supply to the secondary loop from the primary loop enters into the right-most T, then the circulator would be to the right of the right-most T. You would probably want the expansion tank between the right-most T and the secondary circulator. You might want the air extractor there too, although it might make sense to have it in the primary loop.



    Maybe if you post a drawing of what your are talking about it would clarify matters.
  • Brad White
    Brad White Member Posts: 2,398
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    I cannot picture it either.

    A sketch or better yet a photograph would help a lot.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"



    -Ernie White, my Dad
  • bkoz
    bkoz Member Posts: 3
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    primary secondary piping

    Here is the diagram of the primary secondary piping that the customer has. Hope it is helpful and clear enough to see the diagram.
  • Brad White
    Brad White Member Posts: 2,398
    edited February 2011
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    Plenty of flow but

    possibly not enough temperature.



    What you diagrammed is what I pictured but wanted to be sure. Do you know the relative flow rates?



    I would move the circulator to pump out of the first tee, presuming that

    flow rate is less than the main boiler circuit flow rate.



    What I see happening, picture this on the diagram:



    The boiler flow enters from the left. The first thing it sees is an incoming return from the secondary circuit, followed two feet down by the exiting "supply" circulator secondary branch.

    That first incoming return branch is cooler by the delta-T of the secondary circuit, say for discussion, 20 degrees cooler and it dilutes the main flow that will shortly leave the main. And also for discussion, say the boiler loop circulates 10 gpm and the secondary branch, 5 gpm.



    By the numbers: Boiler flow enters from the left at 160F, 10 gpm.  In comes a cooler return, 5 gpm at say optimistically 120F.  A "T-mix" (temperature mix) occurs at that tee.



    The main piping between the tees now sees 15 gpm at 146.7 degrees. Naturally, the secondary tee can only extract this temperature, pre-diluted. As the system warms up, (it will take some time now on the secondary side), it eventually will get to a balance point where the return is a bit warmer. Regardless, the exiting supply to the secondary circuit will always be cooler than the boiler circuit.



    Moving the circulator to exit from the first branch should solve the issue.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"



    -Ernie White, my Dad
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
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    Also ...

    If those Ts are two feet apart, the primary loop should be 5-inch diameter pipe. I assume it is not, so the Ts should be closer together; i.e., not more than 5 pipe diameters apart.
  • bkoz
    bkoz Member Posts: 3
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    primary/secondary piping

    Can I keep the pump located in the same place but reverse the flow?Or should I resolder the pump after the 1st tee in the secondary loop to pump into the convectors?

    Thanks for your response and help.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
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    Reversing the pump.

    Remember that I am not a professional.



    I will use the term boiler loop to describe the loop with the boiler in it, and the term load loop to describe the loop with the baseboard, radiators, or whatnot in it, because others use the terms primary and secondary differently.



    It seems to me that you will have two problems. If you reverse the pump in the boiler loop, the boiler will probably not like that at all. I have a mod|con and it would not condense if I were to reverse the pump.  If you reverse the pump in the load loop, it might help, but you will have other problems.



    Also, somewhere you said the closely spaced Ts were about 2 feet apart. If that is piped in 1 1/4 inch pipe, they should be 5 inches apart for best results, so if you are fixing that anyway, you might as well put the pump in the right place at the same time.



    Where is the expansion tank in this system? Where is the air separator? Where is the pump with respect to the expansion tank? These all must be correct.
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