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Which is better?

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Zman
Zman Member Posts: 7,569
edited July 2018 in THE MAIN WALL

The mixing assembly on the left is very commonly seen. I have been wondering if the one on the right is better. It has less resistance on the inlet of the circ, effectively making it closer to the PONPC. The valve will also have less pressure pushing against it being it is on the other side of the radiant panel.
Thoughts?
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein

Comments

  • HVACNUT
    HVACNUT Member Posts: 5,833
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    Left drawing, hot- bottom run, mix- top run, cold- bull.
    Right drawing, hot, top run? mix- bottom run? cold- bull?
  • Zman
    Zman Member Posts: 7,569
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    HVACNUT said:

    Left drawing, hot- bottom run, mix- top run, cold- bull.
    Right drawing, hot, top run? mix- bottom run? cold- bull?

    On the right drawing the horizontal line out of the mixer should be blue.
    I don't understand your comment.
    Both designs do the same thing.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 22,129
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    Is it a motorized 3 way?

    Thermostatic would have a A, B and AB port. Usually A & B across from one another, the AB as the bottom port.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    I understand your question, however, I'd take a guess that the radiant tubing has much more flow resistance than a mixing valve ahead of the circ. That said, the right side puts ALL of the flow resistance at the circ's outlet which is always desirable.

    Ive always piped as shown on the left as that is what is shown in the diagrams which come with the mixers. I'd think the right side would be advantageous, but we are talking low temps with mixing valves with less threat of suction cavitation.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
    Zman
  • hot_rod
    hot_rod Member Posts: 22,129
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    It brings up two important points:

    Avoid any high pressure drop valves on the inlet of a circulator.

    Beware of the Cv on thermostatic valves.

    Thermostatic valves respond best when you pump away from the mixed port, pulling hot and cold thru the valve. You want turbulent flow around that sensing "pill".

    However you are adding pressure drop at the inlet of the cir. If sized by Cv, 1 psi drop at the indicated GPM, it should not be a problem.

    Often times a low Cv thermostatic is used in an application requiring higher gpm, that is where you chance of cavitation is greatly increased. Higher SWT also increase the cavitation potential.

    Here is what the pressure drop would be with a 3 Cv valve and a 6 gpm flow.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Zman
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,239
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    The one on the right has less heat stress on the valve.
    Zman
  • Zman
    Zman Member Posts: 7,569
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    hot rod said:

    Is it a motorized 3 way?

    Thermostatic would have a A, B and AB port. Usually A & B across from one another, the AB as the bottom port.

    It is motorized.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • gennady
    gennady Member Posts: 839
    edited July 2018
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    i would prefer the left one. better control.
    Gennady Tsakh
    Absolute Mechanical Co. Inc.
  • hot_rod
    hot_rod Member Posts: 22,129
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    Very little pressure drop thru a 3 way motorized, hydronic valve, either ball or wafer design, so location should not be an issue, as far as flow restriction.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Zman
  • delphee
    delphee Member Posts: 6
    edited July 2018
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    Not my area so I probably shouldn’t comment, but this is a very interesting question. What you are doing on the right is controlling the return water temperature instead of the supply, which is very different. Under periods of high load (cold room) the panel will be allowed to get much hotter, as the valve will allow the cooled return water to pass with little or no mixing. Under low load (warm room) the flow through the panel will be greatly reduced, but the water will still be very hot where it enters the panel. Moving through the panel very slowly, the water loses its heat and exits the panel at a normal return temperature. So this is very different than the left one, which will try to maintain the panel at a constant temperature. Unless I am not understanding how these valves work, which is very possible!
  • Tinman
    Tinman Member Posts: 2,808
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    Whatever John Seigenthaler recommends, I do. The left.
    Steve Minnich
    Solid_Fuel_ManZman
  • hot_rod
    hot_rod Member Posts: 22,129
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    it depends on what type of valve also. We build 3 way valves as bypass or diverting, used on two pipe air handlers for example..

    I think the valve you are suggesting can modulate, stop at any position to mix?

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Zman
    Zman Member Posts: 7,569
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    delphee said:

    Not my area so I probably shouldn’t comment, but this is a very interesting question. What you are doing on the right is controlling the return water temperature instead of the supply, which is very different. Under periods of high load (cold room) the panel will be allowed to get much hotter, as the valve will allow the cooled return water to pass with little or no mixing. Under low load (warm room) the flow through the panel will be greatly reduced, but the water will still be very hot where it enters the panel. Moving through the panel very slowly, the water loses its heat and exits the panel at a normal return temperature. So this is very different than the left one, which will try to maintain the panel at a constant temperature. Unless I am not understanding how these valves work, which is very possible!

    Both designs will control the temp the same way. The mixing valve controls how much water is recirculated in either design.

    A controls contractor that I have a great deal of respect for always pipes modulating valves on the return, siting that there is less force working against the valve and less resistance between the PONPC and the inlet. I think that I agree with them
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 22,129
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    Not sure what type of valve exactly you are talking about?

    If it's a common 3 way like this tekmar it has a very high Cv of 21. Less than 1 psi drop at 19 gpm. So very little pressure drop thru that valve. 19- 20 GPM in a 1-1/4 pipe is around 4 fps velocity.

    In this type of valve, the Cv doesn't change regardless of the position of the vane inside, so that under 1 psi pressure drop is constant.

    I'm not sure what he means about "force working against the valve"? I've never heard force used in hydronic terms? :)

    If it is the type of valve attached, It's not a shut off or control valve, it just changes flow direction.
    If by "force" he means flow velocity, a 1-1/4 valve at 19 gpm sees 4 fps fluid velocity in either location?

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Zman
    Zman Member Posts: 7,569
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    Hot Rod
    It is a motorized modulating mixing valve for radiant in this case.
    I would normally try to match the cv to the design flow to try to get a the correct "valve authority" as Robert Bean would say. That would put 1 psi of drag between the circ inlet and the PONPC.

    The guy that recommended putting them on the return was probably referring to a modulating diverter valve.

    This is a good conversation.
    Thank you
    Carl
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 22,129
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    I'll bet he is talking about a 3 way control valve. That would be a valve that acts as a modulating temperature control but also has a shut off function, a zone/ mixing valve basically.
    In that case the valve will have a max ∆P rating, sometimes called shutoff pressure. The valve Caleffi offers for that has a 10 bar, 145 psi rating.

    So if you had a commercial application with a pump developing 50 pounds ∆P the valve needs to be able to operate against that pressure, which may be what he means by force?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Steve Thompson (Taco)
    Steve Thompson (Taco) Member Posts: 204
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    Sorry, I have a stupid question (just back from vacation).

    Why mix? Why not differential temp or setpoint temp controlled circ? Both would modulate flow based on temperature demands (load) right?
  • Zman
    Zman Member Posts: 7,569
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    The mixer in this design is supplying water to a zoned high mass radiant system.There are 3 different supply temps piped throughout the building. 31 zones total.The zone valves are spread out over a 16,000 square foot home. I had planned on the Veridian delta p circ set up with proportionate pressure mode.
    I know you Taco guys love delta T. I don't see how I can guarantee the correct flow to each zone as they open and close looking just at delta T in the boiler room.
    PM me if you would like me to send you the entire design.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    16,000 foot home? Wow! Mcmansion
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!

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