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5/8 pipe with the Slant Fin baseboard

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Vad
Vad Member Posts: 55
                  I am planning to connect about 25' of baseboard that have 3/4 pipe in it to a 5/8 PEX AL PEX pipe. Am I going to loose BTU since I am using 5/8 and not 3/4 pipe? I do not know what the flow rate is going to be.

Thank you,

Vad

Comments

  • Al Letellier_21
    Al Letellier_21 Member Posts: 402
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    5/8 pex with baseboard

    It's done all the time but a lot does depend on flow rate and length of run. We have always limited it to 20 feet of baseboard and 100 feet of pex and have never had an issue or a complaint. Whether or not BTU's are lost I don't know, I just know it heats OK
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
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    yes and no

    technically any time you reduce flow you lose BTUs. However it's often so few that you save energy by reducing pumping requirements.



    However if you don't know your heat load or flow requirements, you don't know your pipe size requirements. That's why heat loads are step 1 in the process.



    You would never want more than 1.33 GPM per 10 feet for regular output baseboard (750 BTU/ft at a 10 degree drop). Often you could do half that or even less for only minor reductions in output.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • CMadatMe
    CMadatMe Member Posts: 3,086
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    5/8" Pex

    You can deliver 6gpm or 60,000 btu's on 5/8" Pex. How many baeboard zones has anyone come across that are that big? It's all about pump sizing. I have thousands of baseboard jobs running on 1/2" Pex. Rule of thumb is 50' of board per zone on 1/2" Pex with a standard 007. At a 1gpm flow rate that equates to about 27,000 btu's per zone and that's even heavy for a zone...

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  • Brad White
    Brad White Member Posts: 2,398
    edited September 2010
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    I agree with

    Rob,, Al and HVHEtc. :)



    Here are some related thoughts regarding flow rate, output and long fintube runs.



    1) Low flow rates hurt you in fintube radiation only when you drop below critical velocities and get into the laminar flow range. In English that means that the core water flow passes over a boundary layer formed in the slow lane and transfer to the fin side is reduced. OK, my kind of English.



    Basically if your flow rate is above 0.5 FPS, you will be fine. Incidentally, 1.0 gpm in 3/4" type M tubing will get you to 0.62 fps, per Camerons. You should be fine.



    Assuming your FTR output is 500 to 600 BTUH/LF, your total output will

    be between 12.5 and 15.0 MBH, ideally 1.25 to 1.50 gpm at a 20 degree

    drop. But say you can only get 1.0 gpm against a 15 MBH load. That is a 30 degree drop, not 20 degrees.



    But what does this do to you? Not as much as you think. Say you ideally would have the full 1.5 gpm for a 15 MBH load, 180 in, 160 out. Average water temperature of 170F. It is this AWT on which the output is rated, remember.



    Now say you have 1.0 gpm, same load but a 30 degree drop. 180F in, 150F out.  Your AWT is 165F, not 170F. You only lost five degrees. Not a huge deal. Some loss. If this is a critical room (and they are ALL critical at some point, but just saying), you can boost the supply water temperature by five degrees in colder weather (increase the outdoor reset scale), so that your EWT is 185 and LWT is 155. Same 30 degree drop, but the AWT is back to 170. Worth it for those few hours, if critical.



    2) Long runs: I am with Al on this, I like to keep the runs short, especially if single element. The downside being that the output can drop off at the end and if a long room, can be perceived as chilly, all things being equal. (If this last portion is in front of an insulated wall, much less perceptible. If glass, you run that risk.)



    2A) From the above, you can see a steady degradation in output over length going in one direction. In theory, your center would be your "AWT" or average water temperature, upon which the output is based. Starts hot, say 160F, ends at 140F and should be 150F in the middle for example.



    Now, consider if you make a U-bend at the end and run another element, the return path element, underneath. You will get only 30% more capacity, not double, BUT the average water temperature across each element at each point will be the same. Diagram it and see. It is a good way to compensate for long runs, flow permitting.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"



    -Ernie White, my Dad
  • Ron Jr._3
    Ron Jr._3 Member Posts: 603
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    Not good with the math

    but we often work in Levitt homes that are wrapped with baseboard . 90 to 100 feet are the norm . Alot of the time the baseboard is connected to the boiler using 1/2 copper . Sometimes the whole loop is doubled back , so that's ALOT of 1/2 pipe on one zone . Never had a complaint of not enough heat , even on the coldest days .  Usually it's a Taco 007 . Sometimes a B+G 3 piece .



    BTW , nowadays when we convert these homes from original radiant to baseboard , we use 5/8 pex to connect the baseboard though chops and to the boiler . Same baseboard lengths . No problems so far ............. 
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
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    6 GPM for 5/8" ??????

    6 GPM on 5/8" diameter tubing?



    How does that work when 3/4" maximum recommended flow rate based on a reasonable velocity is 4 GPM????



    Just saying....



    ME

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  • CMadatMe
    CMadatMe Member Posts: 3,086
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    Velocity

    That 4gpm on 3/4" is based on the water moving 3.3ft per second. My number represents a movement of 7.44ft per second in 5/8 pex. I also did say that you would have to size your pump accordingly. Your head would significantly change.



    If I had a run of 100' I would need a pump would be able to move 6gpm @ 33' of head just to get through the pex.

     

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  • NRT_Rob
    NRT_Rob Member Posts: 1,013
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    in order words

    6 GPM through 5/8" PEX is kind of silly and really you rarely would ever want to push more than 2.5-3 GPM before bumping up to 3/4".



    Getting anywhere close to 8ft/sec is a major red flag that you are unnecessarily wasting pump energy.



    Take a deep breath and repeat after me, "Whups, that was kind of silly, sorry. Fingers faster than the brain sometimes.".



    We've all been there. No shame in it ;)
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • CMadatMe
    CMadatMe Member Posts: 3,086
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    Not Silly Rob

    Was answering the question. The fact is that I can move it and it's still under 8ft sec. How many zones have you actually installed that were 60,000 other than waste energy, I'm uncomfortable hydro air systems? Ever run 5/8" to a 2 ton unit?

    Secondly what would be wrong with 4gpm?  That's equiv to 68' of board. Still a silly zone. So what you are saying is that a zone should be no more than 35,000 btu's. That's still silly since the average heat loss of a 3,000 sqft house is 40,000 btu's.

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  • NRT_Rob
    NRT_Rob Member Posts: 1,013
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    I think you know better than this

    but maybe you don't. I'm a little surprised we're even having this conversation.



    head loss increases geometrically with flow. while you can *theoretically* push 6 GPM through 5/8" if you size your pump to do it, it would be a pretty poor design decision to do that, because you'd have a massive energy hog of a pump sitting there wasting energy for the rest of the system's existence for no good reason.



    downsizing the pump would likely pay for the pipe size upgrade on day one and save significant costs every day after that.



    Even at 4 GPM you're burning a foot of pump head every 5' or 6' of pipe in 5/8" pex. If you have an very short run that might be ok, but I wouldn't exactly call that a preferred design specification in the hugely vast majority of systems. To stay at a regular residential circulator sizing you'd have a max loop length of something like 50 feet, which is pretty tight really.



    But you could do 4 GPM, sure, if you're really running the numbers, in very specific circumstances. If you're not, you should go to 3/4". Under no circumstance I can possibly imagine would you ever even approach 6 GPM unless you had a drop dead deadline going, ten minutes to finish, a 2 foot run to do and 5/8" on hand.



    You would even consider for one second sizing a THIRTY THREE FOOT HEAD PUMP to push 6 GPM? really? If so, you should calculate monthly pump running costs someday and reconsider that design consideration.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
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