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Help sizing pex-al-pex for 2 pipe reverse return loop

joshua_3joshua_3 Member Posts: 3
So I had to do basement renovations which led to removing the majority of my existing hydronic heating line feeds(amongst lots of other things). It was fairly haphazardly run, so this wasn't the worst thing. Previous setup was direct return 1" main with mostly 1/2 feeds to the radiators. There are 6 radiators total, 2 on each floor/3 floors. The total BTU output of the rads is just under 38k.

My plan was to run a reverse return loop, and I am planning to use Pex-al-pex. My question is what size the main loops should be - 3/4" or 1". Previously 1" was used. I have attempted to research this, and it sounds like 3/4 pap should carry this? Plan was 3/4 main loops with 1/2 line to the rads off that. Thanks for any insight you can provide.


Joshua

Comments

  • IronmanIronman Member Posts: 5,801
    Generally speaking, a 3/4" line will carry about 4 gpm which = 40k btus. That's based on an estimated head loss of 4ft per 100' of piping.
    There are a lot of variables, so there's no pat answer without knowing all of them.
    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • joshua_3joshua_3 Member Posts: 3
    Thanks for the quick reply. Is there any other info I could add to reduce the major variables? I've generally seen around the 40k number being the high side as well. I'd rather not spend my money oversizing the piping to 1" if it is unnecessary, and I don't foresee needing to add heat anywhere in the future from this loop.

    For what it's worth I'm also having estimates done, and was planning to hand this job off to the pros. The first number came in quite a bit higher than I expected, so I thought I'd at least sketch it out myself. All the runs are pretty exposed and easy to get to, so I'm not afraid of the 'putting it together' part. I do realize though the design is the most important part, and that led me here. Thanks again for any insight.

    Joshua



  • IronmanIronman Member Posts: 5,801
    Mainly, length of pipe, fittings, and any devices (like mixing valves) that add resistance to flow.

    Again, if your total run is not more than 100', then you'll probably be okay with 3/4".
    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • joshua_3joshua_3 Member Posts: 3
    Well, if I'm adding both the supply and return loops in that number, I'm probably closer to 200'. Not a huge house, but 3 floors with rads(2 each floor), and some rads are on opposites sides. Basically running a loop around the ceiling/outside of the basement with feeds popping up or down to the rads from there. While it's all disconnected I've moved a couple around to consolidate the runs, but it's not feasible to place them all on the same side of the house for a shorter run.

    Sounds like fittings are more or less added in as additional feet of tubing for purposes of calculating? If so, I guess that number could be even higher? There are a couple of T's for each rad, and probably a few 90's in spots that might be tight for a bend. Does this mean larger pipe to help lower the resistance? Thanks.

    Joshua
  • IronmanIronman Member Posts: 5,801
    edited October 2016
    "Does this mean larger pipe to help lower the resistance? Thanks."

    Probably not. 3/4" Pex-al-pex has a slightly larger internal diameter than copper. You can actually move up to 6 gpm and still keep the head and velocity manageable.

    You'll probably end up with about 8 - 10' of total head. A Grundfos ups15-58 or Taco 007 can easily move 4 gpm a that head.
    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • Mbyrnes92Mbyrnes92 Member Posts: 10
    What’s going on, I am doing a similar project was wondering if anyone can give me a hand, I want to run a 2 pipe reverse return system I have 5 rads on my first and 5 on my second, here’s a pic let me know if this is a good layout or if I should change anything..thanks 
  • IronmanIronman Member Posts: 5,801
    Pic?
    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • hot_rodhot_rod Member Posts: 13,833
    If you have adequate sized radiators it is quite possible you could run a larger delta T. it's quite common to see 30- 35 or higher∆ on radiator systems.

    500 (f) ∆T
    500 X 4gpm X 20∆= 40,000 as @ ironman mentioned
    Or put another way
    500 X 3gpm x 30∆ = 45,000
    If you have a lot of fittings and tube length, lowering the gpm lowers the head.

    Running a wider delta returns cooler water to the boiler and increases efficiency, also.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • Mbyrnes92Mbyrnes92 Member Posts: 10
    Repiping my house in pex want to run a 2 pipe reverse return system, would this diagram be accurate as to what I want to run? Thanks
  • EdTheHeaterManEdTheHeaterMan Member Posts: 798
    edited October 18
    joshua_3 said:

    Well, if I'm adding both the supply and return loops in that number, I'm probably closer to 200'. Not a huge house, but 3 floors with rads(2 each floor), and some rads are on opposites sides. Basically running a loop around the ceiling/outside of the basement with feeds popping up or down to the rads from there. While it's all disconnected I've moved a couple around to consolidate the runs, but it's not feasible to place them all on the same side of the house for a shorter run.

    Sounds like fittings are more or less added in as additional feet of tubing for purposes of calculating? If so, I guess that number could be even higher? There are a couple of T's for each rad, and probably a few 90's in spots that might be tight for a bend. Does this mean larger pipe to help lower the resistance? Thanks.

    Joshua

    200 ft total Pex piping for the complete system? That is not needed to compute the pump head. To calculate the head loss you only need to know the longest run. Since you are using a reverse return design all the rads will be close to the same length. but you need only measure the piping in the longest run and forget about all the pipes that go to the other radiators.

    So if the third floor is 20 ft above the basement and the pipe in the basement goes around a 15 x 25 basement, the longest run would be from the boiler around the basement to the riser then back down to the basement and back to the boiler.

    15+25+15 around the basement +20 ft up +20ft back down then another +25 ft back to the boiler. That's a total of 120 for the. longest run.

    120 * 1.5 for fittings = 180 total length adjusted for fittings.
    multiply by .04 (this is 4 ft head per hundred ft pipe) 180*.04=7.2 ft head

    As long as you have a pump that can deliver 4 gpm at 7.2 ft head, you are golden with 3/4" main piping

    The Taco 007F that comes with many boilers is more than adequate. you should be fine unless your numbers are much, much larger than my example.
    Mbyrnes92
  • EdTheHeaterManEdTheHeaterMan Member Posts: 798
    @Mbyrnes92

    You should start your own discussion. This can get very confusing trying to keep two conversations going at once.

    But to answer your query, it looks fine. For more info on this, like pipe sizing and the like, please start your own discussion.

    Thanks in advance

    Ed
    Mbyrnes92
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