1. Welcome Hearth.com Guests and Visitors - Please enjoy our forums!
    Hearth.com GOLD Sponsors who help bring the site content to you:
    Hearthstone Soapstone and Cast-Iron stoves( Wood, Gas or Pellet Stoves and Inserts)

Flow velocity in heating loop

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by arbutus, May 14, 2013.

  1. arbutus

    arbutus Feeling the Heat

    Joined:
    Oct 16, 2007
    Messages:
    310
    Loc:
    Michigan UP
    How important is it to actually maintain the recommended 2 ft/sec minimum flow velocity to entrain air bubbles? Some posts, both here and elsewhere seem to indicate a slower velocity is acceptable or even desirable.

    It seems like a pretty good balancing act between loop length, dT, and flow rate for a decent length of either 3/8 or 1/2 inch pex.

    ~900 feet total for the zone.
    ~18k btu design load for the zone

    For 1/2 inch pex, flow needs to be above 1.1 gpm which leaves a long loop length and the resulting high head, several short loops length and small delta T, or short length, low head, low flow.
    3/8 pex drops the required flow nicely, while increasing head for the same loop length.




    Thanks.

    Helpful Sponsor Ads!





  2. BadgerBoilerMN

    BadgerBoilerMN New Member

    Joined:
    Jan 21, 2013
    Messages:
    31
    Loc:
    Minneapolis
    Proper air eliminators and near piping are more important than velocity.
  3. arbutus

    arbutus Feeling the Heat

    Joined:
    Oct 16, 2007
    Messages:
    310
    Loc:
    Michigan UP
    I was looking at the Spirax Jrs.
    Which type do you recommend?
  4. Coal Reaper

    Coal Reaper Minister of Fire

    Joined:
    Aug 10, 2012
    Messages:
    739
    Loc:
    NJ
    i got the supervent. it seams to be working very well but i dont have any experience with others.
  5. BadgerBoilerMN

    BadgerBoilerMN New Member

    Joined:
    Jan 21, 2013
    Messages:
    31
    Loc:
    Minneapolis
    We have used Spirovents (for 20 years), Honeywell PowerVents and lately, Taco Vortex, all with good results.
  6. Highbeam

    Highbeam Minister of Fire

    Joined:
    Dec 28, 2006
    Messages:
    9,409
    Loc:
    base of Mt. Rainier on the wet side, WA
    I noticed the same thing about recommended flow rate range of 2-5 fps and then actual designs where 0.5 gpm is more typical. I sure like the low head loss from lower flows.
  7. arbutus

    arbutus Feeling the Heat

    Joined:
    Oct 16, 2007
    Messages:
    310
    Loc:
    Michigan UP
    Yes!
    High head loss makes finding a pump that operates in the "middle third of its curve" difficult, plus the velocity is great enough that the dT is small. Some of my combinations had less than 10 degrees dT or over 16 feet of head.
  8. BadgerBoilerMN

    BadgerBoilerMN New Member

    Joined:
    Jan 21, 2013
    Messages:
    31
    Loc:
    Minneapolis
    Yes, and lower parasitic pump operating cost as well. Folks get comfort design mixed up with energy. They are related but not the same. If you move a certain amount of water at temperature and it comes back cooler, you are heating.

    The whole 20° delta T has to be rethought to come up with real economy.
  9. arbutus

    arbutus Feeling the Heat

    Joined:
    Oct 16, 2007
    Messages:
    310
    Loc:
    Michigan UP
    What delta T do you shoot for?

    I realize it is a balancing act between pump characteristics, required loop length, head, reasonable supply and return temperatures ...


    Or do you select a small, energy efficient pump and work within its parameters to deliver the required heat?




    I'm starting with a near bare slate. Requirements at this point are sandwich Thermofin U on the 2nd story, Thermofin heavy under the first story, and probably panel radiators in the basement.
  10. BadgerBoilerMN

    BadgerBoilerMN New Member

    Joined:
    Jan 21, 2013
    Messages:
    31
    Loc:
    Minneapolis
    Your question is astute. The answer is yes.

    Working every day with common circulators you learn you use what you have, varied tube size, length, flow, emitters and following the sweet spot of your heat source. I am designing a 20,000 s.f. ag building with groud source heat pumps at the moment. The design water temperature will be 100°F.

    All made easier with my Wrightsoft Roth and Uponor modules.
  11. BoilerMan

    BoilerMan Minister of Fire

    Joined:
    Apr 16, 2012
    Messages:
    1,623
    Loc:
    Northern Maine
    This where a manifold or even a reverse return (basically long manifold which is inherrantly balanced) comes in handy. You little flow and don't have to worry about series piping and the last emitters to recieve cooler water. Lower flows work very well with storage to not mix and use the most available heat by returning cooler water.
    I installed a Spirovent 1-1/4" as my main air elimination this past heating season. It replaced a Watts airscoop, I'd get occational air burping from the boiler when I was in the 190F range, and it would pass right through the Watts scoop only to get hung up in a short low flow baseboard loop and gurgle. With the Spirovent it burps the air out and problem solved. Eventually all the air worked out of the system and no burping at all now, but it never did before and I was not going to put bleeders on my 1/2" baseboard loop. I'm sold on the Spirovent now if you couldn't tell.

    TS
  12. arbutus

    arbutus Feeling the Heat

    Joined:
    Oct 16, 2007
    Messages:
    310
    Loc:
    Michigan UP
    I'm planning on three manifolds, one for each zone.
    Zone 1, 2nd story, (3) 300 ft loops of pex, on 8" centers in Thermofin U sandwich.
    Zone 2, Ground floor, (3) 300 ft loops of pex in Thermofin below the subfloor + one or two panel radiators
    Zone 3, basement, likely (4) panel radiators

    There was another topic on manifolds, but before that started I had planned on the Uponor TruFlow Jr with isolation and balancing valves.

    Water temperature will be less than 140, and the system should keep us warm even at -20F



    Thanks. Your earlier comment was leading, but originally I couldn't figure out why I needed a Grundfos Alpha 15-55 for each zone, when others here were dreaming about using this pump in a system with several zone valves. I thought "All these people can't have 50' runs ..." Then I saw posts regarding low power and off grid applications, some on other forums, and figured that the velocity had to be much lower.

    The dealer in town that I carries Empyre boilers gave me a funny look when I said 1 1/4" pex from the boiler shed to the house. "Naahhh, you won't need that!" Well, I just checked, and it appears that an Alpha 15-55 will move 8 GPM at 8 feet of head just fine, and the 1" pex he thought would be good requires 3x the pump.


    Now I need to study up on near boiler equipment, and primary/secondary arrangements.


    :cool:
    heaterman likes this.
  13. bmblank

    bmblank Minister of Fire

    Joined:
    Jan 17, 2013
    Messages:
    698
    Loc:
    Michigan
    They say don't go more than 250 feet per loop. I'm sure if you increase pump head it wouldn't be a problem, but that's just what they say. Its convenient, too, cause rolls generally com in 250 ft lengths.
    Also, if the loops are the same length then they are balanced from the get go, though you can get balancing manifolds.
  14. jebatty

    jebatty Minister of Fire

    Joined:
    Jan 1, 2008
    Messages:
    4,509
    Loc:
    Northern MN
    Low temp / low flow supply example: My shop has 6 - approx 275' loops, 2.5 gpm total flow, ::DTT=30F, 33,000 btuh; and I supply at 100F via a mixing valve from a 1000 gal storage tank at 192F maximum. I supply the manifold via a plate hx and 1" copper Sides A & B directly from storage, basic air scoops on Sides A & B, and boiler supply to storage is 1-1/4" copper with a Spirovent. Floor is kept at constant 61F +/- 1F. Typical btuh average/day during winter (+25F to -30F) is 12,000 - 18,000. With 100F supply, tank storage maximum is 767,000 btu, which means I have 2+ days of stored btu's. Low temp supply possibility makes a big difference.
    heaterman and BoilerMan like this.
  15. arbutus

    arbutus Feeling the Heat

    Joined:
    Oct 16, 2007
    Messages:
    310
    Loc:
    Michigan UP
    ^ Nice.

    May I ask about the details of your heat exchanger?
    Side B input is 70F, output is 100F.
    What input and output temperature (or gpm) for side A?
    I'm assuming the storage is part of an open system and that is the reason you are using the hx?



    I bet it's real comfortable in there.
  16. jebatty

    jebatty Minister of Fire

    Joined:
    Jan 1, 2008
    Messages:
    4,509
    Loc:
    Northern MN
    The hx is because the floor is anti-freeze and the storage is water. Here is a data clip. The tank is nearly depleted in available btu's, as Side AIN is 103.55 and AOUT is 96.9125, about a 6.5F temp drop. Side BIN is 70.1375 and BOUT is 100.175, 30F temp rise. Side B btu = 33,750; estimated (calculated) flow on Side A would be about 11 gpm (6.5 x 500 x 11 = 35,750). DS18B20 sensors recorded the data.

    Code:
    1272,0000027B7420,"1/3/2011 5:52:02 AM",70.1375,BIN
    1272,0000025BBED8,"1/3/2011 5:52:02 AM",100.175,BOUT
    1272,0000027B8B34,"1/3/2011 5:52:02 AM",96.9125,AOUT
    1272,000002210993,"1/3/2011 5:52:02 AM",103.55,AIN
    
  17. jebatty

    jebatty Minister of Fire

    Joined:
    Jan 1, 2008
    Messages:
    4,509
    Loc:
    Northern MN
    The hx is a 5" x 12" x 30 plate, 1" ports.
  18. BoilerMan

    BoilerMan Minister of Fire

    Joined:
    Apr 16, 2012
    Messages:
    1,623
    Loc:
    Northern Maine
    Jim, I did the same thing as my floor has glycol in it as well.

    TS
  19. maple1

    maple1 Minister of Fire

    Joined:
    Sep 15, 2011
    Messages:
    4,241
    Loc:
    Nova Scotia
    Jim, what are you using for a circulator on the B (DHW) side? And do you have an estimate on the GPM it's putting through the B side of your exchanger?

    I currently in a bit of a quandry, and trying to decide whether to leave my DHW HX as is (short multi-tube sidearm), or improve it either by adding another sidearm on top of the existing one, or replacing all together with a plate HX & circ pump. I really like the simplicity and how the sidearm circulates on its own without a pump (the DHW side at least), but would like to get more heat out of the A side than what I'm getting.

    EDIT: Umm, never mind (I think) - I was thinking you were using the HX for DHW, but after re-reading sounds like in-floor.
  20. heaterman

    heaterman Minister of Fire

    Joined:
    Oct 16, 2007
    Messages:
    3,108
    Loc:
    Falmouth, Michigan
    It seems to be a law among OWB dealers that the only permissible size for underground pex is 1". Period.
    If you need more flow just install a larger circ. ........who cares if it draws 300 watts of power continuously all winter.....
    Karl_northwind likes this.
  21. BoilerMan

    BoilerMan Minister of Fire

    Joined:
    Apr 16, 2012
    Messages:
    1,623
    Loc:
    Northern Maine
    If you think about it, it makes perfect sense. If you have an OWB, you want to eliminate (all or most of) your heating bills. And you don't care about efficiency, so a continous 300Watt draw is not an issue for that mindset.

    TS
  22. heaterman

    heaterman Minister of Fire

    Joined:
    Oct 16, 2007
    Messages:
    3,108
    Loc:
    Falmouth, Michigan
    Seriously.......I have sopken to more than a few who have wondered why their electic bill went up $40 a month after installing the OWB with the 0011 hanging off the back of it.
  23. BoilerMan

    BoilerMan Minister of Fire

    Joined:
    Apr 16, 2012
    Messages:
    1,623
    Loc:
    Northern Maine
    Big wood consumption, $40/month in electricity, and copious amounts of smoke.........all considered to be what is involved in burning wood in a boiler :confused: sad, there is a better (and ALOT cheaper) way, but that is the light that they refuse of plain old don't want to see.......through the smoke.

    TS

Share This Page