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alpha2 max speed

rhlrhl Member Posts: 83
edited September 16 in Radiant Heating
I finally finished my staple up floor heat! It’s 6 loops each loop ~300’ of 1/2” pex-al-pex. 

There are a couple (< 3) repair fittings on each loop. 

At the design load of ~21K BTU, this gives a total zone flow rate of about:

21000/5000 = 4.2 GPM, which is an average flow rate of .7 GPM per loop,
which implies a head loss around 7 ft. 

This is well within the range of an alpha 2 pump, at 4.2 GPM the pump curve is about 14-15ft of head.

If I set the pump to the “high fixed speed” setting, I’m only getting the alpha2 to display 2GPM, and it getting between .25-.5 GPM per loop. 

Shouldn’t I be capable of pushing the water much much faster given this setup? 

What am I missing? 

Comments

  • hot_rodhot_rod Member Posts: 13,737
    My calc shows 1/2, 300' at .7 gpm = 10' head
    Don't trust that readout on any of the ECM circulators, it is a calculation not actual measured flow rate, I have seen them off 60% when checked against a lab quality flowmeter.

    I have one running on my display right now, shows 0 GPM but the Quicksetter next to it indicates 4 gpm and it is clear plastic tube, so I see the movement and flow speed :)

    Without a reliable flowmeter you really do not know the actual flow rate is.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • rhlrhl Member Posts: 83
    Nevertheless the flow rates shown on the 6 caleffi balancing valve is around .25 GPM 

    that is ~1.5 GPM on the pump. 

    Hopefully it’s all *somewhat* reliable.

    Is my alpha2 undersized? This is at high fixed speed. 
  • rhlrhl Member Posts: 83
    @hot_rod fwiw I’m using the formula from this PDF:

    http://www.taco-hvac.com/uploads/FileLibrary/SelectingCirculators.pdf

    k = 0.0394
    c = 1.1 (overestimate)
    f = .7

    this yields:

    H = .0233*L

    at 300 feet that’s about 7ft. 

  • rhlrhl Member Posts: 83
    @hot_rod also we are splitting hairs a bit on 7ft vs 10ft. This is the duty point for the design load situation as per above:

  • hot_rodhot_rod Member Posts: 13,737
    I would say you number is correct, here is the PPI Calc result. This is just for the 300' loop, any other piping valves, in that circuit? The manifolds themself have some pressure drop across the actuator and flowmeter.

    If .5-.6 gpm gets the job done, I would not worry about getting exactly .7. Most flowmeters have an accuracy tolerance =or-.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • rhlrhl Member Posts: 83
    edited September 16
    I’m using a 4 way mixing valve with a Cv of 3.5

    there are a few fittings on the pipe, then a pump, then a brass nipple into a caleffi 6 port manifold.

    each loop is <= 300’ of pipe and each loop may have a few repair couplings.

    On what is likely the largest loop, I appear to be getting < .5gpm, reading around .25gpm.

    Right now we don’t have the insulation up or anything, I’m just trying to decide if there is something wrong or not. 

    I am just somewhat surprised that at fixed speed 3 I’m only getting 2GPM and not a higher flow rate, so the pump (while crazy efficient), is using more energy than the above diagram, by 2x and is outputting half the flow. 
  • Jamie HallJamie Hall Member Posts: 13,203
    Find some way to measure the pressure across the pump. You can do all the arithmetic on the piping you want to, but the pump isn't lying to you: if you have a flow rate of x, you have a head of y -- or on the other hand, with a head of y, your flow rate will be x. There is no substitute for actually measuring what the pump is really doing.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • rhlrhl Member Posts: 83
    Fair point. I'll look into this.
  • hot_rodhot_rod Member Posts: 13,737
    @Jamie Hall is correct, gauge it up and see. Get a good quality gauge, not too high of a pressure scale. Use the same gauge on both sides, or a ∆P gauge for most accurate result.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • rhlrhl Member Posts: 83
    edited September 17
    Will this isolate what the true head loss is on the loop of pex?

    Could I just measure the Delta-P on the drain cock of the manifold with all but one zone closed?

    EDIT: would it work to take a pipe off the drain cocks with a tee and just stick an ordinary pressure gauge on it, sort of like configuration #4 ?
  • Jamie HallJamie Hall Member Posts: 13,203
    edited September 17
    Depending on where you put your pressure taps, you can measure the head loss on pretty much anything... the point of putting them directly on opposite sides of the pump is that that allows you to look at the pump curve and get the flow rate.

    Then, of course, you can put additional taps -- and gauges -- at opposite ends of a loop or whatever you want to measure.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
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