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Reasonable head loss on series loop

Hi. Not sure this should be in radiant or not, but I didn’t see a “hydronic” section. What is a reasonable head loss for a series of 3/4” pex with 6 baseboard units in series? Based on gpm (0.8) and pump size (1/25 hp taco) and my independent pressure gauge measurements, the non-boiler (loop) head losses are on the order of 5-7 feet, giving 3/4” equivalent length of 1150 feet (includes elbows etc) This seems incredibly high for a loop that is heating a ~1000 sqft place. Trying to figure out if there is a construction somewhere before it gets dry walled up.

Comments

  • Jamie HallJamie Hall Posts: 10,145Member
    You're math doesn't add up. You've got 0.1 foot-pounds/second in your flow. Which is a good bit less than the nominal pump size...
    Jamie



    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
  • humnan1st0humnan1st0 Posts: 6Member
    edited January 27
    At 0.8 gpm, the pump should overcome ~8.3 ft of head. The boiler loses about 3 ft at this rate. I checked this against inlet/outlet pressure measurements and it adds up correctly. What I am left with is ~5 feet of loss at 0.8 gpm over what I understand to be a relatively short distance. When I add up the theoretical losses including elbows, I get ~600 ft, not the 1150 that results. Is there something I’m missing??
  • humnan1st0humnan1st0 Posts: 6Member
    Seems like your calcs are off. I get something like 0.3 psi drop from 0.1 ftlb/sec, not
  • GordyGordy Posts: 9,263Member
    Your equivalent length calculations are way off.
    A reasonable flow rate is from 1 gpm to 4 gpm for baseboard.
    A reasonable headloss is one that would let a reasonably sized circulator of choice pump that flow rate.



  • humnan1st0humnan1st0 Posts: 6Member
    That’s what I’m saying. There shouldn’t be this much head. There is no reasonable pump for this situation. Why? It seems like the must be some grave head loss there...
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,466Member
    Are you calculating the head base on pipe and fittings or getting it from pressure gages?

    Head loss is usually /100 ft of pipe if that has any bearing
  • GordyGordy Posts: 9,263Member
    I guess I'm not understanding the issue. You said you are getting .8 gpm. Is this assumed, or read by a flow meter?

    What size taco circulator? 007? Or other. 1/25 hp is a broad range of circulators.

    How are you taking your pressure differential readings? Should be right at the suction, and discharge of the pump.

    Pressure Gauges 1, or 2?

    A 007 at .8 gpm is seeing 9.5' of head.

    3/4" pex tubing is .34psi per 100'.

    How many 5 gallon buckets of fittings were used? Were they expansion fittings?

  • humnan1st0humnan1st0 Posts: 6Member
    Thanks for the responses. The 0.8 gpm is from the boiler. I understand it may not be highly accurate but is ball park.

    The taco circulator is the 007-f5. The pump pushes water through the heater then through the loop.

    Pressure readings were taken just downstream (d/s) of the pump, just d/s of the heater, then at the return u/s of the heater. I don’t anticipate that the pressure readings are highly accurate either, but they corroborate that the pump is operating correctly.

    3/4” hepex table show 0.4 ft head loss per 100 ft at 0.8 gpm and 150F, not 0.34 psi (0.78 ft / 100 ft).

    Using 0.4 ft/100ft and assuming a 5 foot head loss, I get a total of 1250 ft of piping. Which is 10x the house perimeter. Like I said, this seems waaaaaay too high.
  • GordyGordy Posts: 9,263Member
    edited January 28
    Time out.......what’s “the heater”?
    Boiler?
    Tankless?
    There is a difference.
  • GordyGordy Posts: 9,263Member
    If it’s a tankless water heater you are using as a boiler there is your issue. They have an extremely restrictive HX compared to a boiler.

    If it’s not piped primary secondary there is the other problem.
  • humnan1st0humnan1st0 Posts: 6Member
    Yes. I didn’t specify but the heater/boiler I referred to is neither, it’s a tankless. The product specs show head loss of 3 ft at 1 gpm and about 6 ft at 2 gpm. As I said in my posts I am considering this loss in my calcs.
  • Jamie HallJamie Hall Posts: 10,145Member
    May I humbly suggest you go right back to square one. Measure -- with a pair of accurate gauges -- the pressure change across the pump (not somewhere else in the system -- it can get very confusing). Refer to the pump's characteristic curve to get the gallons per minute which the pump -- assuming it's in decent condition -- is delivering (a note: if the pump inlet pressure is low -- near 0 psig -- it may not be delivering its rated flow). Then proceed from there.
    Jamie



    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
  • GordyGordy Posts: 9,263Member
    If it’s a tankless domestic hot water heater it’s head loss is much higher than that.
  • Jamie HallJamie Hall Posts: 10,145Member
    A tankless could easily have 20 to 30 feet of headloss at one gpm -- not 3.
    Jamie



    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
  • GordyGordy Posts: 9,263Member
    My tip off was the gpm read out in “the heater”. Boilers do not have this feature as it is not all that reliable.

    If your chart for the tankless is saying 3’ of head for 1gpm. The 007 is trying to push way more than that through it. The 007 has a steep curve, not designed for high head loss applications. As you can see.

    Typically when piped primary secondary it takes a grundfos 26-99, or similar curved circulator on the primary loop for the tankless to get the flow needed. Then the 007 can manage the secondary heating loop flow.

    A tankless depends on domestic water pressure to get the flow it needs. You have to mimic that.
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