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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 Hall
    Jamie Hall Member Posts: 18,929
    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...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • humnan1st0
    humnan1st0 Member Posts: 6
    edited January 2019
    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??
  • humnan1st0
    humnan1st0 Member Posts: 6
    Seems like your calcs are off. I get something like 0.3 psi drop from 0.1 ftlb/sec, not
  • Gordy
    Gordy Member Posts: 9,514
    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.



  • humnan1st0
    humnan1st0 Member Posts: 6
    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-Ed
    EBEBRATT-Ed Member Posts: 11,995
    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
  • Gordy
    Gordy Member Posts: 9,514
    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?

    JohnNY
  • humnan1st0
    humnan1st0 Member Posts: 6
    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.
  • Gordy
    Gordy Member Posts: 9,514
    edited January 2019
    Time out.......what’s “the heater”?
    Boiler?
    Tankless?
    There is a difference.
  • Gordy
    Gordy Member Posts: 9,514
    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.
  • humnan1st0
    humnan1st0 Member Posts: 6
    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 Hall
    Jamie Hall Member Posts: 18,929
    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.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Canucker
  • Gordy
    Gordy Member Posts: 9,514
    If it’s a tankless domestic hot water heater it’s head loss is much higher than that.
  • Jamie Hall
    Jamie Hall Member Posts: 18,929
    A tankless could easily have 20 to 30 feet of headloss at one gpm -- not 3.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Gordy
    Gordy Member Posts: 9,514
    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.
    humnan1st0