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radiant floor pump upgrade. gpm, watts, head, etc.

zuul Member Posts: 1
edited November 2020 in Radiant Heating
In my home, I recently finished a great room with hydronic heating in a polished concrete slab (800sq ft, 5" thick, 20' ceilings). Its 30F outside now and working fantastic. The system is 4 parallel circuits of ~200ft 1/2" barrier pex, a Takagi instant hot set to 120F, and a Taco 0015e3 3-speed pump (plus other misc stuff on my panel). I'm controlling the pump via Ecobee thermostat with a 0.5deg differential setting, which is keeping me within 1.5 or 2 degrees throughout the day, and only cycling maybe 6 times a day, which I'm happy with.

I'm getting 1.8 GPM on high speed, but wanted 2.5 - 3 GPM. I probably underestimated the pressure drop on the Takagi heater. I am having difficulty figuring out how to get to 3GPM without guessing again. The reason I want a little more flow, is for one my loopcad designed for it and I think I'll need it on very cold days, and two, I want a little less temperature difference in the floor, which a higher flow should accomplish.

If I look at the performance curves on my Taco 0015e3 pump (https://www.ecomfort.com/manuals/taco-a7fd4eba7f6381c494cf4c6d925b7182.pdf) , I see that since I'm running 1.8 GPM, I probably have 17.5 ft head. Here's where I'm not sure about my approach: If I look at another pump that is geared towards high head, like the Taco 009 (curves https://s3.amazonaws.com/s3.supplyhouse.com/manuals/1347286893605/79538_PROD_FILE.pdf) , can I simply look at what GPM is gives at the same 17.5 ft head point? In that case it would be about 6.5 GPM (too much).

That brings the next question. Is it OK to use a too-powerful pump and simply turn the pex circuit balancing valves down a bit to keep the flow down to 3 GPM? And would doing that reduce the amps draw of the pump since its flowing less?



  • Zman
    Zman Member Posts: 7,550
    I am sure the issue is the pressure drop in the Takagi. Have you been able to track down the curves?
    Repiping it primary/secondary would be one way to fix the problem.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Jamie Hall
    Jamie Hall Member Posts: 22,947
    "can I simply look at what GPM is gives at the same 17.5 ft head point? In that case it would be about 6.5 GPM (too much)". No, sorry, it doesn't work that way. As the flow increases, the head loss also increases -- almost (but not quite) as the square of the flow. You need to draw the system head vs. flow curve, and overlay that on the pump head vs. flow curve, and the system will operate where they intersect.

    Not surprised at the low flow with the Takagi. Very nice instant hot water heater. Never meant to be used for a heating system.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EBEBRATT-Ed Member Posts: 15,345
    Find out the pressure drop of the Takagi at the mfg flow rate. The mfg can tell you that

    Then with the B & G system sizer app or wheel find the PD in feet of head at the flow your putting through the heater, Once you have this new PD ad it to the head loss of your pipe valves and fittings for the circuit with the most resistance
  • GroundUp
    GroundUp Member Posts: 1,868
    You're correct, it is the high pressure drop of the Takagi causing the low flow. With 200ft loops, you're going to be limited to about .9-1 GPM per loop regardless of circ so upsizing it to something like an 009 would still max out around 3.5-4 GPM anyway no matter what the curve says. Don't ask me why, I don't know but I've asked dozens of well respected hydronics techs including several on this page and nobody seems to be able to answer it. Long story short, either upsizing the main circ or repiping as primary secondary are your options with the Takagi
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    Primary/secondary is the best option. 
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!