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Circulating Pump Speed?

I have a recently installed hydronic floor heating system with two Grundfos UPS15-58FC circulating pumps. They are variable speed (3) and one is set to HI and one set to LO.

Seems like a mistake by the installer, shouldn't both be the same?

Does the speed of the pumps influence:
1. How quickly the system heats up?
2. Efficiency?

Thanks for all the help!

Additional info: 1 floor, 1000 sq. ft, natural gas combi boiler.


  • HomerJSmith
    HomerJSmith Member Posts: 1,564
    edited February 2021
    The speed of the pump doesn't have any thing to do with "How quickly the system heats up." Flow is the component of the sys that has the most impact aside from temperature in how quickly a sys heats up. Of course, running a pump at a higher flow, all thing being equal, will put more heat energy into a space.

    The speed of the pump increases the pressure to overcome the resistance that the tubing and fittings presents. So, one pump can be at one speed and another at a different speed, but the flow would be the same in both circuits because both circuits have different resistances. For the speeds to be the same, both circuits would have to be identical in terms of resistance.

    Running a pump at a lower speed is more efficient because it uses less electricity. But as far as boiler efficiency, not so much.
  • pecmsg
    pecmsg Member Posts: 2,898
    In floor radiant heat is S L O W. Do not use set backs. 

    EBEBRATT-Ed Member Posts: 12,010
    as @HomerJSmith said

    If the installer did the right thing he selected the circulators based on the water flow needed to heat the space and to overcome the resistance of the tubing and fittings.

    Not unusual to have different circulators or different speeds
  • Zman
    Zman Member Posts: 7,330
    If you know the number of loops and the length of the loops on each circ, we could give you more advice.
    If you want to get a little nerdy on your own, measure the temp difference in the supply and return pipe attached to each circ. Do this after the circ has been running for a while. If the difference in temp is >10 degrees, try the circ on a higher speed.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • GroundUp
    GroundUp Member Posts: 1,406
    Are there two zones or is one of the pumps doing the boiler loop while the other runs the floor loops in a single zone arrangement? If the latter, it makes perfect sense that they would be set at different speeds because they're doing different jobs. A photo of the system would be incredibly helpful