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Pump sizing why is it so important?

Why you don't want too much.

1) Velocity noise as you mention.

2) Excessive wear at very high velocity--particularly from poorly reamed copper tube. If TRVs are used they have to work harder to slow the velocity and their generally indefinite lifespan becomes more definite...

3) Excess electric consumption. More water moved=more effort required=more electricity.

4) Heat transfer problems. As velocity increases, temperature drop decreases and average temperature increases. The lowest possible average temperature to meet demand (within ability of the boiler) will be the most efficient. Efficiency at the boiler HX decreases. Distribution loss increases. <I>[Excuse the decrease/increase mixup in the original. I must be getting dyslexic!]</I>

Reasons you don't want too little:

1) Air removal may be difficult at extremely low velocities.

2) As velocity decreases, temperature difference increases while average temp decreases. Let the average temp fall too low and your emitters may not be able to liberate heat rapidly enough and the boiler will short-cycle unnecessarily as it "bounces" off high limit.


  • RoosterBoy
    RoosterBoy Member Posts: 459

    hay guys besides having high velocity in your pipes and the noise that go's with it.

    why is it better to pick a pump that is just right for the job. why is it bad to over size

  • Michal
    Michal Member Posts: 213
    few reasons

    well efficiency of pumps change if they are over sized or under sized, wire sizes change accordingly to motor size, also with improper flow through a coil or heat exhanger you will not get your rated capacity for heating, too fast and your over loading a coil. You might also circulate water too slow and get water too cold coming back to the boiler thus causing thermal shock. There are noise issues as well. You try to size a pump accordingly to match your demand, but you leave a littl;e slack so it can "ride the curve" just in case there are some unaccounted for losses in head, or added gpm load taht might have been mised. But these misses are minor not major #'s, if they are off by a significant number, then the pump should be resized. These are a few things. I am sure there are many more reasons?
  • RoosterBoy
    RoosterBoy Member Posts: 459

    so in general the right size pump helps with keeping that delta t at 20 to 30 deg difference coming back to the boiler right?

    if my pump is to big my return temp will be to high and will cause the boiler to short cycle.

    sound right?


  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    so in general the right size pump helps with keeping that delta t at 20 to 30 deg difference coming back to the boiler right

    "Helps" is the operative word in that sentence, but yes.

    if my pump is to big my return temp will be to high and will cause the boiler to short cycle

    Yes, it could lead to shortened cycles because the radiation is now warmer, has greater output and can satisfy a call more rapidly. In my mind, that problem isn't as great as the decreased boiler HX efficiency and increased transmission loss.

  • RoosterBoy
    RoosterBoy Member Posts: 459

    mike thanks so much your always a good help to me I have learned allot from you since i came to this form

    thanks allot
  • hr
    hr Member Posts: 6,106

    high head pumps are generally 2 to 3 times the cost:)

    As far as delta t, ideally onthe radiant loops you want a tight delta t, some consider 10- 15 degesss ideal for ultimate comfort. Ocassionally i will drive commercial truck repair, etc, slabs to a 30 degree delta t. It allows for longer loop lengths and less pump.

    But wide delta t may be better on your boiler loop, storage, or buffer. The wider delta T allows more energy storage, and allows smaller pipe size. It will help with boiler short cycling also. Be aware of how wide of a delta T the boiler manufacture will allow. You need to keep an eye on the return temperature to cast or copper boilers.

    With a condensing boiler return temperature is not a problem, and the lower the better for efficiency.

    Different delta t for different applications.

    Dan does such a wonderful job of explaining all things hydronic! This link will lead you to a good piece on delta t leverage.


    hot rod

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