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Just Don't Get Modcon Boiler Pump Charts!



  • Eastman
    Eastman Member Posts: 927
    edited April 2013
    Here's something...

    This is kind of a silly website and a bit out of context but, here you go:

    -Flow in Relationship to Tees-

    "Now with all the above explained let's look at what is happening in the system. We will look first at the primary loop drawings 1 - 3. There are two rules we must remember when piping p/s.  The first is "What enters a tee exits a tee". It has nowhere else to go and cannot stay in the tee. If 10 gpm enters a tee it must exit at 10 gpm. It does not have to exit in equal flow rates out of the tee as you will see later. But, for ease of explanation let's assume as the 10 gpm enters the tee, 5 gpm goes out the branch and the other 5 gpm moves between the tees."
  • Eastman
    Eastman Member Posts: 927
    edited April 2013
    book recommendation

    I would recommend the latest edition of John Siegenthaler's Modern Hydronic Heating.

    But honestly, you don't need to read a book for this.  Grab a 1 gallon jug.  Fill it up with water.  Go outside.  Go to the top of a small hill.  Dig a small trench down one side and another going down the other side. (Make sure they come together at the top.) Dump the water into the trenches where they join at the top of the hill.  You dumped one gallon, some went left and some went right.  But together there must still be one gallon of water on the ground.  Go back home and fill the jug again and dump it again.  Do this once a minute.  You are dumping one gallon per minute onto the ground.  Some goes left some goes right.  But together there must be 1 gpm flowing down the hill.  Not 2 gpm.  That would imply that 1 gpm is magically appearing from somewhere else.  Notice that the size of the trenches, the opening on the jug, all of that is irrelevant.

    If you can cram 6gpm into a tee, then 6 gpm total is going to come out.  Perhaps the tee explodes, or perhaps some supernatural being is playing tricks, but barring all that, what goes must come out.


    By the way, did you see the other thread I started on this topic?  Several more people have weighed in now.  Everyone unanimously concurs that if 6 goes in then 3 and 3 are coming out.  So hopefully you are truly convinced now.
  • Eastman
    Eastman Member Posts: 927
    edited April 2013

    At this point, perhaps you're wondering --"what again was this significance of all these flow rates?"

    Here's the deal, along time ago when you were doing estimates for head loss @ target flows, you were doing them wrong.  Most of the examples on the internet are for pri/sec systems, and most of the advice you have been given has been too general and doesn't address your specific system.  Not only were you using a rule of thumb that has a huge margin of error, you were plugging in the wrong numbers.

    You wanted to do something that is not covered in the manual. (ditch the pri/sec)  Can you do it?  If you use really rough worst case estimates and then plug in the wrong worst case numbers, it gives the impression that it could only work at the highest possible boiler deltas.

    But....  If you are going to do something that is not covered by the installation manual, should you be using rough rule of thumb estimates in the first place?  (Never mind plugging in the wrong info.)  H*ll no.  Estimates are based on more complicated and fundamental equations.  There are lower level, more fundamental, more complicated, and infinitely more precise equations that describe these relationships.  You should be using those equations.

    Now, people might complain, "That's too difficult.  How can you recommend that the laymen attempt such calculations."  Don't worry.  That's why this site exists.  If you can't do it I will.

    Moving on....  I calculated the head loss accurately (not an estimate) along time ago for 6gpm.  (3 in each branch)  The common ecm residential circs all can easily exceed that flow rate through your system.  Directly pumped.

    Conclusion:  Weather you decide 10, 20, or a 35 degree delta is necessary --all of the common adjustable ecm circs will be able to hit the necessary target flows.  Nothing is being damaged.  My only two concerns are that at a 35 degree delta, a)there is a small risk bubbles could accumulate and airlock the system, and b) much more likely, your home won't heat evenly like it is now.

    Personally, I'd go with a bumble bee, just because it would be fun to play with the delta T function.  And also, I ?believe? it can be swapped out with your 007 in case it gets hit by lightning or something crazy like that.
  • tom3holer
    tom3holer Member Posts: 45

    I just called Taco and confirmed that the Bumblebee is a direct bolt in replcement for the 00 series. The data sheets do show a 1/16" difference in flange distances but the tech rep confirmed it is a direct replacement.

  • ced48
    ced48 Member Posts: 468
    Hey, I Finally Get IT!

    The problem my peanut sized brain was having was thinking that GPM and velocity were one in the same. My thinking that the water in the smaller pipes had to be traveling at about the same rate of speed was correct, but not relevant. GPM is how much water, volume, is being delivered per minute, not how fast. I really want to thank Eastman for sticking with me on this, and for of all the time he put into my problem. Also, thank you Chris and all the rest.
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