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Grundfos pump size for HTP Elite 150 k boiler

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rosy
rosy Member Posts: 9
in Radiant Heating
last month the Grundfos (GS) 26/64 that fed the 2011 Elite 150k boiler failed, and in a pinch we installed a 14/42 for temporary use. THe boiler is piped as a secondary circuit with 3' of pipe on the intake and output with closely spaced T's and two 90 degree turns (not sweeps) to give us very little resistance) on a secondary/Primary system (1.25" copper pipe all the way around) and the Delta T decreased by about three degrees to 17 rather than 20, to get back to 20 we temporarily throttled back the input and output ball valves to about half. A Grundfos "rep" (sales) said that we needed to reinstall the 26/64 because of head pressure (17 called for in the manual, and 10 GPM flow) and that we needed to run the 26/64 pump wide open to get the necessary head and our Delta T of 20 would return.

However it at least seems as though our problem is too much water flow since the 14/42 is rated at about 15-17 GPM , and our supply and return on the HTP comes with 1" NPT which are sized up immediately after exiting the boiler. When we choke back the H20 we get a rise in DElta T as one might guess. What I do not fully understand is the precise role that head plays in the game here assuming we have TOO LITTLE of it. Is it simply the case that lacking enough head that the H20 cannot fully exchange heat with the boiler heat exchanger? Note that a 15/58 3 speed is arguably close on the curve to providing both 10 GPM and the required head at speed three. What Can't we just use this instead of what seems like a monster of a grundfos pump in the 26/64.

My theory is that we are just moving too much water through the boiler too fast.

RElated question. If that rep is correct --Then when we take a new HTP ELite 150 and use the 0-10 DC on a variable speed pump and its the same heat exchanger (allowed by the factory manual), that pump is varying water speed at GPM by measuring intake/output temps anywhere from the minimum to the maximum. Besides the fact that head could affect flow - can anyone explain if it's not too much trouble, the lack of resistance through a boiler and heat exchange - assuming that we do not have enough head (is there ever enough head?)

THanks,

Dave

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  • GroundUp
    GroundUp Member Posts: 1,907
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    There is no "required head". There will be a pressure drop through any given equipment at a given flow rate. I'm not going to dig up an Elite 150 manual, but I'm guessing what's going on is that there is a pressure drop of about 7.4 PSI (aka 17 ft of head) at 10 GPM flow rate through the boiler. Now if you look at the curve of a 15-42 (there is no such thing as a 14-42), you will see that at 10 GPM, it only has about 7 ft of head to offer while the 26-64 has about 18.

    Okay I dug up a manual- minimum flow through the boiler is 10 GPM and that makes a pressure drop of 12ft. Still too much for the 15-42 to handle. The Elite has a 10:1 turndown ratio, so what you are seeing with the lower delta is a lower firing rate than what you were seeing when you were watching it with the old circ, or the old circ was already near death when it was set to a 20 degree delta. Basically, in order to meet the 10 GPM demand at high fire you will need to overcome 12ft of head. In order to hit that curve, you're looking at another 26-64 or a 26-99 on speed 2. One can move into the 0-10 or even a delta T circ if you're feeling fancy, but personally I'd go back to the good old 26 series and make it happen.
  • rosy
    rosy Member Posts: 9
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    I appreciate your input. And I follow. I had that right in front of me too, the curve. But the boiler says 100% and I think she means it. Additionally, when we choke the intake and output the delta t rises. I have a nasty feeling when I open it up, I’m going to find s dirty HE and when I think about the one zone (warning!; some viewers may find the next words horrifying) with entran 2, I have a sinking feeling the HE has a nice coat of black on it, oil that is, black gold (bev hillbillies reg), anyway don’t forget the 1.25” tubing. Also let’s assume your absolutely correct; wouldn’t my delta t b higher if the water was taking longer to go through the boiler?

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