Variable Speed Pump with a low head loss system

Robert_H

<ol><li>I am possibly obsessing on nothing and am in desperate need of being shoved in the right direction. The good news is the system is going in, some radiators are in place, radiant ceiling and manifold are in and pressure tested. However, I’m concerned about my original pump choice.



The system basics are:

42000 BTUH

Vitodens B2HA-19

Using ODR

Viessmann Low Loss Header with DeltaT pump on system loop, Home Run piping.

Cast Rads with TRVS

One small radiant ceiling

Rads oversized to match Radiant Ceiling temp. No mixing or zone valves.



I originally planned to run a bumblebee on Delta-T. But the  worst case head loss  in the system is 2.4’ at 4.2 GPM on a 0 degree day. (using half inch tube on the rads)

That puts me just above the low speed curve. Meaning to me that most of the time I’m going to be riding that curve and getting no benefit from the Delta-T mode. I might even be over pumping a lot of the time (if that’s an issue).  My understanding that I should be above the lowest speed curve to take full advantage of variable speed.

I’m I correct in that basic understanding?



Assuming I’m right I looked at the Grundfos Alpha and Wilo ECO circulators and my entry point on those curves was not better.  So back to my system, Trying to get above the curve:

Being trained in electronics long ago the first thing that came to mind was to add a resistor, in this case a valve of some sort to add head. Probably a silly idea.  

I relooked at my piping and was using ½” fostapex with longest round trip run at 60 feet.  The highest rad load is 4750 BTUH.  As best I can tell, 3/8” tube will meet flow requirements and increase the head somewhat.

I ran heat loss at 0, 30 and 50 degrees ODT and calculated head loss at those temps for the 3/8 and ½ tube. Ill attach a Bumble bee curve with that data plotted.



With the 3/8” tube, 0 and 30 degrees are above the pump curve which looks like an improvement.

Doing some research I thought of using a pressure differential valve on the system manifold to allow the pump to run in its zone. Would this help?



Any comments and advice will be appreciated.

Robert



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CMadatMe

You Didn't Need

To pipe with a low loss header. Could have piped direct. Since you did, best option for a system pump is a B&G Vario.



http://documentlibrary.xylemappliedwater.com/wp-content/blogs.dir/22/files/2012/07/A-139A.pdf

Robert_H

Auto version

Thanks for the reply Chris.

What about the Auto, would you use that in this case or stick with the fixed speed?