From Small Things…

“…Mama, big things one day come.”

Another classic from our man Dave Edmunds.  Try getting that out of your head the rest of the day.

Last time we started looking at how a typical Delta-P “variable-speed” circulator would work in a zone valve system (click here to review).  So far in our example, there’s nothing “variable-speed” about it.

We left off with three zones calling and a design-condition (coldest day of the year) load of 60,000 BTUH, or 6 GPM.  We found that the Delta-P Wilo Stratos-Eco, under those conditions, would still be running at full speed.  In addition, it would be delivering 9 GPM, some 50% more flow than the 6 GPM required.

In addition, the Stratos-Eco would be doing this every day of the winter, regardless of outdoor temperature or BTUH load.

Now let’s take a look at what happens with only the two smaller zones calling.

Let’s presume now that only 2 zones are calling, with a total combined load of 30,000 BTUH.  That’s 3 GPM worth of required flow at a 200 designed-for Delta-T. Further, the worst-case head loss of the remaining zones is just under 4’ of head.  That operating point is below as Point C.

Last time we said this type of Delta-P circulator operates on a fixed pump curve.  It’s funny looking, but it’s determined when you adjust the red programming dial.

So looking at reality, where will the system operate with only the two small zones operating?

Again, it’s where the two-zone system curve line intersects the down-slanting portion of the pump curve line – at approximately 4½ GPM at nearly 8’ of head.

This also represents the first time in our system the circulator has actually “varied” its speed.  Also note that once again we’re getting 50% more flow than we actually require.

Now let’s get down to just 1 zone calling, with requirements of 1 GPM (10,000 BTUH) and only 2½’ of head (it’s a little zone!).  The requirement shown below is Point D, but the actual operating point would be at nearly 2 GPM and 5’ of head – roughly double what is required.

And again, this circulator will run on that performance curve all the time.  It’ll run that speed in January.  It’ll run that speed in October.

The examples shown are “design conditions,” the so-called “coldest day of the year.”  When it’s warmer than that out – as it is nearly 98% of the time – the circulator is still going to run at those same speeds even though the heating loads (and required flow rates) will be drastically lower.

Tell me again how this is a “variable-speed” circulator?

It’s more of a “how-many-zones-are-open” speed circulator. In the example shown, with the combination of zones shown, the circulator will run at 3 distinct speeds.

With different combinations of zones calling the circulator will run at other speeds. It will always, however, run somewhere on that fixed, albeit funny-looking, pump curve.

Because of its electronics, the pump will slow down as zone valves close, but the speed at which it ultimately runs has nothing to do with the actual flow rate required by the system at that given point in time, nor does it have anything to do with the actual head loss of the system at that given point in time.

The system just runs on that line.

All the time.

Understand that no one is saying this circulator is a piece of junk.  It’s a high quality product made by a world-class manufacturer for a very common European application: panel radiators in a parallel piping system using thermostatic radiator valves.

In a series loop zone valve system the Stratos-Eco, with right-out-of-the-box programming, is functional, but we can do better.

Next time we’ll discuss what would happen if we actually set the programming dial to match the actual head loss of the system and change that funny looking pump curve.

And we’ll leave you with another one of Dave’s best…

“…knowing it ain’t really smart.”


One Response to “From Small Things…”

  1. Thanks John! Gonna send this one off to a few of my delta p lovers. Gotta see what the rebuttal is!

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