Midnight At The Oasis

“Send your camel to bed…”

Maria Muldaur hit #6 in the spring of ’74 with this one.  I can remember staying up late to watch her on The Midnight Special.

Don’t think my 13-year-old self slept well that night…

There is an oasis in hydronics, you know.  It’s the spot where the pump curve and the system curve intersect.

It’s also known as the actual operating point of the system.  You can do all the diligent design that you want in order to figure out the exact flow rate you need and the exact head loss of the piping system, but once you slap in a fixed speed circulator (either single speed or 3-speed), the system works where the circulator says.

That’d be midnight at the oasis…

Let’s say we have a 90,000 BTUH zone valve system.  Some quick calculations tell us the system requires 9 Gallons Per Minute of flow, with a head loss of  7 feet.  Let’s plot it on a pump curve chart:

From the looks of it, we could select a Taco 007 or a 008.  If we chose the 0015 3-Speed circulator, the chart below says Medium speed would do the trick.

Next, using a math formula (click here to review) we’re able to plot out something called the System Curve. The system curve is a theoretical (sort of) curve, showing system head loss at a bunch of different flow rates.  As flow goes up, so does head loss.  When you plot the curve you can figure out where the system curve and pump curve intersect.

And that’ll tell you where the system will actually operate.

Who knew?

So here’s the real world operating point. With a 007, we’re getting roughly 9½ GPM and roughly 7½ feet of head.

Pretty close, no?

With a 008, we’d get roughly 9¾ GPM and just over 8 feet of head.

Want an easy way to figure system curves? Read on…

The Taco FloPro Designer software (free download here) has a neat little tool that will do the math for you, provided you know what you’re looking for.

Once you open up the program, click on the Tools menu, and then select the Pump Laws item, as shown here:


To use the Pump Laws tool, input your calculated flow rate (9 GPM) in the Flow 1 box, and your calculated head loss (7′) in the Head Loss 1 box.

Next, simply input other flow rates to find what the head loss would be through that particular piping system.

So here’s what we’ve come up with:

2 GPM – .35 feet of head

4 GPM – 1.38 feet of head

4.5 GPM – 1.75 feet of head (½ the flow, ¼ the head!)

6 GPM – 3.11 feet of head

Just for kicks, let’s keep going:

11 GPM – 10.46 feet of head

14 GPM – 16.94 feet of head

18 GPM – 28 feet of head (twice the flow, 4 times the head!)

If we plot these points on the pump curve chart and connect the dots, we get the “finger print” of the system.  We also get an idea of what will happen to the system if we oversize or undersize the circulator.

If we use a 006 for this job, we’d get a little less than 7.5 GPM at around 4.5 feet of head.  Would it work?  For most of the winter, absolutely.

When it gets really cold? No dice.

What about using a 3-speed?  Low won’t do it, but Medium or High surely would.

Seems like the simple choice, right?  A 3-speed circulator is the only one you’ll ever need, right?

Well, yes and no.

Remember, we’re dealing with a system, and Webster’s defines a system as “a regular interacting or interdependent group of items forming a unified whole.”

It all has to work together.

We’ll throw zone valves into the equation next time.  It should be interesting…

Not as interesting as this hit from ’74…

Yep – Jim Stafford’s “classic” hit #3 that spring.



2 Responses to “Midnight At The Oasis”

  1. what a coincidence John, Maria Muldaur , performed at Ninigret Park in Charlestown RI this weekend at the Rhythm and Root festival.

  2. Small world! She certainly has a unique talent and a pretty rich background. Didn’t realize she’d been so prolific over the years, including a grammy nomination just a few years ago.

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