Kung Fu Fighting

Wanted this to be my wedding song 10 years ago. The bride thought otherwise…

Carl Douglas and his unlikely worldwide #1 hit from the Fall of that musical Hall Of Fame year, 1974.

Last time out we shared how system curves and pump curves interact, and that systems always operate where the system curve intersects the pump curve.  Even if we calculate system requirement at, say, 9 GPM at 7 feet of head, once you slap a fixed speed circulator on the system it’ll most likely never work there.

Let’s presume we have a zone valve system.  Last week we showed where the system operates with different circulators when all zones are calling.  But what would happen with only one zone calling?

Well, we know the system will have to work somewhere on the performance curve of the chosen circulator. Based on last week’s blog, let’s presume we’re using the Taco 0015 3-speed (or any common, steep-curved 3-speed circulator – Grundfos, B&G, etc – their performance curves are pretty similar). The performance requirements were 9 GPM at 7′ of head:

The red line represents the theoretical system curve with all zones calling, and the blue circles represent the operating points with the circulator running at Medium Speed and at High Speed.

But what happens when only 1 zone is calling?

Let’s assume the smallest zone on the system requires only 2 GPM under design conditions, and that the zone has a head loss of 5′.  Using the Taco FloPro Designer software Pump Laws tool (click here to review), we can calculate the following points on the theoretical system curve for that particular zone:

1 GPM @ 1.3′
2 GPM @ 5′
3 GPM @ 11′
4 GPM @ 20′
5 GPM @ 30′

Let’s plot these points on the chart, connect the dots and see what happens:

As you can see, the operating point  at Medium Speed is 3.5 GPM at around 15′ of head.  At high speed it’s 3.75 GPM at roughly 17.5′ of head.

No doubt about it – this circulator is gonna deliver the BTU’s this system needs.  You want “safe?” You got it!

So what’s the downside?

Take a look at the head pressure – either 15′ or 17.5′, depending on which speed you select (we didn’t pick low speed – it didn’t meet the system requirements with all zones calling).  Zone valves closing against that kind of pressure differential do not go quietly into the night.

They tend to bang like crazy!

Which generally makes customers to complain like crazy.

Which leads to kung-fu fighting.

That’s the obvious problem (a pressure differential bypass valve may mask it)  The other problem is silent, hidden and sinister.

And it can cut overall economy of operation off right at the kneecaps.

And we’ll tackle it next go round!

Till then, rock on with this David Essex gem from March of ’74…

“And where do we go from here?”

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