But have you noticed some changes in the way your multi-zoned systems operate when you use these little circulators?

For example, let's say someone installs three electric zone valves on a packaged boiler which came with a little circulator. When any one zone calls, everything is fine. But when two zones are calling, things begin to get a bit noisy.

Both zone valves are open and the little circulator is on. Suddenly one zone valve closes, but it doesn't close quietly. It's accompanied by an annoying water-hammer and the next thing you know, the customer is on the phone asking if his boiler is going to explode.

To understand what's happening here, you have to cut through the marketing chatter and look only at the engineering.

Operating performance curves are the roads on which the circulator must travel when it's operating in a system. They show us that as resistance (Head) increases, flow will decrease. Of course, the opposite also applies: As resistance decreases (in other words, as valves open), flow will increase.

The Series 100 has a very flat curve. That's because it runs at 1750 rpm. It's designed to move a lot of water against a system with low "head" resistance (typically, systems with 3/4", 1" and 1-1/4" piping).

Wet rotors have a steep curve. This is characteristic of all high-speed circulators. This type of circulator will move less water, but at a higher pressure. It can handle, for instance, the higher resistance and lower-flow requirements you'd find in the small tubes of a radiant or solar panel.

See? Different applications require different circulators. It's engineering, pure and simple.

Now let's take a look at the problem mentioned earlier. Why do the zone valves sometimes bang when the little circulator shuts down?

The reason becomes clear if you can imagine the water flowing through the pipes. Two 3/4" zones are open so we have about 8 GPM moving out to the two zones. We can safely say this because a 3/4" copper pipe can handle a maximum of about 4 GPM. That's why baseboard is rated at 4 GPM; it's the most water that can move through a 3/4" pipe without making a whistling, velocity noise.

Knowing this, we can say that wet rotor circulators will be operating at about this point on its performance curve.

The Series 100, as you can see, would also be operating at this same point were it serving this system. However, look at the difference in the curves.

Look at the 20% rise in pressure the little water lubricated circulator must go through to get back to 4 gm. That rise represents nearly a full pound of circulator pressure. This pressure increase is usually what causes the zone valve to bang as it shuts.

You see, some zone valve manufacturers use a rotating valve disk to close their valve. As the disk swings into the onrushing flow from the high-speed circulator, the velocity across the valve increases.

Then, just before the disk seats, the velocity from the circulator peaks and the valve bangs shut. The bang is caused by the high-pressure water that suddenly has the brakes put on it.

But now look at the Series 100 curve. The difference is apparent; the Series 100 has a "flat" curve. This is characteristic of most 1750-rpm circulators.

Because of the flatness of the curve, the Series 100 can drift beck to 4 GPM without creating a rise in pressure. And since the Series 100 doesn't build excessive pressure as the flow needs of the system change, the velocity of the water doesn't increase. That means the zone valve doesn't bang when it closes.

This is something that's rarely mentioned in the marketing of small high-speed circulators. Their steep curves make them fine for single-zone duty, but they pale by comparison to the Series 100 when it comes to systems zoned with several valves.