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Trap Doors and Ghosts

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Did you know that closely spaced tees on a Hydronic system create two trap doors? The further apart these tees are the more open the doors will be. Open the doors and the ghosts will haunt you!

A friend of mine’s favorite saying is “The Rules of Economics will never outweigh the Laws of Physics.” Wow, think of that, no matter how you are trying to shave costs, no matter how much you underbid the job, you just can’t fool the rules of physics!

In a hydronic heating system, we deal with the Laws of Physics. They include, but are not limited to; Hot water rises (Archimedes’ Principle). High Pressure goes to low pressure (Second Law of Thermodynamics). Heat goes to lack of heat (Second Law of thermodynamics). Lighter objects (air) float (Archimedes’ principle). Increase the working pressure of water in a boiler and air bubbles get smaller (Boyle’s law). As we increase the temperature of water in a boiler, air comes out (Henry’s Law). As we heat air bubbles in a system, the bubbles get larger (Charles law). Every employee tends to rise to his level of incompetence (Peter Principle). Lastly, if anything can go wrong, it will (Murphy’s Law)!

Hydronic systems are all about movement of water. First in the boiler we transfer energy into the water, circulate that water so we can drop off that energy, and come back to the boiler to do it all over again. In today’s modern systems, we use circulators to move the water. A circulator is NOT a pump it’s a circulator! It moves water by creating high pressure on the outlet and low pressure on the inlet. High pressure goes to low pressure, alas water movement! For more information on how circulators work, I suggest you pick up a few books on the subject written by Dan Holohan, John Siegenthaler, or Robert Bean, or attend one of John Barba’s and David Holdorf’s schools at Taco.

Now I have to admit that I am just a plumber. What I write about is what I have learned by taking things apart and putting them back together. All this talk about “Primary Secondary Piping” on boilers is both confusing and challenging, which really makes me want to play. So when hearing of these “closely spaced tees” that hydronically “unlink” two loops I start to ask “WHAT??” Now it’s really time for me to take things apart and put them back together to figure all this out. In other words, “it’s playtime!!!!”

ACCA Fig1 (1)To simplify primary/secondary piping you have two loops connected, each loop with different flows, neither affecting the other. Look at the piping diagram to the right. You will see a boiler loop (boiler circulator), Zone 1 loop (AH1 circ), Zone2 loop (AH2 circ), and Zone 3 loop (RH1 circ). To prevent flow in one loop being caused by another loop we are told to install these loops to the primary loop using closely spaced tees.

What’s so special about closely spaced tees and why does this stop flow? Think of these tees as having trap doors on the branches! The closer the tees are together the harder the doors are shut. The doors are locked when they are no more than four pipe diameters apart (*) AND when they are more than 18” (**) from a change in direction.

When following these distance rules, the water in the main loop is being circulated by the boiler circulator and gets to the zone tees(*) the water sees no pressure difference between the two tee branches and flows right by them, just like someone walking down a hallway walking past two tightly closed doors.

You will get flow in that secondary loop caused with any distance between the zone tees greater than four pipe diameters. I have to reinforce this because of the confusion caused by a manufacture stating in their literature that this distance should be “less than 12 inches.” The greater the distance past four pipe diameters the more pressure difference between the tees, the more open the door is, the greater the ghost flow.

Now about this “**” 18 inches! When water changes direction in an elbow, or in a tee, it creates a swirl of pressures. Think of it like the chocolate vanilla swirl cone you used to eat as a kid! However, in a hydronic system this swirl is areas of high and low pressure. This swirl will also contain high and low temperatures if this change of direction is at a junction tee. It will take 18 inches for this fluid to calm down and blend back to one. If your tees are within this swirl area, each tee will see different pressures. High pressure goes to low pressure and that my folk’s is flow!

To keep the ghost flows out, you have to keep the doors closed! Next time we discuss why Gravity is not kind!

Steve Wieland
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Posted In: ACCA Now, Hydronics

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