Give Me Some Flow
May 14, 2010
The GlitchOverview:An installer sets up a zoned distribution system as shown below. To keep things simple, he uses the same size piping for the headers and all the distribution circuits. The heat source is a boiler with high flow resistance. The distribution system operates at water temperatures where sustained flue gas condensation will not be an issue.
To prevent heat migration, he installs a spring-loaded check valve at the beginning of each zone circuit, just above the header. To control differential pressure, he installs a partially open globe valve across the ends of the headers. He sets the globe valve for one-turn open.
Exercise: Can you identify and describe at least six details that should be changed?
The FixHere’s what’s wrong and how to correct it:
1. The high-flow resistance boiler will create significant variation in zone flow rates depending on which zones are open. If the boiler has low thermal mass, there could be an additional problem with flow starvation under low-load conditions.
2. The partially closed ball valve is a very poor excuse for proper control of differential pressure across the headers. The variable-speed, pressure-regulated circulator is now the state-of-the-art method for providing such control in systems using valve-based zoning.
3. Headers should always be generously sized to minimize pressure drop along the length of the header. This plays a big part in providing hydraulic separation between the load circuits.
4. Any “flow-sensitive” boiler supplying a system using zone valves should be decoupled from the distribution system through some means of hydraulic separation. In the fix drawing, this is accomplished with a pair of closely spaced tees. It could also be accomplished using a hydraulic separator or a buffer tank between the boiler and the distribution system.
5. It’s never a good idea to install check valves near fittings that creates high turbulence. This is the case with the check valves shown in the glitch drawing. There should always be at least 12 diameters of straight piping upstream of any check valve to calm turbulence that otherwise will cause the check valve to rattle.
However, heat migration in this system is eliminated by simply installing the zone valves on the supply side of the zone circuits. Do this and there is no need of installing the check valves.
6. Purging valves on each zone circuit are missing in the glitch drawing. They are shown in the fix.
7. The circulator location in the glitch drawing, in combination with the likely pressure losses of the piping system, puts the circulator in a “pumping toward” rather than “pumping away from” situation with respect to the expansion tank.