The Fix
Here’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.
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