The Glitch
Overview: A copper tube boiler is
installed as shown with six zone circuits
controlled by zone valves. The installer got a great price on a “contractor
pack” of 3/4-inch copper tees, so the supply and return manifolds are built
with 3/4-inch copper, as are all the zone circuits. The circulator is installed
on the return side of the boiler.
Exercise:There
are at least four errors in this design. Can you spot them?
The Fix
Copper
tube boilers are “flow sensitive.” They should not be operated with low flow
rates, such as might occur when only one zone circuit is operating. The
correction is to install the boiler as a secondary circuit with its own
circulator that maintains constant flow through the boiler regardless of flow
in the distribution system.
Other corrections include:
1. Moving
the zone valves to the supply side of the zone circuits to prevent heat
migration.
2. Ensuring there is at lest twelve pipe diameters (12D) of
straight pipe on the inlet of all circulators to reduce turbulence.
3. Installing
a differential pressure valve to limit differential pressure gain across the
headers under partial load conditions.
4. Sizing the headers for a flow
velocity of 2 feet/second when all zone circuits are on. This keeps the pressure
drop along the header very low and helps prevent changes in zone flow rates as
other zones turn on and off.
