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Configuring an HVAC Water System

268 The HVAC World

Chilled water systems should be equipped with manual air vents at
the high points of the system. These air vents should be equipped
with air-collection chambers below them. The chemical feeder should
be installed at the central energy plant for complete treatment of the
entire water system. Larger systems might be equipped with auto-
matic chemical feeders. Chemical feeders offer a better and in many
cases less expensive method of removing air from these systems than
depending on a mechanical separator, which cannot eliminate all the
oxygen from a water system. Reiterating, mechanical separators can-
not remove dissolved air!
Hot water systems can use mechanical separators to remove some
of the air, as is shown in Figs. 3.6a and 3.7a. Seldom should they be
located on the discharge from boilers. Little additional air removal is
achieved on most installations, and the location on the discharge pipe
from boilers may be cumbersome. The use of dip tubes on boilers,
manual air vents, and a chemical feeder may produce a more econom-
ical and better method of removing air from hot water systems than
large mechanical separators.

9.9 Control of Return Water Temperature
As we began this chapter with system temperature difference, so we
will leave it. Return water temperature is one of the most important
operating values for a chilled or hot water system. It tells the operator
just how good a job the control system and coils are doing in convert-
ing energy from the chillers or boilers to the air or water systems
that are cooling or heating the building. This is such a basic criterion
that it should be addressed early in the design of a chilled or hot
water system. Individual coils should be equipped with thermometers
or insertion plugs on their return connections, as described in Fig. 9.12a;
zone or building returns should be equipped with temperature transmit-
ters, as also shown in this figure. This information should be displayed
and recorded at the central data-acquisition point for the entire system.
There is a great argument ongoing in the HVAC industry about
how to control return water temperature in loop-type systems such
as chilled and hot water systems. The proper method of controlling
return temperature is through the correct selection of control
valves and heating and cooling coils. The use of return temperature-
control valves, as shown in Fig. 9.12b, is a quick way to solve many
system operating problems, but it is fraught with its own problems.
For example,

1. The return temperature-control valve adds head loss to the entire
system and is therefore an energy waster.

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