30 November 1998
RETROFIT OF EXISTING HVAC CONTROL SYSTEMS
1. INTRODUCTION. When determining whether to retain or replace existing control systems (in whole
or in part) in the retrofit of existing HVAC systems, the designer must evaluate the applicability of the
design guidance provided in this manual. The reason for this evaluation is that deviation from this
guidance may be necessary in certain circumstances to prevent adverse impacts on the operation and
performance of the retrofitted HVAC systems. Examples of control system situations that require such
evaluation are as follows:
a. Reuse of existing valves, where their sizes may affect the existing or new hydronic systems
and their pump sizing.
b. Reuse of existing dampers, where their sizes may affect the existing or new air handling
systems and their fan sizing.
c. Replacing three-way valves with two-way valves and vice versa and its effect on hydronic
systems and their pump sizing.
d. Partial retrofit, where only the final elements such as dampers, valves, and operators may be
left in place.
e. Retrofits involving economizer control loops.
f. HVAC systems that may not match the systems shown in Chapter 4 or their variations in
2. VALVE SIZING AND ITS EFFECT ON HYDRONIC SYSTEMS. Quite likely the guidance provided in
this manual for the sizing of control valves differs from the design criteria on which the existing control
valves (in a retrofit project) were selected. Consequently, the designer must compare the pressure drop
across the existing control valve with the pressure drop for a control valve based on the sizing
requirements of this manual. If the existing valve sizes do not meet the pressure drop requirement for
sizing valves in accordance with this manual, it may be that the sizing of the existing valves was based
on pressure drop through the valves lower than required by the manual. The designer must then
determine if the existing pumping system can provide adequate flow throughout the system with new
valves (sized in accordance with this manual) in place. If not, the existing pumping system will have to
be upgraded or replaced if the valve sizing pressure drop guidance of this manual is applied.
3. DAMPER SIZING AND ITS EFFECT ON AIR HANDLING SYSTEMS. Evaluation of control dampers
in a retrofit project is similar to control valve evaluation, because changing the size of an existing
damper would change the damper's pressure drop and in turn affect fan air volume delivery. Also, the
damper actuators might have to be retrofitted if the evaluation shows a change in the damper's pressure
4. REPLACEMENT OF 3-WAY AND 2-WAY VALVES. Whenever there is a change in the type of
control valve in a retrofit project (either from a 2-way valve to a 3-way valve or vice versa), the designer
must make additional pressure and flow evaluations. If the change is from 3-way to 2-way, the pressure
could increase significantly with a significant pumping system flow decrease as the valve closes.
Conversely, the change from 2-way to 3-way could cause significant pressure decrease and flow