Preferred FlexFit 110 User Manual

1-4 Description – Features
FlexFit – Linkageless Control – Revision 1.0
PREFERRED
UTILITIES MFG CORPORATION
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E.g. due to fuel valve and air damper non-linearity, a 10-degree air damper change might correlate to a 5-degree fuel valve
change. In this example, the fuel servo must run at one-half the speed of the air servo to remain "on the curve" during a load change.
Additional information on the pacing logic is provided in section "Servo Description and Operation" on page 2-21.
Tandem Oil/Gas Valve Servo
This option allows a single servo to drive both the oil (fuel 1) and gas (fuel 2) control valves. The FlexFit maintains separate
curves for Standby, Purge, and Ignition positions and separate fuel/air ratio curves for fuel 1 and fuel 2. This allows the user to leverage
existing linkage or direct mount to a dual gas/oil valve.
Oxygen Trim
Oxygen measurement is used to continuously adjust (trim) the fuel/air ratio. Oxygen trim saves fuel by ne-tuning the burner to
operate safely and reliably at reduced excess air levels throughout the burner ring range. It also allows the controller to compensate
for external environmental changes that affect burner operation (i.e. ambient temperature, fuel heating value, viscosity, etc.). Low ue
gas oxygen will cause an alarm and can be elected to shut down the burner after an adjustable time delay.
Firing Rate Setpoint and Control
The FlexFit can operate in automatic mode where the ring rate is determined by an operator-selected setpoint; the FlexFit
will automatically use PID control to maintain that setpoint. The ring rate PID compares the pressure or temperature setpoint to the
measured pressure or temperature and adjusts the ring rate demand accordingly. The ring rate PID is active if the ring rate mode
is in local/automatic, remote 4-20mA SP, or remote Modbus SP.
The FlexFit can also operate in the manual mode where ring rate is determined by an operator-selected value, a local 135-ohm or
P7810 demand, a remote 4-20mA FR, or a remote Modbus FR. The remote ring rate modulation method is determined by parameter
"P3.4.6 Remote Modulation" on page 4-74.
Call for Heat
The Call For Heat (CFH) start and stop command is determined either by local setpoint deviation or a remote input (e.g. a lead-lag
controller). When setpoint deviation is selected, the FlexFit will generate a CFH when the boiler outlet is a user-dened temperature
(or pressure) below the setpoint. Alternatively, the CFH is removed when the boiler outlet is a user-dened value above the setpoint.
For more information on CFH, see section "Call for Heat (Option)" on page 4-72.
Domestic Hot Water Override
The Domestic Hot Water (DHW) override feature forces the exit water temperature setpoint to be greater than or equal to the
DHW setpoint. Some boilers are used primarily for building space heating, but they also provide heat for DHW tanks. When outdoor
weather is warm, there may be no space heating load and the boilers will shutdown. DHW override ensures hot water will still be
available by providing a call for heat when the DHW setpoint is greater than the current ring rate setpoint.
Warm Standby Option
When this option is selected, the warm standby logic starts and stops the boilers using a boiler shell temperature (or pressure)
switch.
Some boiler installations, such as those for manufacturing plants, research facilities, or medical facilities cannot tolerate a
sustained low header pressure (or temperature). Larger boilers require a long, slow warm-up cycle. Therefore, some facilities require
that one or more boilers be kept on warm standby.
Warm standby boilers are periodically red at low re until their boiler outlet is almost as high as the operating pressure or
temperature; then the boilers are shut down. When the outlet drops to a lower threshold, the cycle repeats. If a boiler is kept warm,
it can rapidly be brought up to modulate as needed.
Cold Start WarmUp Cycle Option
Because of the stress created by thermal expansion, some boilers require a slow warmup if the boiler is cold. Parameter "P3.6.1
Cold Start Warmup Option" on page 4-76 steps the ring rate up in stages in response to the boiler outlet temperature (or pressure).
An override timer can cause the ring rate to increase to the next ring rate step, even if the boiler outlet has not yet reached the
step setpoint. Consult the boiler manufacturer for the proper ring rate and setpoint step sizes and the proper override time for your
specic boiler.
An in-progress warmup cycle can be canceled by placing the ring rate in manual. This cycle occurs only once per boiler start-up.
The cold start warmup cycle ends automatically and the PID logic begins modulating the burner if the boiler outlet condition
exceeds a eld selectable value or becomes equal to the ring rate setpoint.
A low re hold option is also available. To minimize thermal stresses, some boilers require that the burner hold at low re (0%)
after lighting off. The FlexFit will release the burner to modulate when the boiler shell temperature (or pressure) increases to setpoint

Preferred FlexFit 110 Specifications

General

Brand	Preferred
Model	FlexFit 110
Category	Controller
Language	English