Watts tekmar 294 - System Operation - PID and Proportional Details; PID Stage Delay; Proportional Staging Parameters; Boiler Group Sequencing

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IOM-T-294 2205 © 2022 tekmar
Interstage Differential
Select the temperature drop from the prior boiler turning on
to turn on the next boiler stage. The first boiler turns on when
the boiler supply temperature drops the Interstage Differential
below the boiler target. The next boiler must drop by twice the
Interstage Differential to turn on the next boiler. The same
pattern is repeated for all available boilers.
Interstage On Delay
When a boiler stage is turned on, the Interstage On Delay time
must elapse before the next boiler is able to turn on.
Interstage Off Delay
When a boiler stage is shut off, the Interstage Off Delay time
must elapse before the next boiler is available to turn on again.
Minimum Stage Run Time
When a boiler stage turns on, it must remain on for the
duration of the minimum stage run time. This prevent boiler
short cycling.
Minimum Stage Off Time
When a boiler stage turns off, it must remain off for the
duration of the minimum stage off time. This prevents boiler
short cycling.
Boiler Group Sequencing
The boilers are divided into two groups:
• Condensing Boiler Group
• Non-condensing Boiler Group
Each boiler group allows the selection of sequential or parallel
sequencing. Boilers in the condensing group are always the
first to fire followed by boilers in the non-condensing group.
Sequential Sequencing
With Sequential sequencing, the control will turn on the fewest
boilers possible to meet the load. The control will bring a boiler
up to 100% firing rate before bringing on an additional boiler at
it’s minimum firing rate. If operating multi-stage boilers, sequen-
tial staging alway fires stages in the order of 1, 2, 3, and 4.
100% 60% Off Off
Parallel Sequencing
With Parallel sequencing, the control will turn on the most
boilers at the minimum firing rate. The control turns on the
first boiler at the minimum firing rate and does not increase its
modulation. If more boiler output is required, additional boilers
will turn on at the minimum firing rate until all available boilers
are on. If still more boiler output is required, all boilers are mod-
ulated in parallel until they reach 100% firing rate. If operating
multi-stage boilers,
the control stages all of the low stage out-
puts in all of the boilers first. Once all of the boilers are operating
on their low stages, the control then operates the second stage
in each boiler in the same order. The same process is repeated
for the third and fourth stages respectively.
40% 40% 40% 40%
Condensing and Non-Condensing Boiler Groups
Operating a boiler plant that contains both condensing (high
initial cost) and non-condensing (lower initial cost) boilers allows
the boiler plant to achieve nearly the same operating efficien-
cies as operating all condensing boilers but at a much lower
capital cost to the building owner. High system efficiency can
be achieved as long as the condensing boilers are the first to
operate in the firing sequence. During mild weather, the lead
condensing boilers operate at lower boiler temperatures and
achieve their peak boiler efficiencies while the non-condensing
boilers are rarely operated. During very cold weather, the boiler
target temperature is often above the boiler’s condensation
point and the condensing and non-condensing boilers operate
together at roughly the same efficiency level.
When a condensing boiler is operating, it is desirable to operate
the boilers without a boiler minimum temperature being applied
to the boiler target. This allows the condensing boiler to operate
at its maximum efficiency. When a non-condensing boiler is
operating, a boiler minimum temperature should be applied
to the boiler target to prevent damage to the non-condensing
boiler’s heat exchanger from sustained flue gas condensation.
The 294 supports operation of a condensing and a non-con-
densing boiler group. A condensing boiler group is created
when at least one boiler is selected to be condensing and
a non-condensing boiler group is created when at least one
boiler is selected to be non-condensing. The condensing
boiler group is always sequenced on first, followed by the
non-condensing boiler group.