AR No. ___ - Install Boiler Stack Damper
Estimated Gas Energy Savings = **.* MMBtu/yr
Estimated Gas Cost Savings = $***/yr
Implementation Cost = $***
Simple Payback = *.* years
Recommended Action
A damper can be installed on the stack of the boiler in the _________ area to reduce convective
heat losses from the boiler chamber during periods when the boiler is not firing. This would
reduce energy usage by the boiler, particularly during periods of low load (intermittent firing).
Background
The boiler considered is a ______________ Btu/h input, natural draft type unit. It has a vertical
stack that is about ___ feet tall (total height from body of boiler to outlet). Based on
measurements taken during the audit visit, the temperature of the air inside the boiler chamber
during periods when it is not firing (periods considered for damper use) is approximately _____
F. Flow of this warm air up the stack is due to thermal buoyancy forces (acting on the hot air
passing over the boiler tubes), and due to wind forces. The phenomenon driving the buoyancy
forces is known as stack effect and that due to wind forces will be referred to as wind effect.
Anticipated Savings
Annual energy savings due to reducing convection losses are directly proportional to the average
volumetric flow rate of air escaping through the boiler stack during periods when the boiler is on
line but not firing. The total average flow rate from the stack opening, Q, is based on the flow
rates due to wind and stack effects, as follows1:
Q  Qs  Qw
2
2
with
Qs  C1  A 
Tb  To
h
2 g 
To
K
Qw  A  Vw 
2
where
Qs
Qw
C1
Tb
1
=
=
=
=
C w Tb

K To
flow due to stack effect, cfm
flow due to wind effects, cfm
unit conversion factor, 60 s/min
temperature of air within the boiler during periods when the boiler is not firing (at
1989 ASHRAE Handbook of Fundamentals, p. 2.10, eq. 32; p. 14.4, eq. 7 and 8.
To
A
g
h
K
Cw
=
=
=
=
=
=
Vw =
stack entrance level), measured during the site visit, R
average outdoor air temperature when the boiler is not firing2, R
area of stack opening, ft2
gravitational constant, 32.17 ft/s2
stack height, ft
loss coefficient for stack opening and fittings3, no units
effectiveness of stack with reference to wind forces, no units
This represents the estimated effect of the angle at which the wind flows across the
stack opening. For diagonal cross winds (typically the case across the face of the
stack) the suggested value is Cw = 0.25 to 0.35. To be conservative, Cw = 0.25.
average wind speed during periods when the boiler is not firing, 800 ft/min
The flow rates are estimated as follows:
Qs  (60)(**) 
* * * * * *
**
 (2)(32.17) 
 * * * cfm
***
**
and
Qw  (**)  (**) 2 
** **

 * * * cfm
** **
The total average flowrate of air from the boiler stack is thus estimated as follows:
Q  ( * * *) 2  ( * * *) 2  * * * cfm
The heated air lost up the boiler stack must be made up by cold air from outside the building
envelope, and thus savings are based on the difference between the air temperature at the boiler
stack and the average outdoor air temperature for the period considered. The energy savings, ES,
and annual cost savings, ECS, due to installing a damper on the boiler can now be estimated as
follows:
ES =
Q    C p  C 2  H  FR  ( Tb  To )
C 3  EFF
ECS  ES  avoided cos t of natural gas
where
ρ =
Cp =
density of escaping air (at measured temperature), lb/ft3
specific heat of air, Btu/lb R
2
From Typical Meteorological Year (TMY) weather data.
3
1989 ASHRAE Handbook of Fundamentals, p 2.10, Table 3; and Chapter 32.
C2 =
H =
FR =
C3 =
EFF =
conversion factor, 60 min/h
annual hours that boiler is hot (on-line) but not firing, h/yr (i.e., hours during which
damper could be used)
fraction of air flow that can be reduced by damper, typically 90%
conversion constant, 1,000,000 Btu/MMBtu
steady state boiler efficiency, no units
The hours can be estimated as the difference between the annual hours during which the boiler is on
line and the annual hours of boiler firing. Boiler firing time is estimated as the ratio of the gas usage
rate and boiler rated input capacity. The boiler is on line for about ___ h/day, ___ days/wk, ___
wk/yr, for a total of about _____ h/yr, and the boiler gas usage is estimated as ______ MMBtu/yr.
As discussed above, the boiler input rating is ____ MMBtu/h. Therefore,
H  * * * h/y 
* * * MMBtu/yr
 * * * h/yr
* * * MMBtu/h
Thus, the annual energy and cost savings are estimated as follows:
ES 
(* * *)(* * *)(0.24)(6 0)(* * *)(0.9)( * * * - * * * )
 * * *.* MMBtu/yr
( 1,000 ,000 )( * * * )
ECS  (* * *.* MMBtu/yr) ($ * * * /MMBtu)  $ * * * /yr
Implementation Cost
A satisfactory damper could be purchased and installed for about $____. This price includes the
purchase and installation of an automatic industrial grade damper assembly. The suggested
assembly would sense when the boiler is firing, and then open and close the damper as appropriate.
The cost savings of $____/yr would pay for the estimated implementation cost of $____ within
about __ months.
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