ASHRAE Journal - November 2013 - 10

letters

Church Project

The Author Responds

October's column, "Lessons From
Church Project" by Leslie Beu, recommends the use of hot water supply temperature reset for this project. The author
contends in Figure 6 that outdoor reset
of hot water supply temperature should
be used and expanded in this church or
school. My experience with over 1,000 religious buildings is that outdoor reset does
not work well and does not save energy,
so long as the buildings are intermittently
occupied. The author does not provide any
technical, analytic, or metered energy data
to support her claims.
The best example is morning warmup
on a mild day after setback during cold
weather. When the mass of the building is
cold (say 55°F), the outdoor temperature
is 60°F or 70°F, and the reset schedule
says the hot water temperature is very low
(say 80°F to 100°F as proposed in Figure
6), the building will never reach comfort
temperature in any reasonable time. To
reach comfort temperature after setback
in intermittently occupied buildings in a
reasonable time requires high hot water
supply temperatures.
There is no evidence that any savings from
hot water reset compensates for the inability to reach comfort after setback. The result
too often is that when comfort temperatures
cannot be achieved in a reasonable time, the
operator is tempted to disable setback and
maintain comfort temperatures around the
clock or change the reset schedule, thereby
wasting energy efficiently.
It is also possible that rooms nearer the
boiler will likely become warmer than
those farther away, leading to uneven
interior temperatures. A form of proportional temperature control can be accomplished by staging boilers or burner firing
rates according to return water temperature, rather than outside temperature.
This ensures full boiler horsepower at the
beginning of the heating cycle.
Outdoor temperature reset may work
well in continuously heated buildings, but
not in ones that are intermittently heated,
especially with days of setback.

Thank you for sharing your opinion and
providing an avenue for us to continue
this important discussion. I'm going to try
to address your concerns, while further
explaining how this particular church
benefited from the condensing boiler
with hot water reset. This building shows
marked improvement in comfort with
the lower hot water supply temperatures,
while allowing the boiler to operate at
higher efficiency in the condensing range.
First, I agree that hot water temperature reset cannot be applied universally
in every situation to achieve energy savings and maintain comfort, as there are
many factors to consider. You indicated
that based on your extensive experience
with church facilities, outdoor reset does
not work and does not save energy in an
intermittently occupied building. Without
direct knowledge of the particulars, you
assigned occupancy and setpoint characteristics in your example that differ from
this specific church.
The other important factors to be considered that contributed to this project's success were the climate, building envelope/
construction, system type, system functionality, as well as addressing previous comfort
issues. This large congregation finds many
uses for the building, which is occupied
daily each week and does not have days
of setback. High solar loads are prevalent
year-round in this semi-arid high plains
desert region next to the Rocky Mountains.
The block masonry construction contributes to large building envelope mass and
solar energy storage, which helps it hold its
temperature overnight.
Still, this building is not allowed to reset
as low as 55°F as your example indicated.
The building is able to reach comfort in a
reasonable period after its more moderate
setback temperature of 62°F, while taking advantage of boiler reset during more
mild outdoor temperatures.
Overcoming comfort issues was a critical project goal, which was achieved while
limiting excessive heating and overshooting to save energy. Unfortunately, project
cost restrictions resulted in existing leaky
valves and worn-out damper actuators

Larry Spielvogel, P.E., Fellow ASHRAE, Bala
Cynwyd, Pa.
10

ASHRAE JouRnAl

ashrae.org

N ovem ber 2013

that still remain. Supplying cooler hot
water temperatures is advantageous and
better supports the remaining marginally
functioning system components.
Boiler supply temperature reset and
unoccupied space setback schedules, setpoints, limits, and details often require
ongoing optimization and fine-tuning
based on unique building characteristics
and operational challenges. If a building
cannot take advantage of lower return
water temperatures without negatively
affecting comfort or building operation,
a condensing boiler should probably not
be selected. My column emphasized that
installing a condensing boiler itself does
not achieve the energy savings, but operating it with effective setpoints is required
to see the actual savings.
Your concerns contribute to my point
that critical thinking is required to correctly
apply (or not apply) certain strategies to
specific systems and conditions. An engineer shouldn't provide a one-size-fits-all
energy solution, nor negate a solution
purely based on assumptions or experiences that don't account for all of the facility
and operational specifics.
Leslie Beu, Denver

Big Plant, Small Space
In July's "Big Plant in a Small Space" by
Blake Ellis, in Figure 4 the chilled water
expansion tanks (and makeup) are shown
connected to the upper and lower return
mains. I am wondering if there should
also have been a makeup and expansion
tank connection to the "headered" loop,
which facilitates the use of any pump to
serve any chiller.
Asif Kadiani, P.E., Member ASHRAE, Springfield, Ill.

The Authors Respond
We don't believe the makeup water/
expansion tank should be connected to
the cross-tie loop (the loop facilitating
the use of any pump to serve any chiller).
The cross-tie loop is connected on the
discharge side of the chilled water supply
pumps. The makeup water/expansion
tank is connected to the CHWR headers,
which are on the suction side of the chilled
water supply pumps. The suction side of

ASHRAE Journal - November 2013

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