ASHRAE Journal - March 2022 - 40

TECHNICAL FEATURE
FIGURE 2 Relative efficiency versus load at Site 3.
0.014
0.012
0.01
0.008
0.006
0.004
0.002
5 - 10
10 - 15
15 - 20
Portion of Capacity (%)
FIGURE 4 Relative efficiency versus load at Site 1.
In all cases, the relative efficiency is somewhat insensitive
to the load on the system. To convert relative
efficiency to true efficiency, the actual heat losses from
all the spaces would have to be known within a few
percentages.
Notice that in Figures 2, 3 and 4, it appears that the
efficiency might be increasing under conditions of low
load. It is unlikely that efficiency is increasing then
because of the following compounding factors. First, the
assumption of the base HDD can affect plots of relative
efficiency. The assumed base HDD has a greater relative
effect on lower loads. However, the base HDD used was
well supported by theory and practice. Second, under
low loads, solar radiation has a greater relative effect.
Detroit tends to have sunnier skies when it is warmer.
Therefore, the fuel use could be influenced by that relationship.
It was considered whether to throw out data
that looked high, which is the data with high relative
efficiency values at low demand. That would only impact
two points in Figure 2, have minimal effect on Figure 3,
but in Figure 4 it could remove all data lower than 12.5%.
Overall, the efficiency results would still look insensitive
to demand. The remaining data would be flat over a
wide range.
Another possibility is that the modulation of the
boilers is having a dramatic effect on the results. That
could partly explain the two larger systems, which are
modulated, but the data shows that the smaller system's
efficiency is insensitive to load, too. Modulation will be
discussed further in the modeling section.
Figures 2, 3, and 4 all show the precise range that is
important to look at for seasonal efficiency difference.
If there was a significant difference in seasonal boiler
efficiency, then Figures 2, 3, and 4 should all decrease at
40
ASHRAE JOURNAL ashrae.o rg M A R CH 2022
1.4
1.2
1.0
0.8
0.6
0.4
0.2
5 - 7.5
10 - 12.5
7.5 - 10
12.5 - 15
15 - 17.5
17.5 - 20
20 - 22.5
22.5 - 25
FIGURE 3 Relative efficiency versus load at Site 2.
25 - 30
20 - 25
0.14
0.12
0.1
0.08
0.06
0.04
0.02
5 - 10
10 - 15
15 - 20
20 - 25
Portion of Capacity (%)
Portion of Capacity (%)
low loads. Conventional wisdom is that the reduction in
efficiency should be roughly 20% to 50%.1 That variation
should start somewhere around 30% of boiler capacity
and drop down to 5% of capacity. That is not supported
by the data presented for these sites.
Modeling
A model was created to explore theories related to the
setpoint effect (Figure 5). The variables were the inside
temperature, heat loss, modulation ratio and system status.
The parameters were outside temperature, indoor
setpoint and allowable deviation, basis temperature
differential for boiler design, and building loss rate per
unit time per degree of temperature difference.
The forms of heat loss modeled were conduction and
convection. Radiant heat loss is nonlinear, but it is low
for typical temperature ranges, so it was neglected.
Most conductive and convective heat losses are linearly
related to the temperature difference. Convective
heat loss due to air exchange with outside air can vary
with wind speed, but on average wind speeds don't
vary much. Therefore, heat loss was calculated as the
assumed loss ratio times the difference between inside
and outside temperatures.
Relative Efficiency
Relative Efficiency
Relative Efficiency
http://ashrae.org
ASHRAE Journal - March 2022

Table of Contents for the Digital Edition of ASHRAE Journal - March 2022
