ASHRAE Journal - March 2014 - 41

Technical FeaTure
Fea

fan horsepower as independent variables (Equation 1).
Based on the regression statistics (adjusted R-Squared of
0.922), it appears fan runtime and fan horse power values were good predictors of the potential energy savings
from the advanced RTU controller retrofit. Note that for
this analysis, fan horsepower was actually measured and
the nameplate reading was not used. Using the nameplate reading will introduce some uncertainty but could
still be valid.

cost savings were $744, $1,489 and $2,233 for the three
considered utility rates, with a corresponding average
installed cost of $4,172, resulting in average payback
period of six, three, and two years, respectively. Wang,
et al.,8 provides additional details on the payback period
calculation accounting for the metering and monitoring
packages.
For individual units, the simple payback period
varies from nine months to 10 years at a utility rate
of $0.15/kWh. This variation in payback period largely
depends on RTU size and RTU runtime. The units with
Annual Electricity Savings (kWh) = 0.382 ×
Fan Runtime + 3,688 × Fan Horsepower
(1) the shortest payback period were either large units (e.g.,
greater than 15 tons [53 kW]) or had the longest annual
Payback Analysis
runtime (operating hours per year).
The simple payback analysis
Figures 5 and 6, on the other
table 1 Advanced RTU controller costs.
helps building owners understand
hand, show the average payback
rtu caPacity
suPPly-Fan
controller
controller
the financial impact of RTU con(tons)
size (HP)
($)
labor ($) period for varying utility rates and
troller retrofits and justify their
≤5
1
2,200
750 three different utility incentive
investment. Table 1 provides the
>5 and ≤10
2
2,600
750 scenarios. These units had varycosts associated with the advanced
ing capacities (between 5 and 25
>10 and ≤15
3
3,500
750
controller retrofit for a single
tons [18 and 88 kW]) and varying
>15 and ≤20
5
4,000
750
RTU at varying capacities and
annual runtimes (between 4,004
> 20 and ≤25
7.5
4,142
750
corresponding supply fan sizes.
and 8,736 hours). As you can see,
The controller cost varied because the size of the VFD
for units smaller than 10 tons (35 kW), the average paydepended on the size of the supply fan motor, which was
back period was three years or less for utility rates above
included in the controller cost. Labor rate was assumed to $0.12/kWh when incentives were offered (three years at
be $125/hour. This may vary based on market conditions.
25% incentives and two years at 50% incentives, respecBased on the costs outlined in Table 1, a simple payback tively), and $0.16/kWh without incentives.
period was calculated for the advanced controller, based
For units larger than 15 tons (53 kW) (Figure 6), the
on the projected normalized annual energy savings.
average payback period was less than three years, with
Three utility rates were used: $0.05/kWh, $0.10/kWh
or without incentives. Although not shown in either
and $0.15/kWh. Across all units, the annual average
figure, for units between 10 tons (35 kW) and 15 tons

10
9
8
7
6
5
4
3
2
1
0

Figure 6 Average payback period for units with capacities greater than 15 tons
(53 kW), for varying runtimes and utility rates.

3

≤10 Tons (No Incentives)
≤10 Tons (25% Incentives)
≤10 Tons (50% Incentives)
Payback (year)

Payback (year)

Figure 5 Average payback period for units with capacities less than 10 tons (35
kW), for varying runtimes and utility rates.

0.05

0.07

0.09

0.11

0.13

0.15

0.17

0.19

>15 and ≤25 Tons (No Incentives)
>15 and ≤25 Tons (25% Incentives)
>15 and ≤25 Tons (50% Incentives)

2

1

0
0.05

0.07

0.09

utility rate ($/kWH)

0.11

0.13

0.15

0.17

0.19

utility rate ($/kWH)

March 2014

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ASHRAE Journal - March 2014

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