1. Update On Energy Consequences of Filtration
Jeffrey Siegel,
The University of Texas at Austin
IGERT: Indoor Environmental Science and Engineering

2. Goal Update you on filter energy research (ASHRAE RP-1299)
Thoughts for future filter energy research
General filter-related research update

3. Central Question
Does higher pressure drop filter mean more energy in smaller (i.e., residential) systems?
I told you “maybe not” 3.5 years ago
Since then, lots of field work and controlled tests
Answer “likely not”
Provide evidence to support this answer

4. Conventional Wisdom
“A dirty filter will slow down air flow and make the system work harder to keep you warm or cool – wasting energy.”1
“Clogged, dirty filters block normal air flow and reduce a system's efficiency significantly…. Keeping the filter clean can lower your air conditioner's energy consumption by %.”2
1http://
2http://apps1.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cfm

5. Energy Implications of Filtration
Primary effects
Less flow, less fan energy
AC capacity and efficiency degradation
Secondary effects
Changes in duct leakage
Changes in coil/component fouling
Tertiary effects
Changes in sensible/latent balance
Changes in comfort/thermostat setting

6. Physics Part I As flow ↑, electricity required ↑
Precise relationship is complicated
Remember “work”
Force over a distance = work
If distance = 0, work = 0

7. Physics Part II Every fan has curves, every system has curves Fan
Pressure (IWC)
Fan
Efficiency (W/CFM)
Flow (CFM)

8. Physics Part III
Intersection of fan and system curve gives actual flow and pressure
System
Fan
If you add a pressure drop
ΔPfan ↑
Flow↓
Amounts depend on fan and system curve
System
Pressure (IWC)
Flow (CFM)

9. Energy Consequences of Filters Large Commercial Buildings
Fan
Coil
Constant
Flow
Flow
Return Duct
Supply Duct
Atmospheric Pressure
Atmospheric Pressure
Low
MERV
Constant Airflow
Energy Consumption ↑
Pressure
Drop
Fan
Power
High
MERV

10. Energy Consequences of Filters Residential and Light-Commercial Buildings
Fan
Coil
Flow
Return Duct
Supply Duct
Atmospheric Pressure
Atmospheric Pressure
Low
MERV
Flow?
Cooling Capacity?
Pressure
Drop
Fan
Power?
High
MERV
How does overall energy consumption change?

11. Experimental Investigation Study Description
3 Filter Efficiencies
Low, Medium, and High-MERV
Occupied Field Sites
17 residential & light-commercial systems
1 visit per month for a year (~270 total visits)
Influenced by climate and occupant behavior
Unoccupied UTest House
2 independent systems continuously monitored for 4 months
Controlled thermostats
Binned analysis isolates climate and occupant impacts

12. Field Sites

13. Field Data Collection One visit per month for 12 months
3 months low-efficiency (MERV 2)
3 months mid-efficiency (MERV 6-8)
3 months high-efficiency (MERV 11-12)
3 month repetition period (any MERV)
270 total monthly site visits
55 residential visits in cooling mode
60 light commercial visits in cooling mode
Every visit: 15 minutes fan only
Cooling season: 24 hours normal operation

14. Field Measurements Cooling Fan Only Air flow T/RH After coil pressure
Before coil pressure
Fan Only
After filter pressure
Before filter pressure
T/RH
Source:

15. Field Equipment Energy Logger Air flow Accuracy ±5-7%
Pressure transducer
Accuracy ±1%
Voltage tap
Temp//RH
Logger
Accuracy
T = ±0.35°C
RH = ±2.5%
CT Accuracy ±1%
Wattnode Accuracy 0.45% of reading
Current transducer
Accuracy ±1%
Power meter
Accuracy ±0.45%

16. UTest House Measurements
Unoccupied manufactured home at PRC
2 systems continually monitored at 10-second intervals
Controlled thermostats
~50 days high-MERV
~50 days low-MERV

17. 3 Research Questions 1. What is the impact of filter MERV?
Captures real filter loading
2. What is the impact of filter pressure drop?
Ignores filter MERV
3. What is the range of energy consequences?
Moving from low-MERV to high-MERV

18. Residential: Fan Only Filter pressure drop and airflow ratesQ Q
March 5, 2010
Brent Stephens

19. Light-Commercial: Fan Only Filter pressure drop and airflow ratesQ Q Q
March 5, 2010
Brent Stephens

20. Median Changes in Airflow Rates Moving from low-MERV to high-MERV
Residential
Light Commercial

21. Median Change in Fan Power Draw Moving from low-MERV to high-MERV
Residential
Light Commercial
Median ≈ 2% ↓

22. Impact of Filter Pressure Drop Fan Only (n = 218)
Use a filter with 2x the pressure drop and expect a…
Change in Airflow Rate (%)
Change in Fan Power Draw (%)
6-8% decrease in airflow
1-3% decrease in fan power draw
Cooling mode relationships were similar

23. Range of Energy Consequences Moving from low-MERV to high-MERV
Average Change in Daily Energy Consumption
High MERV = More Energy
Δ kWh per ton per day
High MERV = Less Energy
Median Δ = -0.3 kWh/ton/day

24. Field Results: Energy Consequences
Median impact of high efficiency filters
0.3 kWh/ton/day less electricity use w/ high-MERV
Mean decrease of 0.8 kWh/ton/day
Lots of scatter
Filter effects small compared to climate and thermostat settings
Standard deviation = 4.4 kWh/ton/day
Other more important prevalent factors
Refrigerant charge, low airflow, duct leakage, and improper sizing

25. UTest House Results 2 Systems Fan curves Binned analysis
Overall energy consumption
March 5, 2010
Brent Stephens

26. UTest House
Upflow HVAC
Downflow HVAC

27. UTest House

28. UTest House

29. UTest House Fan Curves Test House System #1 Test House System #2
Upflow
Test House System #2
Downflow

30. UTest House Analysis
Control for indoor entering wet bulb and outdoor dry bulb temperatures
32-bin analysis
Outdoor Dry
Bulb, °F (°C)
73-77 (23-25)
77-81 (25-27)
81-84 (27-29)
84-88 (29-31)
88-91 (31-33)
91-95 (33-35)
95-99 (35-37)
(37-39)
Entering Wet
Bulb, °F (°C)
63-64 (17-18)
64-66 (18-19)
66-68 (19-20)
68-70 (20-21)
8 bins
4 bins

31. UTest House Results Binned Analysis
132
High-MERV vs.
Low-MERV
(Average Change)
Filter ΔP ↑ 4x
Airflow ↓ 9%
Fan Power ↑ 3%
Outdoor Unit Power ↓ 0.5%
Total Power ↑ 0.1%
ΔT across coil ↑ 6%
ΔW across coil ↑ 5%
Total Capacity ↓ 4% 67
+153
110 57
Pressure
(Pa)
-21
+192
-82
Filter
Fan
Coil
Supply Duct
Flow
Low-MERV
High-MERV
Avg Flow = 996 CFM
Avg Flow = 909 CFM

32. UTest House Results Daily Energy Consumption Trends
Test House System #1
Test House System #2

33. Overall Findings Weak link between energy and filter pressure drop
Some impacts are positive, some are negative
Impact of filter often lost in the noise
Other prevalent system issues, not filter issues
Constricted returns
Cheap fans
Low refrigerant charge
Duct leakage

34. Conclusions Field Sites UTest House
Lots of scatter
Median energy consequences of filters were small
Less than 1 kWh/ton per day
UTest House
No significant difference in HVAC energy consumption due to MERV 11 filters
Enhanced IAQ by better filtration does not appear to have significant energy penalties in smaller HVAC systems

35. More Information Questions/Comments: jasiegel@mail.utexas.edu
Stephens, B., Novoselac, A., Siegel J.A Energy Implications of Filters in Residential and Light-Commercial Buildings. ASHRAE Transactions, 116(1),
Stephens, B., Novoselac, A., Siegel J.A.  The Effects of Filtration on Pressure Drop and Energy Consumption in Residential HVAC Systems. HVAC&R Research, 16(3),
Stephens, B., Novoselac, A., Siegel J.A Energy Implications of Filters in Residential and Light-Commercial Buildings (RP-1299). American Society or Heating, Refrigeration and Air Conditioning Engineers, Atlanta, GA, 387 pp.

36. What about Commercial Systems?
Bigger buildings do not typically have constant speed fans
Fan increases speed (RPM) to deliver correct flow
Energy consequences are much clearer and can be significant Fisk (2002) Indoor Air, Matela (2006) Facilities Engineering Journal
But…

37. Fan Curves
ASHRAE Handbook of HVAC Systems and Equipment (2004)

38. Impacts in Commercial Buildings
Shapes of fan and system curves are critical
Efficiency curve
Change in intersection can result in higher or lower efficiency
In steeper portions of fan curve
Change in filter pressure will result in a small change in fan speed (but a big change in pressure)
In flatter portions of fan curve
Change in filter pressure may result in a big change in fan speed (but a small change in pressure)
How important is filter to overall system pressure drop?

39. My Request When you have time to kill: Make data available!!!!
In single filter systems
Successively tape up filter and measure pressure drop, flow, and fan power
In multiple filter systems
Successively bag filters and same measurements
Make data available!!!!
I’m doing it for 16 big box retail stores

40. Effect of Filter Pressure Drop
Filter Pressure Drop (Pa)
Flow Rate (CFM)

41. Fan Power
Fan Power (W)
Flow Rate (CFM)

42. Fan Efficiency (combined)
Flow Rate (CFM)
Beko et al. (2008) Indoor Air %; Fisk et al., (2002) Indoor Air – 67.5%

43. Big Picture
Energy consequences of filtration are important, but poorly understood
I hope that
I have raised a reasonable doubt about the connection between higher efficiency filters and increased energy use in smaller systems
I have convinced you that we need more data on larger systems

45. Other Relevant Research
In-situ filter efficiency tests
Retail store ventilation and indoor air quality
Test method for ozone emission from in-duct air cleaners
Research idea: Spread of mold and radiation in Northern Japan