Measured Performance of Residential Gas Water Heaters Dan Cautley ACEEE Hot Water Forum May 13, 2010
Today s topics Intro Efficiency & operating costs Flow in venting system, added infiltration Vent spillage Loads & delivery temperature Misc practical issues
Introduction Funding DOE State Technologies Advancement Collaborative WI Dept of Administration, Div of Energy Services (public benefits) Material contribution from AO Smith corp Other participants include LBNL, BNL, and ACEEE Objectives of ECW field monitoring Measure conditioned-air loss Determine efficiencies (old & new) Investigate spillage
Existing Water Heaters 10 homes in S. Wisconsin Natural gas water heaters (WH) WH vintage: 1977 to 2003 (8) 40 gallon, (2) 50 gallon Variety of household sizes & usage
Condition of 10 existing water heaters Mineral buildup: 2 severe (inches thick) Dip tubes 2 nonexistent 1 broken off at top (in transit??) 2 had longitudinal cracking 5 intact Anode rods: 8 were nonexistent
New water heaters (1) FVIR atmospheric tank type (short-term test) (4) power vent 40 gallon (1) 50 gallon condensing efficiency storage type (4) non-condensing tankless (1) condensing efficiency tankless
Primary measurements Hot water flow Cold & hot water temperature Burner run time (& gas flow in some) Vent pressure, temperature Zone pressure, outdoor temperature CO2 (& CO) in vent, flue, & room Oxygen (part-time)
Efficiency Combustion (steady state, recovery) η Energy to water / Fuel energy Instantaneous Input-output (EF) Hot water delivered / Fuel energy Over time (e.g. daily) Other (draw different boundaries)
Combustion Efficiency 0.84 Site A (non-fvir) Combustion effiency 0.82 0.80 0.78 Measurement period for steady-state efficiency 95th percentile 75th percentile 0.76 median 25th percentile 5th percentile 0 5 10 15 Elapsed minutes of firing Data are for 105 firing episodes, and are grouped in 5-second bins of elapsed firing time. Bins with <25 data points are omitted.
Input-output efficiency 1.5 Site D (non FVIR) Daily gas input (therms) 1.0 0.5 0.0 0.0 0.2 0.4 0.6 0.8 1.0 Daily hot water output (therms)
A 45.7% 76.3% B C 42.9% 46.3% 81.5% 80.4% D E 35.1% 37.6% 77.1% 80.0% E (FVIR) 24.2% 75.7% F (FVIR) 59.3% 81.0% G 14.2% 80.3% H 51.5% 81.2% I 33.7% 79.0% J -4.8% 73.4% -10% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% Median steady state on cycle thermal efficiency Median off cycle thermal efficiency
Instantaneous efficiencies (old WH) 0.90 0.85 Measured comb eff Measured in-out slope GAMA Rec Eff 0.80 Recovery efficiency 0.75 0.70 0.65 0.60 0.55 0.50 A natural draft B natural draft C natural draft * D natural draft E natural draft G natural draft H natural draft I natural draft J natural draft E FVIR F FVIR *
1.00 0.95 0.90 0.85 0.80 0.75 0.70 0.65 0.60 Instantaneous efficiencies (new WH) Measured comb eff Measured in-out slope GAMA Rec Eff C power vent D power vent G power vent J power vent F power vent, condensing A tankless E tankless H tankless I tankless B tankless, condensing Recovery Efficiency
Energy factor: data vs GAMA 0.80 0.75 0.70 EF from data GAMA EF Energy Factor 0.65 0.60 0.55 0.50 0.45 0.40 B natural draft C natural draft D natural draft E natural draft G natural draft H natural draft I natural draft E FVIR C power vent D power vent G power vent J power vent
Estimated stack & jacket losses 25,000 20,000 Est jacket loss Off-cycle stack loss (23 hrs) Losses (Btu/day) 15,000 10,000 5,000 - A natural draft B natural draft C natural draft D natural draft E natural draft G natural draft H natural draft I natural draft J natural draft E FVIR F FVIR
Operating costs: 65 F temp rise, $1/therm,.13/KWH $350 Tank (1977-2009) $300 Power-vent (new) Power-vent, condensing (new) 298 274 $250 $200 Tankless, non-condensing (new) Tankless, condensing (new) 176 237 214 198 181 249 239 209 $150 $100 116 95 96 154 147 123 109 159 $50 65 59 $0 25 50 75 100 Gallons per day
Estimated operating costs by fuel (75 gal/day) $300 $250 Electric Gas $200 $150 $100 $50 $0 Tank (1977-2009) Power-vent (new) Vertex (new) Tankless, noncondensing (new) Tankless, condensing (new)
Vent system flows
Airflow through water heater flue and draft diverter Daily vent airflow (cfm @ 60F) 12 10 8 6 4 2 Site B (non-fvir) Flue Draft diverter 0 0 20 40 60 80 Daily outdoor temperature (F)
Water heater vent flow (average)
Heating penalty from loss of conditioned air - method Measure off-cycle and on-cycle vent flow (stoichiometry) Fit linear model of flow and vent pressure Find relationship of vent pressure (flow) with outdoor temperature Estimate contribution to infiltration Sum infiltration x temp diff for heating season
Estimated annual heating infiltration load penalty from natural-draft water heaters (for Madison, WI), by operating mode contribution A B C D E (FVIR) E F (FVIR) G H I J for Madison, WI 0 5 10 15 Estimated annual infiltration load (therms) WH Off-cycle WH On-cycle WH off-cycle, HTG on-cycle (shared venting) WH on-cycle, HTG on-cycle (shared venting)
Vent spillage
Vent pressure during main burner firing 0 Vent pressure (Pascals) -2-4 -6-8 -10-12 -14 minimum draft requirement per WI weatherization program For outdoor temperatures between 21 and 40F 0 2 4 6 8 10 E (non-fvir) G (non-fvir) E (FVIR) D (non-fvir) B (non-fvir) I (non-fvir) C (non-fvir) J (non-fvir) A (non-fvir) F (FVIR) (smoothed data) Elapsed firing minutes
Vent pressure slope with Tout A (non-fvir) B (non-fvir) C (non-fvir) D (non-fvir) E (non-fvir) E (FVI R) F (FVIR) G (non-fvir) I (non-fvir) J (non-fvir) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Change in vent pressure per 10F change in outdoor temperature (Pascals)
Spillage on startup A1 B1 C1 D1 E1 Site E2 F2 G1 H1 I1 J1 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Firing events with spillage
Positive vent pressure (off cycle) Site A1 B1 C1 D1 E1 E2 F2 G1 H1 I1 J1 0.12% 0.07% 0.02% 0.16% 0.10% 0.03% 0.00% 0.01% 0.00% 0.00% 1.91% 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 1.2% 1.4% 1.6% 1.8% 2.0% Vent pressure reversal (off cycle)
+6 Site G, February 20, 2009 +4 +2 spillage event 0-2 -4 0 6 12 18 24 +2 +1 0-1 -2-3 0 10 20 30 +2 +1 0-1 -2 0 10 20 +10 Site G, March 3-4, 2009 +5 0-5 spillage event -10 0 6 12 18 24 6 18 24
Loads and delivery temperature
Fraction of hot water drawn in events up to X gallons, all sites
Delivery temperature dropoff in healthy water heater 140 Site D, water heater ID 1 Hot water temperature (F) 120 100 80 60 0 10 20 30 40 Cumulative gallons
Temperature dropoff with volume, atmospheric vs new power vent No dip tube No dip tube Power Vent
Delivery temperature, tankless unit 140 Site I, water heater ID 5 Hot water temperature (F) 120 100 80 60 0 10 20 30 40 Cumulative gallons
Delivery temperature, condensing tankless unit 140 Site B, water heater ID 6 Hot water temperature (F) 120 100 80 60 0 10 20 30 40 Cumulative gallons
Practical installation and operating considerations
Venting Power vent and tankless condensing units PVC venting. Can run 100+ feet with proper vent size Tankless non-condensing Can run 35 ft (1 elbow), 23 ft (3 elbows) Concentric aluminum-in-pvc vent pipe, very expensive
Gas meter and piping Tankless water heaters have inputs of 150,000 to 200,000 BTU/hr (some even higher) Gas piping and gas meter must be sized to meet maximum demand
Minimum flow Tankless water heaters require a minimum flow rate before burner will start, typically about 0.5 gal/min. Many dishwashers draw 0.5 gpm or less when filling, and will not trigger water heater operation no easy fix Incidental draws and leaks do not carry an energy penalty with tankless water heaters
Major leak: up to 100 gal/day
Thank You!