Maximizing the Efficiency of Condensing Boilers - Cx Associates
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The document discusses maximizing the efficiency of condensing boilers in both new and existing commercial and residential buildings. Key learning objectives include understanding the relationship between hot water temperature, outdoor air temperature, and heating loads to achieve optimal efficiency. The presentation emphasizes practical strategies, such as controlling hot water temperature based on outdoor conditions to maximize condensing hours and improve overall boiler performance.
![Relationship Between OAT, Load, and
HWT
Qconduction = (U x A x ∆T)
Qinfiltration = (1.08 x CFM x ∆T)
Qload = (U x A x ∆T) + (1.08 x CFM x ∆T)
Qload = ∆T[(U x A) + (1.08 x CFM)]
45](https://image.slidesharecdn.com/bbdboilerefficiencypresentationfinal-cxassociates-140205122343-phpapp02/85/Maximizing-the-Efficiency-of-Condensing-Boilers-Cx-Associates-45-320.jpg)
![Relationship Between OAT, Load, and
HWT
Qconduction = (U x A x ∆T)
Qinfiltration = (1.08 x CFM x ∆T)
Qload = (U x A x ∆T) + (1.08 x CFM x ∆T)
Qload = ∆T[(U x A) + (1.08 x CFM)]
Qload = ∆T x k
46](https://image.slidesharecdn.com/bbdboilerefficiencypresentationfinal-cxassociates-140205122343-phpapp02/85/Maximizing-the-Efficiency-of-Condensing-Boilers-Cx-Associates-46-320.jpg)
![Relationship Between OAT, Load, and
HWT
Qload = Qterminal_heat
Q = ∆T x k
k = Q / ∆T
K = (6,400 BTUH) / (70°F – (‐10°F))
K = 80 [BTUH/Δ°F]
57](https://image.slidesharecdn.com/bbdboilerefficiencypresentationfinal-cxassociates-140205122343-phpapp02/85/Maximizing-the-Efficiency-of-Condensing-Boilers-Cx-Associates-57-320.jpg)
Maximizing the Efficiency of Condensing Boilers - Cx Associates
- 3. Introduction • Condensing boilers are commonly specified and installed in buildings today for efficiency benefit. – System selection and setpoints are key to achieving rated efficiency!!! 3
- 5. Introduction • The concepts and recommendations in this presentation are applicable to . . . – Both new construction and existing buildings. – Both commercial and residential buildings. 5
- 6. Introduction • The concepts and recommendations in this presentation are applicable to . . . – Both new construction and existing buildings. – Both commercial and residential buildings. • $0 capital cost opportunities!!! 6
- 7. Introduction • Learning Objective #1: – Be able to explain the impact of hot water temperature on condensing boiler efficiency. – Understanding efficiency ratings. • Learning Objective #2: – Be able to explain the relationship between outdoor air temperature, heating load, and heating hot water temperature. 7
- 8. Introduction • Learning Objective #3: – Be able to estimate hot water temperature reset setpoints that maximize condensing hours and satisfy heating loads. • Learning Objective #4: – Be able to size terminal heating equipment for maximum condensing hours. 8
- 9. Introduction • Learning Objective #5: – Describe operation of indirect DHW – Relate boiler HW temperature back to efficiency – Describe non‐heating season impacts. 9
- 10. Condensing Boiler Basics • Learning Objective #1: – Be able to explain the impact of hot water temperature on condensing boiler efficiency. – Understanding efficiency ratings. 10
- 11. Boiler Efficiency Ratings • All boilers are not rated equally Boiler Capacity (BTU/H) Rating Method (%) <300,000 AFUE – Annual Fuel Use Efficiency According to ASHRAE Standard 103 300,000 < Efficiency 2,500,000 Thermalto ANSI Z21 According >2,500,000 Combustion Efficiency According to ANSI Z21 11
- 12. Condensing Boilers ‐ 101 • Hot water, not steam • Typically used with Nat Gas or Propane • Up to 15% more efficient than a non‐ condensing boiler. • The lower the return water temperature, the more efficient the boiler. 12
- 14. Condensing Boilers ‐ 101 Water vapor contains a LOT of energy. • 1 LB of water requires: – 1 BTU to raise the temperature 1 ○F • 1 LB of water requires: – 970 BTUs to turn it into steam (with no temperature change) Photo by Scott Akerman 14
- 15. Condensing Boilers ‐ 101 • Burning fuel makes water vapor (steam). • This cannot be avoided. • Turning that steam into water (CONDENSING) releases energy back into the process. 970 BTU / lb 15
- 18. Condensing Boilers ‐ 101 • Vapor from natural gas combustion begins to condense at roughly 130 ○F. • It’s NOT all or nothing. 18
- 20. Condensing Boilers ‐ 101 • Notes on boiler construction • Different materials require different water treatment. • Cast Aluminum, Stainless Steel, Possibly Cast Iron • pH, Chlorides, alkalinity, cleanliness, etc. 20
- 22. Putting this Knowledge to Work • Keep your return water temperatures as low as you can for as long as you can. • How? – Make informed decisions about hot water temperature control – Make informed decisions about heat producing terminal devices. 22
- 31. Relationship Between OAT, Load, and HWT Qload Qload = Qconduction + Qinfiltration 31
- 34. Relationship Between OAT, Load, and HWT Qload = Qconduction + Qinfiltration Qconduction = (U x A x ∆T) 34
- 35. Relationship Between OAT, Load, and HWT Qload = Qconduction + Qinfiltration Qconduction = (U x A x ∆T) Qinfiltration = (1.08 x CFM x ∆T) 35
- 36. Relationship Between OAT, Load, and HWT Qload = Qconduction + Qinfiltration Qconduction = (U x A x ∆T) Qinfiltration = (1.08 x CFM x ∆T) Where: Qload = U = A = ∆T= CFM = 1.08 = 36
- 37. Relationship Between OAT, Load, and HWT Qload = Qconduction + Qinfiltration Qconduction = (U x A x ∆T) Qinfiltration = (1.08 x CFM x ∆T) Where: Qload = Heating Load (BTUH) U = A = ∆T= CFM = 1.08 = 37
- 38. Relationship Between OAT, Load, and HWT Qload = Qconduction + Qinfiltration Qconduction = (U x A x ∆T) Qinfiltration = (1.08 x CFM x ∆T) Where: Qload = Heating Load (BTUH) U = Overall heat transfer coefficient (BTUH/ft2‐ ˚∆T) A = ∆T= CFM = 1.08 = 38
- 39. Relationship Between OAT, Load, and HWT Qload = Qconduction + Qinfiltration Qconduction = (U x A x ∆T) Qinfiltration = (1.08 x CFM x ∆T) Where: Qload = Heating Load (BTUH) U = Overall heat transfer coefficient (BTUH/ft2‐ ˚∆T) A = Envelope area (ft2) ∆T= CFM = 1.08 = 39
- 40. Relationship Between OAT, Load, and HWT Qload = Qconduction + Qinfiltration Qconduction = (U x A x ∆T) Qinfiltration = (1.08 x CFM x ∆T) Where: Qload = Heating Load (BTUH) U = Overall heat transfer coefficient (BTUH/ft2‐ ˚∆T) A = Envelope area (ft2) ∆T= Difference between space temperature and outdoor air temperature CFM = 1.08 = 40
- 41. Relationship Between OAT, Load, and HWT Qload = Qconduction + Qinfiltration Qconduction = (U x A x ∆T) Qinfiltration = (1.08 x CFM x ∆T) Where: Qload = Heating Load (BTUH) U = Overall heat transfer coefficient (BTUH/ft2‐ ˚∆T) A = Envelope area (ft2) ∆T= Difference between space temperature and outdoor air temperature CFM = Infiltration airflow (ft3/minute) 1.08 = 41
- 42. Relationship Between OAT, Load, and HWT Qload = Qconduction + Qinfiltration Qconduction = (U x A x ∆T) Qinfiltration = (1.08 x CFM x ∆T) Where: Qload = Heating Load (BTUH) U = Overall heat transfer coefficient (BTUH/ft2‐ ˚∆T) A = Envelope area (ft2) ∆T= Difference between space temperature and outdoor air temperature CFM = Infiltration airflow (ft3/minute) 1.08 = Air heat capacitance and unit conversion (BTUH‐min/ft3‐hr‐°F) 42
- 44. Relationship Between OAT, Load, and HWT Qconduction = (U x A x ∆T) Qinfiltration = (1.08 x CFM x ∆T) Qload = (U x A x ∆T) + (1.08 x CFM x ∆T) 44
- 45. Relationship Between OAT, Load, and HWT Qconduction = (U x A x ∆T) Qinfiltration = (1.08 x CFM x ∆T) Qload = (U x A x ∆T) + (1.08 x CFM x ∆T) Qload = ∆T[(U x A) + (1.08 x CFM)] 45
- 46. Relationship Between OAT, Load, and HWT Qconduction = (U x A x ∆T) Qinfiltration = (1.08 x CFM x ∆T) Qload = (U x A x ∆T) + (1.08 x CFM x ∆T) Qload = ∆T[(U x A) + (1.08 x CFM)] Qload = ∆T x k 46
- 53. Relationship Between OAT, Load, and HWT Tiers Steam Encl. and Mounting 215°F Catalog Fin Fin Depth and Centers Height Factor Tube Size Designation Fin Size per ft. Thickness Height (in.) (in.) (in.) 1.00 3‐1/4" 3/4" C3/4‐33 SQ. 32 0.020" 14A 1 18‐7/16 1050 53 200°F 0.86 900 Hot Water (Avg.) 190°F 180°F 170°F 160°F Factor 0.78 0.69 0.61 0.53 820 720 640 560 150°F 0.45 470
- 54. Relationship Between OAT, Load, and HWT Hot Water (Avg.) Steam 200°F 190°F 180°F 170°F 160°F 150°F 215°F Factor Factor 0.86 0.78 0.69 0.61 0.53 0.45 1.00 Catalog Designation C3/4‐33 54 BTUH/LF 1050 900 820 720 640 560 470
- 55. Relationship Between OAT, Load, and HWT Hot Water (Avg.) Steam 200°F 190°F 180°F 170°F 160°F 150°F 215°F Factor Factor 0.86 0.78 0.69 0.61 0.53 0.45 1.00 Catalog Designation C3/4‐33 55 BTUH 10,500 9,000 8,200 7,200 6,400 5,600 4,700
- 56. Relationship Between OAT, Load, and HWT Hot Water (Avg.) Steam 200°F 190°F 180°F 170°F 160°F 150°F 215°F Factor Factor 0.86 0.78 0.69 0.61 0.53 0.45 1.00 Catalog Designation C3/4‐33 56 BTUH 10,500 9,000 8,200 7,200 6,400 5,600 4,700
- 58. Relationship Between OAT, Load, and HWT Q = ∆T x k Use value of k to calculate Q at 60°F OAT. 58
- 60. Relationship Between OAT, Load, and HWT Q = 800 BTUH Hot Water (Avg.) Steam 200°F 190°F 180°F 170°F 160°F 150°F 215°F Factor Factor 0.86 0.78 0.69 0.61 0.53 0.45 1.00 Catalog Designation C3/4‐33 60 BTUH 10,500 9,000 8,200 7,200 6,400 5,600 4,700
- 64. Controls – Determining the Right Reset Schedule • Learning Objective #3: – Be able to estimate hot water temperature reset setpoints that maximize condensing hours and satisfy heating loads. 64
- 65. Determining the Right Reset Schedule • What is a “reset schedule”? – Means of controlling the Hot Water Supply Temp (HWST) based on the Outdoor Air Temp (OAT) • Example of Typical Reset Schedule OAT HWST OATMIN HWSTMAX 180 OATMAX 65 0 60 HWSTMIN 120
- 66. Determining the Right Reset Schedule 1 – Initial Reset Schedule 1 Condensing begins at 40 deg OAT 66
- 68. Determining the Right Reset Schedule • Initial reset schedule used 0 deg OAT as a design condition – maximum HWST. OAT HWST OATMIN 0 HWSTMAX 180 OATMAX 60 HWSTMIN 120 • Revise to reflect the actual OAT design condition OAT HWST OATMIN HWSTMAX 180 OATMAX 68 ‐11 60 HWSTMIN 120
- 71. Determining the Right Reset Schedule • It gets colder than ‐11. ‐20 is more likely the actual condition for which the system was designed. • Increases condensing hours from 3,650 to 4,580 per year. • 66% of possible hours OAT HWST OATMIN HWSTMAX 180 OATMAX 71 ‐20 60 HWSTMIN 120
- 72. Determining the Right Reset Schedule 1 – Initial Reset Schedule 2 – MIN OAT from 0 to ‐11 3 – MIN OAT from ‐11 to ‐20 1 2 3 Condensing begins at 33 deg OAT 72
- 74. Determining the Right Reset Schedule • Went from 52% to 66% of possible hours by simply changing the MAX OAT to the actual design OAT. • Next step – are 120 HWST and 60 OAT right? • 120 HWST results in a return temp of between 100. • Most condensing boilers will accept an 80 degree or lower entering water temp. • Use 100 deg F HWST. 74
- 75. Determining the Right Reset Schedule • Next step – are 120 HWST and 60 OAT right? • 120 HWST results in a return temp of between 100 and 110. • Most condensing boilers will accept an 80 degree entering water temp. • Use 100 deg F HWST. • What about 60 OAT? 75
- 76. Determining the Right Reset Schedule • Example – 10 ft x 14 ft office with an 8 ft ceiling. • Heating Load = 2,100 BTUH • 70 deg indoor, ‐20 deg outdoor • Designed with 4 ft of active finned tube radiation. • At design conditions we need 525 BTUH/LF 76
- 77. Determining the Right Reset Schedule • Finned tube output needed ‐ 525 BTUH/LF AWT 180 170 160 150 140 130 120 110 100 90 BTUH/LF 652 576 501 425 378 312 246 189 142 104 • 170 deg AWT satisfied the load. • This selection satisfies design heat loss conditions using 180 deg HWST. 77
- 78. Determining the Right Reset Schedule • Earlier we decided to use a minimum 100 deg HWST. AWT 180 170 160 150 140 130 120 110 100 90 BTUH/LF 652 576 501 425 378 312 246 189 142 104 • Finned tube output at 100 deg HWST or 90 deg AWT = 104 BTUH/LF • 4 Feet 416 BTUH capacity. 78
- 79. Determining the Right Reset Schedule • Earlier we decided to use a minimum 100 deg HWST. AWT 180 170 160 150 140 130 120 110 100 90 BTUH/LF 652 576 501 425 378 312 246 189 142 104 • Finned tube output at 100 deg HWST or 90 deg AWT = 104 BTUH/LF • 4 Feet 416 BTUH capacity. • At what OAT does the output match the load? 79
- 81. Determining the Right Reset Schedule • • • • • • 81 416 BTUH capacity at 90 deg AWT Earlier we concluded Q = k x ∆T Q = 2,100 BTUH ∆T = 70 – (‐20) = 90 k = 23.33 Use this k to find ∆T based on matching 416 BTUH capacity to heating load.
- 82. Determining the Right Reset Schedule • • • • 82 Q = k x ∆T We’re assuming that at 65 degrees OAT, the Q = 416 BTUH and k = 23.33 heating load to maintain 70 deg IAT is exactly zero. 416 / 23.33 = 18 deg ∆T 65 OAT – 18 deg ∆T = 47 deg OAT
- 83. Determining the Right Reset Schedule • • • • Q = k x ∆T Q = 416 BTUH and k = 23.33 416 / 23.33 = 18 deg 65 – 18 = 47 deg OAT Capacity of finned tube at 90 deg AWT will satisfy the building load at 47 deg OAT. 83
- 85. Determining the Right Reset Schedule 1 – Initial Reset Schedule 2 – MIN OAT from 0 to ‐11 3 – MIN OAT from ‐11 to ‐20 4 – MIN HWST revised, MAX OAT matched. 1 2 3 4 85 Condensing begins at 14 deg OAT
- 89. Determining the Right Reset Schedule Summary • No change to boiler or selected finned tube. • No change in MAX HWST • Used simple approach to determine the optimal reset schedule. • Result: Increase in condensing hours from 52% to 93% at no additional first cost. 89
- 91. Adjusting an Existing Reset Schedule • Step 1 – Lower the lowest HWST. • Step 2 – Lower the MAX OAT 3 to 5 degrees at a time, over a period of days or weeks. • Step 3 – Lower the MIN OAT using what you know about your building. – The last time it was ‐10 or ‐15 outside, was your building satisfied? OAT HWST OATMIN HWSTMAX 180 OATMAX 91 ‐20 47 HWSTMIN 100
- 93. Fin‐Tube Selection Qload = 6,400 BTUH Catalog Designation Steam 215°F Factor 1.00 C3/4‐33 10,500 AWT = 130˚F 93 200°F Hot Water (Avg.) 190°F 180°F 170°F 160°F Factor 150°F 0.86 0.78 0.69 0.61 0.53 0.45 9,000 8,200 7,200 6,400 5,600 4,700
- 94. Fin‐Tube Selection Qrated = Q215˚F x CFAWT x CFw_flow x Cfheight Where: Q215˚F CFAWT CFw_flow Cfheight 94 = Catalog capacity = Correction factor for average water temperature = Correction factor for water flow rate = Correction fact for mounting height
- 95. 95
- 96. 96
- 97. Fin‐Tube Selection Qrated = Q215˚F x CFAWT_EAT x CFw_flow x Cfheight Qrated = (10,500 BTUH) x (0.33) x CFw_flow x Cfheight 97
- 98. 98
- 99. 99
- 100. 100
- 101. Fin‐Tube Selection Qrated = Q215˚F x CFAWT_EAT x CFw_flow x Cfheight Qrated = (10,500 BTUH) x (0.33) x (0.931) x Cfheight 101
- 103. Fin‐Tube Selection Qload = 6,400 BTUH Qrated = 3,226 BTUH (130˚F AWT, 10 ft of fin‐tube) Hot Water (Avg.) Steam 200°F 190°F 180°F 170°F 160°F 150°F 215°F Factor Factor 0.86 0.78 0.69 0.61 0.53 0.45 1.00 Catalog Designation C3/4‐33 103 BTUH 10,500 9,000 8,200 7,200 6,400 5,600 4,700
- 104. Fin‐Tube Selection Qload = 6,400 BTUH Qrated = 3,226 BTUH (130˚F AWT, 10 ft of fin‐tube) Hot Water (Avg.) Steam 200°F 190°F 180°F 170°F 160°F 150°F 215°F Factor Factor 0.86 0.78 0.69 0.61 0.53 0.45 1.00 Catalog Designation C3/4‐33 BTUH 10,500 9,000 8,200 7,200 6,400 5,600 4,700 • Need twice the amount of fin‐tube! 104
- 105. Fin‐Tube Selection Qload = 6,400 BTUH Qrated = 3,226 BTUH (130˚F AWT, 10 ft of fin‐tube) Hot Water (Avg.) Steam 200°F 190°F 180°F 170°F 160°F 150°F 215°F Factor Factor 0.86 0.78 0.69 0.61 0.53 0.45 1.00 Catalog Designation C3/4‐33 BTUH 10,500 9,000 8,200 7,200 6,400 5,600 4,700 • Need twice the amount of fin‐tube! • Gain only 510 additional hours of condensing operation. 105
- 106. Indirect DHW • Learning Objective #5: – Describe operation of indirect DHW – Relate boiler HW temperature back to efficiency – Describe non‐heating season impacts. 106
- 109. The Domestic Hot Water Demon • Physics dictates that the heat source must be hotter than the DHW. • Most off‐the‐shelf controllers use 180 deg F source hot water – Non‐adjustable. – Forget about condensing. – Need 130 deg return to START condensing. • BMS Controlled systems have more flexibility. – Mostly a trial and error process 109
- 110. The Domestic Hot Water Demon Non‐Heating Season • Burlington, VT: • 1,765 hours where no heating is needed. • Optimized reset schedule allows for 8,160 hours of condensing operation. • BUT – You need DHW all year round. • When the boiler makes 180 deg water you lose the efficiency gains we just got for free! 110
- 111. The Domestic Hot Water Demon Non‐Heating Season • Boiler short cycling is a known efficiency killer. • Patterson Kelly has noted measured reductions of 15% to 40% in efficiency attributable to short cycling. • Boilers are nearly always sized for the heating load and not the DHW load. • This leads to a lot of… 111
- 115. Conclusion • System selection and control setpoints (hot water reset schedule) are key to achieving condensing boiler efficiencies. – Keep your return water temperatures as low as you can for as long as you can. – Ensure that terminal heating is sized for condensing temperatures. – Separate heating boilers and DHW boilers. 115