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Hydronic Boiler Efficiency
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Boiler Efficiency
What We Will Discuss:•Types of Efficiency•Testing Standards•What makes a boiler “condensing”?•First Cost•Applications
Types of Efficiency
• Combustion Efficiency
• Thermal Efficiency
• Seasonal Efficiency
• AFUE
Types of Efficiency
• Combustion Efficiency - The measurement and calculation of how well a boiler is burning a specific fuel and how much heat is lost to the flue. Some of the factors included in the calculation are: excess air content, net flue temperature, CO levels• Does not consider heat transferred to the
system.• Determines whether a boiler is “Condensing” or
“Non-Condensing”
Types of Efficiency
• Thermal Efficiency – Gross energy output divided by energy input.• Measures the heat transferred to the medium to
be heated. Generally a “steady state” measurement
• ANSI Standard Z21.13 is a thermal efficiency “steady state” test with 80ºF inlet water and 100ºF ΔT
• In practice Thermal Efficiency is less than Combustion Efficiency
Types of Efficiency
• Seasonal Efficiency – A term used to describe efficiency when the effects of sizing, cycling etc. are considered. • Will typically be lower than thermal
efficiency• Could be considered “real world” efficiency• No test standard for commercial
Types of Efficiency
• AFUE – DOE test standard intended to account for operating and standby losses (seasonal efficiency)• Applies to boilers with 300,000 BTU inputs
and less• Similar to the EPA millage ratings. Can be
used for comparison but will probably not match the actual installation
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Where the Heat From Combustion Goes
• Into the water via the heat exchanger• Out through the stack • Jacket losses• These three add up to 100%
What Is “Condensing”
• During the combustion process some of the energy contained in the fuel is locked up in the creation of water vapor
• This energy (latent heat) cannot be transferred to the medium to be heated without cooling the flue gas to a point below the dew point
• Condensing boilers are built to be able to extract the latent heat contained in the flue gas
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Attainable Combustion Efficiencies
• 84% Maximum “safe” efficiency for non-condensing vent, not highly dependent on water temperature
• 88.3% Maximum efficiency without condensing in the boiler. Requires a vent that can withstand condensing flue gas
• Up to 98% on low temperature systems such as pool heating and water source heat pumps with boiler at low fire. Requires condensing vent.
• Condensing / High Efficiency- highly dependent on inlet (Return) water temperature
10
Relationship of Dew Point, CO2, and Combustion
Efficiency
Source: 2000 ASHRAE Systems and Equipment Handbook
Operation above this line condensing in boiler and/or flue
Operation between lines condensing in flue
Operation below this line no condensing in either the boiler or flue
Normal CO2 range for Nat. Gas = 8.5 - 9.0
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Flue Loss Calculations
80.00
85.00
90.00
95.00
100.00
70 90 110
130
150
170
190
210
230
250
270
290
310
330
350
370
390
410
Flue Temp
Eff
icie
nc
y
10%
88.3
Non-Condensing410ºF = 82%140ºF = 88%270ºF for 6%
Condensing135ºF = 89% 90ºF = 98% 45ºF for 9%
Chart produced by GAMA(Gas Appliance Manufacturers Association)
To Condense or Not To Condense??
• Requires special flue material
• Requires special boiler construction
• Requires the right system conditions. Return (boiler inlet) needs to be low enough for condensing.
• Condensing boilers cost more to install and require the right conditions in order to see the savings
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Raytherm 82% Eff. $1,000
MVB 87% Eff. $1,590
XTherm 96% Eff. $2,272
Comparative Cost based on a 2,000 MBH Unit
First Cost by Order of Magnitude
Condensing Boiler Application Considerations
To Maximize a Condensing Boiler’s Abilities:
•Inlet temperatures below 120ºF
• Water source heat pumps
• In-floor radiant heat
• Reset schedules
•Frequent “cold starts” even when the “warm” temperature is
higher than 120ºF
•Maximize system ΔT
•DHW when piped correctly