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

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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