Hydronic Boiler Efficiency

1. Boiler Efficiency
What We Will Discuss:
•Types of Efficiency
•Testing Standards
•What makes a boiler “condensing”?
•First Cost
•Applications

2. Types of Efficiency
• Combustion Efficiency
• Thermal Efficiency
• Seasonal Efficiency
• AFUE

3. 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”

4. 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

5. 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

6. 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

7. 7
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%

8. 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

9. 9
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. 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

11. 11
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
Efficiency
10%
88.3
Non-Condensing
410ºF = 82%
140ºF = 88%
270ºF for 6%
Condensing
135ºF = 89%
90ºF = 98%
45ºF for 9%
Chart produced by GAMA
(Gas Appliance Manufacturers
Association)

13. 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

14. 14
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

15. 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