Energy Efficient Buildings
Furnaces and Boilers
More than 85% of homes in the U.S. are heated with furnaces or hot water boilers.
Furnaces burn fuel to heat air which blown by fans though ducts into the conditioned
space. Boilers burn fuel to heat water which is pumped through radiators, heat
exchanges or through the floor of the conditioned space.
Residential furnace
Source: www.nyserda.ny.gov
Atmospheric hot-water boiler
Furnace/Boiler Fuel
Most furnaces and boilers burn natural gas, fuel oil or liquefied petroleum gas (LPG) as
fuel. Natural gas is composed of about 89% methane, 8% ethane and 3% propane. Fuel
oil is diesel fuel #2. Liquefied petroleum gas is primarily propane. Properties of these
fuels are shown below.
Density
Lower Heating Value
Higher Heating Value
Natural Gas
0.042 lb/ft3
21,500 Btu/lb
23,900 Btu/lb
Fuel Oil
53 lb/ft3
18,500 Btu/lb
19,700 Btu/lb
Propane (LPG)
4.2 lb/gal
84,300 Btu/gal
91,600 Btu/gal
During the combustion process, fuel molecules combine with oxygen and reform as
water and carbon dioxide. The heat of reaction is the heat transferred from the
combustion chamber during the combustion reaction. The heat of reaction depends on
phase of water in the products of combustion.
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If the H2O in the combustion gasses leaves as vapor, it carries latent energy with it. In
this case, the energy transferred from the fuel is lower, and is given by the Lower
Heating Value (LHV).
If the H2O in the combustion gasses leaves as a liquid, then the latent energy in the H20
was removed from the combustion chamber. In this case, the energy transferred from
the fuel is higher, and is given by the Higher Heating Value (HHV).
The dew-point temperature of H2O vapor in the products of natural gas combustion is
about 140⁰F. Thus, if exhaust temperature is greater than 140⁰F, exhaust leaves as
vapor and heat of reaction is the LHV. If exhaust temperature is less than 140⁰F, H2O
condenses to liquid giving up heat, and heat of reaction is the HHV. For this reason, high
efficiency furnaces and boilers use large heat exchangers to reduce the temperature of
the exhaust below 140 F to utilize the entire HHV of the fuel. This step change from LHV
to HHV increases the efficiency of these devices.
Combustion Efficiency
Combustion is a chemical reaction in which energy is released when oxygen reacts with
fuel molecules to produce carbon dioxide and water. Understanding combustion is
important for understanding how to maximize the efficiency of fuel burning furnaces
and boilers.
Excess Air
Assuming that natural gas is made up of 100% methane, the equation for combustion of
natural gas with atmospheric air is:
CH4 + 2 (O2 + 3.76 N2)  CO2 + 2 H2O +7.52 N2
In this equation, just enough oxygen is supplied to break apart the fuel molecules (CH 4)
and create CO2 and H20. The nitrogen in air does not react. The minimum amount of air
required for complete combustion is called the “stoichiometric” air. In this equation,
“stoichiometric” air is achieved when 2 moles of atmospheric air combine with one
mole of methane. Combining 3 moles of atmospheric air with one mole of methane
would result in “excess” air.
The ratio of the mass of air required to completely combust a given mass of fuel is called
the stoichiometric air to fuel ratio, AFs. AFs can be calculated for the combustion
equation shown above using the molecular weights of the fuel and air.
AFs = mass air / mass fuel = [ 2 (16 x 2 + 3.76 x 14 x 2) ] / [ 12 + 1 x 4 ] = 17.2
Thus, for combustion of methane (natural gas), AFs is about 17.2 lb-air/lb-ng.
2
The quantity of air supplied in excess of stoichiometric air is called excess combustion
air, EA. The fraction of excess combustion air can be written in terms of the
stoichiometric air to fuel ratio, AFs, the combustion air mass flow rate, mca, and natural
gas mass flow rate, mng.
EA = [(mca / mng) / AFs] – 1
Excess combustion air dilutes the combustion gasses and lowers the temperature of the
gasses, which results in decreased efficiency. However, insufficient combustion air is
even more problematic since insufficient combustion air results in unburned
hydrocarbons escaping in the exhaust. Unburned hydrocarbons in the exhaust waste
fuel energy, are a safety hazard, and result in smog formation. Thus, it is much better to
error on the side of excess air than insufficient air. The recommended quantity of
excess combustion air for furnaces and boilers is 10%. This ensures sufficient
combustion air so that all hydrocarbon molecules are combusted, while minimizing the
reduction in combustion efficiency.
Temperature of Combustion
The temperature of combustion (Tc) is the temperature of the products of combustion if
all energy from the reaction enters the products. An energy balance on the combustion
of natural gas gives:
mca hca + mng hng – mp hp = 0
(mca Cpa Tca) + mng (Cp,ng Tng + hr,ng) – mp Cpp Tc = 0
where m is mass, h is enthalpy, Cp is specific heat, T is temperature, the subscript ca is
combustion air, the subscript ng is natural gas, and the subscript p is product. The
specific heats of air, natural gas, carbon dioxide and water are about:
Cpa ≈ Cpng ≈ Cpp ≈ .26 (Btu/lb F) = Cpp
After substituting Cpp, the temperature of combustion can be approximated as:
Tc = (mca Cpp Tca + mng Cpp Tng + mng hrng) / (mp Cpp)
Where hrng is the heat of reaction of natural gas.
Further, Tca = Tng and mca + mng = mp. Substituting these relations gives:
Tc = Tca [(mca + mng) / mp] + [(mng hrng) / (mp Cpp)]
Tc = Tca + hR,NG / [ {1 + (mca/mng)} Cpp ]
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Tc = Tca + LHV / [ (1 + (1 + EA) AFs) Cpp ]
Thus, the temperature of combustion, Tc, can be approximated from inlet combustion
air temperature, Tca, lower heating value, LHV, excess combustion air, EA, stoichiometric
air fuel ratio, AFs, and specific heat of combustion gasses, and Cpp (~ 0.30 Btu/lbm-F).
Example
Calculate the effective temperature of combustion, Tc, for combustion of natural gas if
the temperature of the combustion air, Tca, is 72⁰F and 10% excess combustion air is
supplied to the burner.
Tc = Tca + LHV / [ (1 + (1 + EA) AFs) Cpp ]
Tc = 72 F + 21,500 Btu/lb / [ (1 + (1 + .10) (17.2)) (0.30) Btu/lb-F ] = 3,670 F
Combustion Efficiency
The combustion efficiency,  is the ratio of useful energy extracted from the
combustion gasses, Qu, to the total fuel energy supplied, Qfuel.
= (Qu / Qfuel)
When the exhaust temperature, Tex, is greater than 140 F, the water formed during
combustion equation remains in the vapor state. Thus, the energy extracted from
combustion gasses as the temperature drops from Tc to Tex is sensible energy. The
change in sensible energy is the product of the specific heat and the temperature
change. In this case, combustion efficiency can be written as:

= [mp Cpp (Tc-Tex)] / mng HHVng
= [(mca + mng) Cpp (Tc-Tex)] / mng HHVng
= [ 1 + (mca/mng) ] [Cpp(Tc-Tex) / HHVng]
= [ 1+ (1+EA) AFs ] Cpp (Tc-Tex)/ HHVng
= [ 1+ (1+EA) AFs ] Cpp (Tc-Tex)/ HHVng
When the exhaust temperature, Tex, falls below 140 F, the water formed during
combustion condenses from vapor to liquid and releases heat in the process. The heat
released during phase change is called latent heat. The latent heat released is:
Qlat = 0
(when Tex > 140 F)
Qlat = HHV - LHV (when Tex < 140 F)
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This formulation of Qlat assumes a step change from liquid to vapor at precisely 140 F.
In practice, the temperatures of individual water molecules vary and the measured
temperature of the gas is the average of the individual molecules. Thus, when the
average temperature of the gas is 140 F, the temperatures of some molecules are
greater than 140 F and the water will be in the vapor state; similarly, the temperatures
of some molecules are less than 140 F and the water will be in the liquid state. Thus,
the latent energy is released over a range of about 10 F, and the formulation above is a
step-change simplification of a continuous process. Using this definition of latent heat,
a more general equation for combustion efficiency is:
 = [ {1 + (1 + EA) AFs} Cpp (Tc – Tex) + Qlat ] / HHV
In summary, the three required input values for computing combustion efficiency are:



entering combustion air temperature Tca
exhaust gas temperature, Tex
excess combustion air, EA
These three values are measured by a combustion analyzer.
Example
Calculate the combustion efficiency of natural gas furnace if the temperature of the
combustion air, Tca, is 72⁰F, the temperature of the exhaust, Tex, is 350 F, and 10%
excess combustion air is supplied to the burner.
From previous example:
Tc = Tca + LHV / [ (1 + (1 + EA) AFs) Cpp ]
Tc = 72 F + 21,500 Btu/lb / [ (1 + (1 + .10) (17.2)) (0.30) Btu/lb-F ] = 3,670 F
Qlat = 0 (since Tex > 140 F)

 = [ {1 + (1 + EA) AFs} Cpp (Tc – Tex) + Qlat ] / HHV
 = [ {1+ (1+0.5) (17.2)} 0.30 Btu/lb-F (3,670 - 350) F + 0 ] / 23,900 (Btu/lb) = 83.0 %
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Example
Calculate the combustion efficiency of natural gas furnace if the temperature of the
combustion air, Tca, is 72⁰F, the temperature of the exhaust, Tex, is 120 F, and 10%
excess combustion air is supplied to the burner.
Tc = Tca + LHV / [ (1 + (1 + EA) AFs) Cpp ]
Tc = 72 F + 21,500 Btu/lb / [ (1 + (1 + .10) (17.2)) (0.30) Btu/lb-F ] = 3,670 F
Qlat = HHV – LHV = (23,900 – 21,500) Btu/lb = 2,400 Btu/lb (since Tex < 140 F)

 = [ {1 + (1 + EA) AFs} Cpp (Tc – Tex) + Qlat ] / HHV
 = [ {1+ (1+0.5) (17.2)} 0.30 Btu/lb-F (3,670 - 120) F + 2,400 Btu/lb ] / 23,900 (Btu/lb)
 = 98.8 %
Efficiency Improvement from Reducing Excess Combustion Air
As discussed earlier, the minimum amount of air required for complete combustion is
called the “stoichiometric” air. Combustion efficiency is maximized when stoichiometric
air is supplied to the fuel. Supplying less than stoichiometric air causes some fuel to be
exhausted without combusting, which increases fuel use, air pollution and the danger of
uncontrolled combustion in the exhaust stack. Supplying more than stoichiometric air
dilutes the combustion gasses, lowers combustion temperature and reduces
combustion efficiency. Thus, best practice is to supply only slightly more oxygen
(combustion air) than the stoichiometric minimum, so that unburned fuel is avoided and
the efficiency penalty from excess combustion air is minimized. The common target of
excess combustion air to guarantee complete combustion is about 10%.
The quantity of excess air in the combustion gasses is sometimes expressed as fraction
oxygen. For methane (natural gas) the conversion between fraction oxygen, FO 2, and
excess combustion air, ECA, are:
FO2 = 2 ECA / (10.52 + 9.52 ECA)
ECA = 10.52 FO2 / (2 – 9.52 FO2)
Thus, 10% excess air produces combustion gasses with about 1.7% O2 content.
Example
A household furnace burns 50,000 Btu/hr of natural gas. Combustion air enters the
burner at 70 F. A combustion analysis of exhaust gasses shows that the flue
temperature is 350 F and the quantity of excess air is 50%. Calculate the current
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combustion efficiency, the combustion efficiency if the excess air were reduced to 10%,
and the resulting fuel savings.
Tc1 = Tca + LHV / [{1 + (1 + EA) AFs} Cpp]
Tc1 = 70 F + 21,500 Btu/lb / [{1 + (1 + .50) 17.2} 0.30 Btu/lb-F] = 2,744 F
Qlat = 0 since Tex > 140 F
 = [{1 + (1 + EA) AFs} Cpp (Tc – Tex) + Qlat] / HHV
 = [ {1 + (1 + .50) 17.2} 0.30 Btu/lb-F (2,744 – 350 F) + 0 ] / 23,900 Btu/lbm
 = 80.5%
The heat delivered to the system, Qout, would be:
Qout = Qf1  = 50,000 Btu/hr x 80.5% = 40,269 Btu/hr
If excess air were reduced to 10%, the combustion efficiency would increase to:
Tc2 = Tca + LHV / [{1 + (1 + EA) AFs} Cpp]
Tc2 = 70 F + 21,500 Btu/lb / [{1 + (1 + 0.10) 17.2} 0.30 Btu/lb-F] = 3,668 F
 = [{1 + (1 + ECA) AFs} Cpg (Tc – Tex) + Qlat] / HHV
 = [{1 + (1 + 0.10) 17.2} 0.30 Btu/lb-F (3,668 – 350 F) + 0] / 23,900 Btu/lbm
 = 83.0%
The fuel energy input, Qf2, would be:
Qf2 = Qout /  = 40.269 Btu/hr / 83.0% = 48,543 Btu/hr
The fuel energy savings, Es, from reducing excess combustion air would be:
Es = Qf1 – Qf2 = 50,000 Btu/hr – 48,543 Btu/hr = 1,457 Btu/hr
Thus, this measure would reduce fuel use by 2.9%
Heat Exchanger Effectiveness
Combustion efficiency is determined by the amount of heat extracted from the
combustion gasses by the heat exchanger. Thus, an understanding of heat exchanger
effectiveness increases understanding of furnace and boiler efficiency.
Heat exchangers transfer heat from a hot stream with entering and exiting
temperatures of Th1 and Th2 to a cold stream with entering and exiting temperatures of
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Tc1 and Tc2. The product of the mass flow rate and specific heat of the hot and cold
streams are called the mass capacitances, mcph and mcpc. A schematic of a
counterflow heat exchanger with these temperatures is shown below.
Heat exchanger effectiveness, e, is the ratio of the actual heat transfer, Qact, to
maximum heat transfer, Qmax.
e = Qact / Qmax
The actual heat transfer is the product of the mass capacitance and the temperature
rise of either the hot or cold stream. The mass capacitance, mcp, is the product of the
mass flow rate, m, and the specific heat, cp.
Qact = mcph (Th1 – Th2) = mcpc (Tc2 – Tc1)
In an infinitely long heat exchanger, the exit temperature of the hot stream would reach
the entering temperature of the cold stream. Similarly, the exit temperature of the cold
stream would reach the entering temperature of the hot stream. The maximum heat
transfer would be limited only by the capacity of the each stream to absorb the heat.
Thus, the maximum heat transfer would be:
Qmax = mcp,min (Th1 – Tc1)
Thus, the heat exchanger effectiveness, e, is:
e = Qact / Qmax = Qact / mcp,min (Th1 – Tc1)
If the heat exchanger effectiveness, mass capacitances and entering temperatures are
known, this equation can be solved to determine the actual heat transfer, Qact, and exit
temperatures of each stream.
Qact = e mcp,min (Th1 – Tc1)
Tc2 = Tc1 + e mcp,min (Th1 – Tc1) / mcpc
Th2 = Th1 - e mcp,min (Th1 – Tc1) / mcph
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Example
Calculate the heat exchanger effectiveness for a furnace in which the combustion gasses
enter the heat exchanger at Th1 = 3,100 F and leave at Th2 = 300 F, and the room air
enters the heat exchanger at Tc1 = 70 F and leaves at Tc2 = 140 F.
e = Qact / Qmax = mch,h (Th1 – Th2) / mcp,min (Th1 – Tc1)
The temperature of the combustion gasses falls by 2,800 F while the temperature of the
room air rises by only 70 F. This means that the mass capacitance of the room air is 40
times larger than the mass capacitance of the combustion gasses. Hence, mcp,min =
mcp,h and
e = (Th1 – Th2) / (Th1 – Tc1) = (3,100 – 300) / (3,100 – 70) = 92.4%
Heat exchanger effectiveness is a function of heat exchanger configuration, surface
area, material and the flow rate and specific heat of the fluids. The most effective
configuration is counter flow, followed by cross flow, followed by parallel flow. With
improved heat transfer effectiveness, the same quantity of heat can be transferred with
smaller flow rates or smaller temperature differences. Alternately, with improved heat
transfer effectiveness more heat can be transferred without increasing flow rates or
incoming temperature differences. Equations for heat exchanger effectiveness are:
Parallel flow: e = [1-exp( -NTU (1+Cr) )] / (1+Cr)
Counter flow: e = [1 - exp(-NTU (1-Cr))] / [1 – Cr*exp(-NTU (1 - Cr))] (use Cr = 0.999
when Cr = 1.0)
Cross flow:
e = 1-exp[(1/Cr)*(NTU0.22)*{exp((-Cr)*(NTU0.78))-1}]
where:
Ch = mh * cph
Cc = mc * cpc
Cmin = min(Ch, Cc)
Cmax = max(Ch, Cc)
Cr = Cmin / Cmax
NTU = UA/Cmin
These equations indicate that heat exchanger effectiveness increases:
 in counter-flow configurations
 as heat transfer surface area increases
 as the minimum mass capacitance decreases
 as the maximum mass capacitance increases
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Older furnaces utilized parallel flow heat exchange, while modern high efficiency
furnaces use employ counter flow heat exchange.
Example
Consider a furnace burning 88,000 Btu/hr of fuel with 10% excess air and supplying
1,200 cfm of warm air to a house. The supply and combustion air enter the furnace at
70 F. The heat exchanger is comprised of four 2 ft x 4 ft plates with combustion gas
flowing downward through channels in the plate and room supply air flowing upward
between the plates. Thermal resistance is dominated by the convection coefficients
(1.719 Btu/hr-ft2-F) on each side of the plates. The density and specific heat of room air
are 0.075 lb/ft2 and 0.26 Btu/lb-F. Using the heat exchanger effectiveness method,
determine the heat supplied to the house and the combustion efficiency of the furnace.
Compare your result with the combustion efficiency calculated using the combustion
method.
The temperature of combustion, Tc, is:
INPUTS
EA = excess air (0=stoch, 0.1 = optimum)
Tca = temperature combustion air before burner (F)
0.10
70
CONSTANTS (FOR NATURAL GAS)
LHV = lower heating value (Btu/lb)
HHV = higher heating value (Btu/lb)
cpp = specific heat of products of exhaust (Btu/lb-F)
AFs = air/fuel mass ratio at stochiometric conditions
21,500
23,900
0.300
17.20
CALCULATIONS
Tc = temp combustion (F) = Tca+LHV/[(1+(1+EA)(Afs))cpp]
3,668
The mass flow rate of the combustion gasses is:
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INPUTS
Qburner (Btu/hr)
HHVng (Btu/lb)
AFRstoch (lba/lbng)
EA
p (at 68 F) (lb/ft3)
CALCULATIONS
Ma = Qb AFRs (1+EA) / HHV (lb/hr)
Mng = Qb / HHVng (lb/hr)
Mtot (lb/hr)
Vtot = Mtot / p (scfm)
88,000
23,900
17.2
0.1
0.075
69.66
3.68
73.35
15.48
The conductance of the heat exchanger is:
U = 1 / (1/h1 + 1/h2) = 1 / (1/1.719 Btu/hr-ft2-F + 1/1.719 Btu/hr-ft2-F)
U = 0.8596 Btu/hr-ft2-F
The area of the heat exchanger is:
A = 8 sides x (2 ft x 4 ft)/side = 64 ft2
The rate of heat transfer is:
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INPUTS
Th1 (F)
mh (lb/min)
cph (Btu/lb-F)
Tc1 (F)
mc (lb/min)
cpc (Btu/lb-F)
U (Btu/hr-ft2-F)
A (ft2)
3,668
1.22
0.300
70
90.0
0.26
0.8596
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CALCULATIONS
Ch = mh * cph (Btu/min-F)
Cc = mc * cpc (Btu/min-F)
Cmin (Btu/min-F)
Cmax (Btu/min-F)
Cr = Cmin / Cmax
N = UA/Cmin
e = [1-exp(N(1-Cr))]/[1-Cr*exp(-N(1-Cr))]
Q = e*Cmin*(Th1-Tc1) (Btu/min)
Th2 = Th1-Q/Ch (F)
Tc2 = Tc1 + Q/Cc (F)
Q (Btu/hr)
0.367
23.400
0.367
23.400
0.016
2.500
0.916
1,208
373
122
72,505
The combustion efficiency calculated using the heat exchanger effectiveness method is:
eff = Q / Qfuel
82.4%
The heat exchanger method predicted that the temperature of the combustion exhaust
would be 373 F. Using, this value, the combustion efficiency calculated using the
combustion method is:
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INPUTS
EA = excess air (0=stoch, 0.1 = optimum)
Tca = temperature combustion air before burner (F)
Tex = temperature exhaust gasses (F)
0.10
70
373
CONSTANTS (FOR NATURAL GAS)
LHV = lower heating value (Btu/lb)
HHV = higher heating value (Btu/lb)
cpp = specific heat of products of exhaust (Btu/lb-F)
Tdpp = dew point temp of H20 in exhaust (F)
AFs = air/fuel mass ratio at stochiometric conditions
21,500
23,900
0.300
140
17.20
CALCULATIONS
Tc = temp combustion (F) = Tca+LHV/[(1+(1+EA)(Afs))cpp]
3,668
hfg = water vapor latent energy (Btu/lb) = (if Tex<140 then hfg=HHV-LHV else
0 hfg = 0)
Efficiency = {hfg+[1 + (1+EA)(AFs)]*cpp*(Tc-Tex)}/HHV
82.4%
Cycling Losses
When furnaces cycle on and off to meet the load, part of the energy supplied by the
natural gas remains in the heat exchanger. Thus, cycling reduces the energy supplied to
the conditioned space and average efficiency. Consider the diagram below. When
room air temperature falls to the thermostat setpoint temperature, the combustion
begins and the heat exchanger begins to warm. After the heat exchanger is sufficiently
warm, the supply air fan starts running and heat is delivered to the room. When the
room air temperature warms to the upper setpoint temperature, combustion ends. The
supply air fan continues to run for a few seconds; however, the supply air fan stops
before the heat exchanger temperature falls to about 100 F. The heat exchanger
continues to cool; however, this heat is lost to the room with the furnace. If this room is
not in the conditioned space, then this energy is lost.
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Source: Energy Engineering, John Mitchell, Wiley Interscience, 1983.
Example
Calculate heat storage losses due to cycling for a furnace controlled by thermostat with
on/off control. The total heat supplied to the house by the furnace, Qf, is 100 x 106 Btu/
5 month heating season. The furnace is rated at Qr = 100,000 Btu/hr. Average on time
during each cycle is 5 minutes. The heat exchanger weights 100 lb. The temperature of
the heat exchanger is 72 F at the start of each cycle and 100 F when the supply air fan
turns off.
Run time hours= Qf / Qr = 100 x 106 Btu/season / 100,000 Btu/hr = 1,000 hrs/season
Heating season hours = 5 months x 30 days/month x 24 hours/day = 3,600 hrs/season
Fraction furnace on time = 1,000 / 3,600 = 28% of time
Number Cycles = Run time hours / Run time per cycle
Number Cycles = 1,000 hours / (5/60) hours/cycle = 12,000 cycles
Qcycling loss = Number Cycles x mhx x cphx x ∆Thx
Qstorage = 12,000 x 100 (lb) x 0.11 (Btu/F-lbm) (100-72) F = 3.70 x 106 Btu = 3.70%
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Pilot Light Losses
Natural gas consumption by pilot lights is about 1 ft3/hour.
Example
Calculate pilot light gas use for the furnace in the previous example.
If furnace is on 28% of time, it is off 72% of the time. Thus, pilot energy use is:
Qpilot light = .72 (3,600) (hours/season) x 1 ft3/hr x 1,000 Btu/ft3 = 2.6 x 106 Btu/season
Qpilot light = 2.6% loss during winter
If it is also on during summer then
Qpilot light = [(.72)3600 + (8,760-3,600)] x 1 (ft3/hr) x 1000 Btu/ft3 = 7.8 x 106 Btu/year
Qpilot light = 7.8% loss
Furnace Efficiency
The preceding sections discuss energy losses in the exhaust gasses, from cycling and
from continuous pilot lights. Combustion efficiency only takes into account the energy
lost in exhaust gasses during steady-state operation. To account for other losses, the
efficiency of residential-size furnaces is characterized using Annual Fuel Utilization
Efficiency, AFUE. AFUE is always less than the steady-state combustion efficiency.
The Path to Energy Efficiency
Over the years, the efficiency of furnaces has improved dramatically. Schematics of midefficiency and high-efficiency furnaces are shown below.
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Traditional Furnace
Condensing Furnace
Furnaces from 1970s had AFUEs between 68%–72%. These low efficiency furnaces were
typically constructed with:
 Natural draft configurations that increased excess air above 10% and enabled a
continuous draft of air to be drawn through the exhaust flue, increasing
infiltration into the house.
 Continuous pilot lights
 Heavy heat exchangers that increased cycling losses
In the 1980s, mid-efficiency furnaces became available with 80%–83% AFUE. Midefficiency furnaces are typically constructed with:
 Induced combustion air fans that reduced excess air and eliminated the
continuous draft of air through the exhaust pipe.
 Electronic ignition (no pilot light)
 Lighter heat exchangers to reduce cycling losses
In the 1990s, high-efficiency furnaces became available with 90%–97% AFUE. Highefficiency furnaces typically have:
 Counter-flow heat exchangers
 A second heat exchanger for condensing flue gases
 Sealed combustion using outdoor air to reduce corrosion due to bleach and
other fumes from indoor air.
 A highly efficient Electronically Commutated Motor (ECM) blower motor
16
These changes have been so effective that the exhaust temperature of high-efficiency
furnaces is typically less than 140 F and water is condensed from the combustion gasses.
To accommodate the condensation, condensing furnaces have a condensate drain to
remove the condensate, and aluminum or steel secondary heat exchangers and pvc
exhaust piping to avoid corrosion. In addition, most high-efficiency furnaces use outside
air for combustion to reduce corrosion from indoor air contaminants and eliminate
combustion-driven infiltration.
The increased heat exchanger area and the addition of a combustion air supply duct
increases friction losses on both sides of the heat exchanger. Thus, high-efficiency
furnaces have bigger blowers and bigger induced draft fans than standard-efficiency
furnaces. A study of high-efficiency furnaces in Madison Wisconsin found that furnace
electricity use increased by about 400 kWh per heating season due to these larger fans.
Recent developments include multi-speed and variable-speed burner fans and blowers.
In these furnaces, the amount and temperature of the air delivered to the house is
varied according to the load. At low loads, the decreased air flow through the heat
exchangers increases heat exchanger effectiveness and efficiency. Multi-speed blower
motors use about the same power at all speeds. However, variable-speed blower
motors substantially reduce fan power at low loads. For example, a fan drawing 400 W
at full speed may draw as little as 65 W at low speed.
Furnace Efficiency Standards
In 2005, the minimum AFUE for non-condensing fossil-fueled, warm-air furnaces was set
at 78%.
In 2007, DOE revised the energy conservation standards for non-weatherized gas
furnaces to 80 % AFUE for residential furnaces manufactured on or after November 19,
2015.
In 2015, DOE proposed raising the minimum AFUE for non-weatherized gas furnaces to
92% for all furnaces manufactured 5 years after the publication of the final rule.
These minimum standards are important, since many manufacturers make furnaces that
barely meet the minimum standards, since many consumers purchase the lowest firstcost furnace available, without regard to cost of fuel over the lifetime of the furnace or
time of residence. However, virtually all manufacturers also make more efficient gas
furnaces. The US EPA gives “Energy Star” ratings to products in top 15% of energy
efficiency. In addition, federal and utility incentives are sometimes offered for
furnaces with AFUE >= 95%.
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Hot Water Boilers
Residential and small commercial hot-water boilers heat water that is supplied to the
building for heating.
Schematics of non-condensing atmospheric and forced-air boilers are shown below. In
both types of boiler, hot combustion gasses transfer heat to the water as the gasses
move upward then out the exhaust flue. These boilers typically employ on/off control,
in which the burner fires whenever the water temperature drops to the lowtemperature set point and turns off when the water temperature rises to the hightemperature set point.
Source: 2008 ASHRAE Handbook HVAC Systems and Equipment.
The cross-flow heat-exchange designs shown above limit combustion efficiency, since
the combustion gasses must leave the boiler at a higher temperature than the boiler
set-point (hot water supply) temperature.
In addition, when open atmospheric boilers are not firing, air is drawn upward through
the interior of the boiler as it warms and becomes more buoyant. This air pulls heat out
of the water and reduces the overall efficiency of the boiler. To reduce this chimney
effect loss, the exhaust flue can be equipped with a stack damper that closes when the
burners are not firing. To be completely effective, the stack damper must be located
below the exhaust flue hood.
Closed forced-draft boilers minimize the chimney effect by sealing the combustion area
with a fan that stops inlet air flow when the burner is not firing. However, as in
furnaces, burner on/off cycling in forced-draft boilers reduces average efficiency. In the
case of forced-draft boilers, the loss occurs because the combustion air fan is powered
on a few seconds before the fuel is ignited to purge the boiler of residual gas which
might explode upon combustion. These purge gasses carry heat out of the boiler. Thus,
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overall boiler efficiency declines as cycling increases. Cycling can be reduced by avoiding
oversizing. Cycling losses can be minimized by the use of modulating burners instead
of on/off burners. Modulating burners control boiler water temperature by modulating
the burner firing rate rather than cycling on and off. As the firing rate decreases, heat
transfer effectiveness increases due to increased contact time between the hot exhaust
gasses and the boiler walls. This reduces exhaust temperature and increases
combustion efficiency. Thus, modulating boilers operate at a higher efficiency at low
loads than at full load.
Condensing Hot Water Boilers
As with furnaces, efficiency increases significantly when water vapor condenses out of
the exhaust gasses. Modern condensing boilers use many of the same techniques used
in condensing furnaces to lower exhaust gas temperature below about 180 F to cause
condensation and improve energy efficiency. As shown below, these furnaces typically
use a combustion fan rather than open air to fix excess air at about 10%. Hot
combustion gasses move in a counter flow direction to the water to increase heat
exchanger effectiveness. The heat exchange are between the combustion gasses and
hot water is larger to increase heat transfer and the heat exchanger is constructed of
aluminum or stainless steel to avoid corrosion.
Need to redraw this figure.
In most boilers, to condense water vapor, the temperature of water returning from the
building and entering the boiler must be 125 F or less. The graph below shows, how
combustion efficiency increases with decreasing inlet water temperature.
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Source: 2008 ASHRAE Handbook HVAC Systems and Equipment.
Traditional non-condensing boilers often operate at a hot-water supply temperature of
180 F. The heat exchange equipment is then typically sized with a 20 F temperature
drop so the design return water temperature is 160 F. Reducing return water
temperature requires that the hot water radiators, fan coil unit and/or the radiant
flooring system be sized to handle these low supply and return water temperatures.
Moreover, supply, and hence return, temperature can also be reduced at low load
conditions. The easiest way to this is with “outdoor air reset”, in which the supply
temperature is highest during very cold weather and peak loads and is reduced during
moderate weather and low loads. A typical outdoor air reset” schedule is shown below.
In addition, outdoor air reset systems typically shut the boiler off during warm weather
about 75 F. Depending on climate, reducing supply (and hence return) water
temperature can increase average boiler efficiency by more than 5%.
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Supply temperature reset ranges typically depend on the type of heat delivery system.
Lower supply and return temperatures drive higher boiler efficiency. Typical ranges are
shown below:
Fan Coil: 140 F to 180 F
Finned-tube baseboard: 120 F to 180 F
Cast-iron baseboard: 100 F to 170 F
Radiators: 120 F to 170 F
Low-mass radiant: 80 F to 140 F
High-mass radiant: 80 F to 120 F
Hot Water Boiler Efficiency Ratings
Residential Boilers
Residential hot-water boilers are rated using annual fuel utilization efficiency (AFUE). In
the 2005 National Energy Security Act, the minimum allowed AFUE rating for a fossilfueled boiler was set at 80%. However, this efficiency rating dramatically overestimates
actual average efficiency because the reported efficiency value is measured with inlet
water temperature of 80 F. The low inlet water temperature inflates the efficiency
rating, since nearly all non-condensing furnaces come with a warning that operation
with inlet water temperature below 140 F will void the warranty. In addition, this rating
is typically determined during full load operation and does not include cycling losses or
heat loss from the boiler when not in firing. Thus, the actual combustion efficiency of
non-condensing furnaces is typically about 10% less than reported.
The US EPA gives “Energy Star” ratings to products in top 15% of energy efficiency.
ENERGY STAR qualified boilers have an AFUE rating of 85% or greater. The Federal
government and utilities often offer incentives for hot water boilers with AFUEs of 90%
or greater.
Small Commercial Hot Water Boiler Systems
A typical small commercial hot water heating system is shown below. Hot water boilers
typically operate at about 180 F and the hot water is returned from the space at about
160 F. The supply water reset control valve can be used to reduce the hot water supply
temperature during warm weather.
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Supply Water Reset
Control Valve
Supply
Pumps
Hot Water
Supply
Circulating
Pumps
Hot Water
Return
Boiler 1
Boiler 2
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