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Home Energy Magazine Online March/April 1996
Combining space heating and water heating in one unit
can provide high efficiencies for both applications, and often costs less
than buying an individual space heater and separate water heater. |
Choosing a Heating System That
Saves Energy
by A.C.S. (Skip) Hayden
If winter heating bills set
more records than freezing temperatures and snowfall, maybe it's time to
replace or upgrade that inefficient heating system. Take a Home Energy
guided tour of the choices before buying. |
A.C.S. (Skip) Hayden is head of Energy Conservation
Technology at the Combustion and Carbonization Research Laboratory (CCRL)
of CANMET in Ottawa, Canada.
When something must be done about an old or inefficient
heating system, the challenge can be daunting. The decision will affect
comfort, energy costs, and the air quality and safety of a home for years
to come. Whatever the reasons for considering a new heating system--adding
living space or replacing a system that is inefficient or just plain falling
apart--this is a long-term investment, and it makes good economic sense
to look carefully at energy efficiency.
As this guide points out, there are a number
of factors that determine the right choice for a particular home. Costs
will include the price of the unit and distribution system, installation
charges, and operating expenses. We will focus on the latter, keeping in
mind that long after the last payment has been made on the new system,
energy bills will keep coming in.
Finding the Perfect Fit
The first step is finding out how much heat is needed.
The heating requirements or heating load of a house depend on climate,
size, and style of house; insulation levels; airtightness; amount of useful
solar energy through windows; amount of heat given off by lights and appliances;
thermostat setting; and other operational factors. Together, these factors
determine how much heat must be put into the house by the heating system
over the annual heating season. This number, usually expressed as Btu per
year, can be estimated by a competent heating contractor. It involves measuring
the house (including the windows), checking insulation levels, perhaps
doing a blower door test, and running calculations to determine how much
heat will be needed in that specific climate. An alternative method to
calculate the house heat load is to use information from energy bills,
as shown in the Heating
Cost Worksheet [in PDF format--to view, download Acrobat Reader free
of charge from Adobe.]
It is counterproductive to invest in a new or
improved heating system, only to allow much of its heat to escape. Be sure
to air seal and insulate effectively before having a heating system sized,
installed, or upgraded (see "Home Energy's
Guide to Insulation," HE Jan/Feb '92, p. 29 and "Air
Sealing in Occupied Homes," HE Nov/Dec '95, p. 33).
Sizing
Contractors often put in a furnace with a higher
capacity than is really needed for the home. This is partly to avoid callbacks
from unhappy customers, or to compensate for leaky ducts or poor house
insulation. However, furnaces and boilers are most efficient when they
have been running for a while, and since oversized furnaces can meet the
demand for heat in a shorter time, they may never achieve their best efficiency.
This is why it is very important to make sure
the contractor does proper load and sizing calculations, instead of using
a rule of thumb that includes a margin of "safety" that oversizes for most
homes. It doesn't make sense to pay more for a bigger furnace or boiler,
yet heat the house less efficiently.
Condensing furnaces (see Heating
Equipment Guide) are the exception to this rule. They are actually
slightly more efficient when they run for shorter periods. Thus a model
that is a little larger than the house heat demand can be installed without
suffering an efficiency penalty.
Go to the Source
Most heating systems use either combustion fuels--natural
gas, propane, oil, or wood--or electricity. It is possible to choose a
combination of these conventional sources, or even other alternatives,
such as solar energy. (Solar energy options will be discussed in detail
in a future article.) Energy prices can vary greatly from region to region,
and in many areas natural gas, which must be distributed by pipeline, is
not available.
Check local prices to see if energy will be saved
by switching energy sources or making modifications to employ more than
one energy source. Table 1 gives the energy content
for the various energy sources, per unit in which they are commonly sold.
It also provides spaces to enter the local prices of energy sources available
in your area. Table 2 lists the typical seasonal
efficiency of common heating equipment used with each energy source. Use
the information in these tables and follow the Heating
Cost Worksheet to determine savings in energy cost from switching heating
equipment.
Central Question: Air or Water?
Most central heating systems today are either forced
warm air or hydronic (hot water). They consist of a heating unit, a distribution
system, and controls, such as thermostats, that regulate the system.
Forced warm air is by far the most common type
of central heating system used in North American homes (See Figure
1). Air ducts and registers distribute heat from a central furnace,
providing heat very quickly. The system can also be used to filter and
humidify the household air, to provide central air conditioning, and to
circulate air for ventilation.
Forced warm air systems have some disadvantages.
Air coming from the heating registers sometimes feels cool (especially
with certain heat pumps), even when it is warmer than the room temperature.
There can also be short bursts of very hot air, especially with oversized
units. Ductwork may transmit furnace noise and can circulate dust and odors
throughout the house. Ducts are also notoriously leaky, typically raising
a home's heating costs by 20% to 30%. Homeowners who are having a forced-air
system put in should demand a tight duct system, and be willing to pay
the contractor to install it properly. With an existing forced air system,
have the ducts sealed before upgrading the equipment (see "Diagnosing
Ducts," HE Sept/Oct '93, p. 26).
Figure 1. In a forced-air system, heat is distributed
through the house by air ducts. Supply ducts bring hot air to the living
space while return ducts return house air to the furnace.
Instead of circulating warm air throughout a
home, a hydronic system pumps hot water from a boiler through pipes and
radiators and then returns it to be reheated and recirculated.
Hot-water heating systems once used large wrought-iron
pipes and massive cast-iron radiators, but for many years now installers
have been using smaller copper piping, slim baseboard heaters, and smaller,
more efficient boilers (see Figure 2). Recently, plastic
piping has been approved for use in some areas as an alternative to the
more expensive copper pipes.
Some advantages of hydronic systems are the ability
to regulate the temperature in each room and to use the same boiler for
domestic hot water. However, the installed cost of hydronic systems is
higher than that of forced-air systems, they can be slow to warm up, and
there is no capability for central air conditioning, air filtering, or
ventilation.
It is usually more economical to stay with an
existing distribution system than to switch to another, unless major modifications
or repairs are needed. However, some homeowners with electric baseboard
heating and high electricity rates will find that a switch to a central
forced air or hydronic system is worth the investment. Also, if central
air conditioning is already installed or if it is planned for the house,
a forced air heating system is a sensible choice for heating, since the
heating and cooling systems can share the ductwork.
Figure 2. A hydronic system uses hot water to distribute
heat through the house. The boiler can be gas, oil, or electric.
Interpreting Energy Efficiency Ratings
All fuel-burning systems (natural gas, oil, propane,
wood) lose some heat in the combustion process. Heat is carried away in
the combustion gases and in the warm air drawn out of the house through
a chimney or vent. The efficiency of the furnace or boiler is the percentage
of heat output that is used in heating the home. For example, if 80% of
the heat value in the fuel is transferred to the house and 20% is lost,
the furnace will have an AFUE (Annual Fuel Utilization Efficiency) of 80%.
Note that this efficiency rating does not include heat losses from leaky
ducts.
Electric space heating equipment using resistance
heating is typically defined as being 100% efficient, because all of the
electrical energy used is converted into heat and there are no combustion
losses through a chimney. The efficiency rating of the equipment does not
take into consideration the losses that took place in the production of
the electricity and its transportation to the house, which account for
its relatively high unit cost.
Heat pumps, another type of electric heating
system, can have efficiencies higher than 100%, since they transfer and
upgrade heat from the outside air or ground, thereby increasing their heat
output without losses. A heat pump's efficiency is given as HSPF (Heating
Seasonal Performance Factor). Dividing the HSPF by 3.413 yields a seasonal
Coefficient of Performance, which is similar to the AFUE for a furnace.
AFUE and HSPF measure seasonal efficiency, which
takes into consideration not only the normal operating losses, but also
the fact that most furnaces rarely run long enough to reach their steady-state
efficiency, particularly during the milder weather at the beginning and
end of the heating season. Steady-state efficiency measures the maximum
efficiency a furnace achieves after it has been running long enough to
reach its peak level operating temperature. This is an important standardized
testing procedure that is also used by a service person to adjust the furnace.
Be sure to look for the seasonal efficiency--the
AFUE or HSPF rating is most useful to a homeowner because it provides a
good indication of how much annual heating costs will be reduced either
by improving existing equipment or by replacing it with a higher efficiency
unit.
Calculating Costs
Capital costs of heating systems can be as low as
$500-$1,000 for baseboard heaters in a small house, and as high as $12,000
for a heating, cooling, and water-heating ground source heat pump in a
large home. Heating contractors can give an estimate of the installed capital
cost of various systems. Certain energy sources may also have additional
ongoing charges for administration, connection, and so forth.
The Heating Cost
Worksheet and
Heating Equipment Guide can help
narrow the choices for either upgrading or replacing a heating system.
Since there are no furnace stores where the different makes and models
can be examined and compared, it will be necessary to obtain manufacturer's
literature from distributors or heating firms. The local utility can provide
information on the cost of purchasing or installing heating systems, and
the estimated seasonal heating costs of different equipment.
It is important to hire a contractor who will
size and install the equipment properly, so that it will operate efficiently.
Before deciding to buy, obtain firm, written bids from several companies
on both the cost of upgrading existing equipment and the cost of buying
and installing a complete new unit along with any other fittings and adjustments
required. These should include changes to ductwork or piping and a final
balancing of the heat supply to the house.
To get an estimate of how many years it will
take for the investment to pay off, divide the costs of buying new equipment
by the energy savings per year calculated in the worksheet. Once it does,
all the additional energy savings are money in the bank. That's one kind
of payback. The other is that from now on winters will be more comfortable,
and annual heating bills will set new records--for low costs!
sidebar
Understanding Energy Source Measurements
All types of heating systems come complete with
their own jargon. This short lesson in heating terminology will make comparison
shopping and the worksheet easier.
The amount of heat a piece of equipment can deliver
is called its capacity, while the amount of energy actually used is called
consumption. Heating units are manufactured and sold by their capacity;
the monthly bills customers receive are for consumption.
Natural Gas. The heating capacity of gas
heating appliances is measured in British thermal units per hour (Btu/h).
(One Btu is equal to the amount of energy it takes to raise the temperature
of one pound of water by 1 degree Fahrenheit.) Most heating appliances
for home use have heating capacities of between 40,000 and 150,000 Btu/h.
In the past, gas furnaces were often rated only on heat input; today the
heat output is given.
Consumption of natural gas is measured in cubic
feet (ft3). This is the amount that the gas meter registers and the amount
that the gas utility records when a reading is taken. One cubic foot of
natural gas contains about 1,007 Btu of energy. Utilities often bill customers
for therms of gas used: one therm equals 100,000 Btu.
Propane. Propane, or liquefied petroleum
gas (LPG), can be used in many of the same types of equipment as natural
gas. It is stored as a liquid in a tank at the house, so it can be used
anywhere, even in areas where natural gas hookups are not available. Consumption
of propane is usually measured in gallons; propane has an energy content
of about 92,700 Btu per gallon.
Fuel Oil. Several grades of fuel oil are
produced by the petroleum industry, but only #2 fuel oil is commonly used
for home heating. The heating (bonnet) capacity of oil heating appliances
is the steady-state heat output of the furnace, measured in Btu/h. Typical
oil-fired central heating appliances sold for home use today have heating
capacities of between 56,000 and 150,000 Btu/h.
Oil use is generally billed by the gallon. One gallon of #2 fuel oil
contains about 140,000 Btu of potential heat energy.
Electricity. The watt (W) is the basic
unit of measurement of electric power. The heating capacity of electric
systems is usually expressed in kilowatts (kW); 1 kW equals 1,000 W. A
kilowatt-hour (kWh) is the amount of electrical energy supplied by 1 kW
of power over a 1-hour period. Electric systems come in a wide range of
capacities, generally from 10 kW to 50 kW.
When converted to heat in an electric resistance heating element, one
kWh produces 3,413 Btu of heat.
This article is part of a series om energy-effiecient
remodeling, which is being funded by the Environmental Protection Agency
and the Department of Energy.
sidebar
HEATING EQUIPMENT GUIDE
Natural Gas and Propane Equipment
Conventional Warm-Air Furnaces
A basic, conventional natural gas-fired warm-air
furnace is shown in Figure 3. The furnace has a naturally
aspirating burner, which means that air for combustion is drawn in from
the surrounding area by the natural forces of hot air rising. The gas and
air burn, forming combustion gases, which give up heat across a heat exchanger
and are exhausted to the outside via a flue pipe and vent. A dilution device,
known as a draft hood, isolates the burner from outside pressure fluctuations
by pulling varying quantities of heated house air into the exhaust. A circulation
fan passes house air from the return ducts over the furnace heat exchanger.
The warmed air then flows into the ductwork for distribution around the
house. These older natural gas systems usually have seasonal efficiencies
of about 60%.
A minor improvement in efficiency comes with
adding a vent damper in the flue exhaust. By closing off the vent during
the off cycle, the damper prevents some of the warm household air from
being drawn up the flue and lost to the outdoors. These furnaces usually
have an electric or electronic ignition. Fuel savings are generally in
the 3%-9% range, relative to a conventional furnace. However, some of this
savings potential can be lost if a conventional gas-fired water heater
is connected to the same chimney. The water heater is still vented and
now has a higher draft imposed upon it, increasing the heat loss through
the water heater.
Figure 3. Conventional gas-fired warm air furnace
with vent damper.
Neither of the previous furnace types meets
current U.S. or Canadian minimum standards for energy efficiency. So those
planning to replace an existing system with a new gas furnace will choose
one of the mid-efficiency or high-efficiency units discussed below. The
seasonal efficiency (AFUE) of the furnace will be listed on an energy guide
label.
The combustion of natural gas produces heat and
some by-products, including water vapor and carbon dioxide. In a conventional
gas furnace, such by-products are vented through a chimney, where a considerable
amount of heat (both in the combustion products and in heated room air)
also escapes. The newer designs have been modified to increase energy efficiency
by reducing the amount of heated air that escapes, and by extracting more
of the heat contained in the combustion by-products before they are vented.
Mid-Efficiency Gas Furnaces with Induced-Draft Fan
Mid-efficiency gas furnaces usually have a naturally
aspirating burner like conventional units. They do not have a continuous
pilot, however, and instead of a draft hood, they are equipped with a powered
exhaust--usually a built-in induced draft fan. They save 15%-25% of the
energy used by conventional gas furnaces.
One word of caution: do not buy a mid-efficiency furnace that is more
than 82% efficient. These systems often have condensation problems in the
furnace or venting system. There is also some concern about the longevity
of high temperature plastic pipe used to vent many of these mid-efficiency
units. For higher efficiency, get a condensing furnace.
High-Efficiency Condensing Gas Furnaces
Condensing gas furnaces (see Figure
4) are the most energy efficient furnaces available, with seasonal
efficiencies between 90% and 96%. They are called condensing furnaces because
the combustion gases are cooled to the point where the water vapor condenses,
releasing additional heat into the home. The resulting liquids (condensate)
are piped to a floor drain. Because the flue gas temperature is low, plastic
piping can be used for venting out the side wall of the house.
Figure 4. High-efficiency condensing gas furnace.
Propane
In general, the same technologies and comments
apply to propane as to natural gas, with slight differences in the efficiencies.
Propane has a lower hydrogen level than natural gas. About 3% less energy
is tied up in the form of latent heat with propane systems than with natural
gas. This means that conventional and mid-efficiency propane furnaces can
be expected to be slightly more efficient than comparable natural gas units.
On the other hand, propane's lower hydrogen content makes it more difficult
to condense the combustion products, so that propane-fired condensing furnaces
will be 2%-3% less efficient than the same unit fired with natural gas.
Boilers
Gas-fired boilers use either a power burner or the
same type of burner as furnaces. A circulating pump pushes heated water
through the pipes and the radiator system. Conventional boilers have seasonal
efficiencies of about 60%.
Condensing gas-fired boilers in hydronic heating systems can have difficulty
condensing in practice, because the return water temperature is above the
dew point of the flue gases. By installing a water-to-water heat exchanger
and storage tank upstream of the boiler, the return water temperature can
be brought below the dew point, flue gases will condense, and efficiency
will improve significantly.
Sealed-Combustion (Direct Vent) Systems
Heating costs may be lowered slightly by reducing
the amount of combustion air drawn from inside the house. One way to do
this is to use outside air, brought in through piping directly to the burner.
This is known as sealed combustion or direct vent and can prevent backdrafting--hazardous
flue gas spillage--caused by exhaust fans or conventional fireplaces in
airtight homes. It can also prevent depressurization of the house caused
by the furnace itself.
sidebar
Oil Equipment
An oil furnace is similar to a natural-gas furnace,
but the dilution device is a barometric damper--a plate that acts as a
valve on the side of the flue pipe. The damper isolates the burner from
changes in pressure at the chimney exit by pulling varying quantities of
heated room air into the exhaust. In many houses, the quantity of air drawn
through the barometric damper is much greater than the quantity required
for combustion and can represent 10% to 15% of the total heat loss in the
house. The burner is a high-pressure gun type, with a blower fan to help
mix the oil and air for good combustion. A conventional oil furnace with
a cast-iron head burner has a seasonal efficiency about 60%. Replacing
the conventional burner with a flame retention head burner will save 10%-15%
on the fuel bill.
Figure 5. Mid-efficiency oil furnace.
Mid-Efficiency Oil Furnaces
Many noncondensing mid-efficiency oil furnaces use
an even more efficient high-static retention burner (see
Figure
5). This type of furnace also features an improved low-mass combustion
chamber (usually ceramic fiber) and passes the hot combustion gases through
a superior heat exchanger that enables the circulating house air to extract
more heat. The barometric damper, with its large requirement for house
air to dilute the combustion gases, has been eliminated in the most efficient
of these designs.
Benefits of a good mid-efficiency furnace are
much lower combustion and dilution air requirements as well as more power
to exhaust the combustion products (both advantages in new, tighter housing);
a safety shutoff in case of draft problems; and a more effective venting
system.
Mid-efficiency oil furnaces can have seasonal efficiencies of 85%-89%
and use 25%-30% less fuel than a conventional oil furnace producing the
same amount of heat.
Condensing Oil Furnaces
While a natural-gas condensing furnace has a significant
efficiency advantage over a mid-efficiency gas furnace, a condensing oil
furnace is only marginally more efficient than a well-designed mid-efficiency
oil furnace. Oil produces only half the water vapor of gas, and so has
much less energy tied up in the form of latent heat; the furnace must work
harder to condense less. In addition, the condensate is much more corrosive
than with natural gas, so the condensing oil heat exchanger must be made
of special materials. For these reasons, a mid-efficiency oil furnace is
a better bet than a condensing oil furnace.
Sealed Combustion
Some newer oil furnaces have optional sealed combustion
to save a bit more energy and prevent backdrafts and spillage. However,
on a very cold winter's day, if the air is not warmed before reaching the
burner, it could cool the fuel oil in some units, causing start-up problems.
Oil Boilers for Hydronic Systems
An oil-fired boiler uses the same types of burners
as an oil-fired forced-air furnace, although the boiler itself is often
somewhat smaller and heavier (see Figure 6). There
is no circulating fan and filter housing as there is in a forced-air furnace.
Instead, most boilers require a circulating pump to push hot water around
the house through the pipes and the radiator system. The seasonal efficiency
of old conventional boilers is similar to that of conventional furnaces
(about 60%).
Figure 6. Oil-fired boiler.
Oil System Upgrade
There are many ways to improve the efficiency of an old oil boiler or furnace.
If the system is oversized (a properly sized oil burner should run 45 to
50 minutes per hour when the temperature outside is the lowest expected
for the area), simply replacing the existing oil burner nozzle with a smaller
one can downsize it. Reduct the nozzle only one size on a conventional
cast-iron head burner, so as not to reduce the firing rate too much. Do
not reduce the size below the minimum firing rate given on the manufacturer's
rating plate.
Flame retention head burners do a much better job of mixing the air
and fuel than old cast-iron head burners. They are now almost standard
on new furnaces and can also be added to most older furnaces. Replacing
the burner can increase the seasonal efficiency of an old oil-fired furnace
by about 15%. Use of a new high-static burner can give even greater savings
and better performance. The nozzle should be reduced at least one size
or even more, and a ceramic fiber combustion chamber liner should be used.
(An experienced contractor should check the system's chimney or flue whenever
any change is made to the system.)
sidebar
Electric Equipment
Most electrically heated homes in North America
use electric baseboards. Baseboards containing electric resistance heating
strips are installed in each room and are individually controlled. Electric
baseboards are cheap to install but expensive to run, although if used
to heat only occupied rooms, they can be less costly than a central electric
furnace.
Central Electric Furnace
In a central electric furnace, house air is blown over electric heating
coils and then distributed through ductwork around the house. This type
of furnace is much simpler than a fuel-fired one, because no combustion
air or exhaust is needed. These units therefore have an efficiency rating
of 100%, meaning that all of the heat created goes into heating the house
air. However, this figure can be misleading. A lot of energy is lost producing
and transporting electricity to the house, and it shows up on the energy
bill. As with all forced-air systems, leaky ducts in a poor distribution
system can lead to hefty additional heat losses. Electricity rates vary,
but in most places a central electric furnace is the most expensive type
of heating system to run.
Heat Pumps
Many forced-air systems use a heat pump instead of an electric furnace
because of its high efficiency and capability to air-condition. A heat
pump is an electrical device that extracts heat from one place and transfers
it to another (see
Figure 7). It transfers the heat
by circulating a refrigerant through a cycle of alternating evaporation
and condensation. A compressor pumps the refrigerant between two heat exchanger
coils. In one coil, the refrigerant is evaporated at low pressure and absorbs
heat from its surroundings. The refrigerant is then compressed en route
to the other coil, where it condenses at high pressure. At this point,
it releases the heat it absorbed earlier in the cycle.
Figure 7. An air-source heat pump during the heating
cycle. Heat pumps use a refrigerant to transfer heat from outside into
the house.
A heat pump can be used for both heating and
cooling. In the summer, it acts as an air conditioner, removing heat from
the air inside the house and transferring it outside. In the winter, the
heat pump operates in reverse, removing heat from the outside air or ground,
and transferring it inside the house. Residential heat pumps are divided
into two major groups: air source (air-to-air) systems, which draw heat
from the air, and ground source (earth energy) systems, which draw heat
from the ground or underground water.
Air Source. Air source heat pumps can
be either add-on, all-electric, or bivalent. Add-on heat pumps are designed
to be used with another source of supplementary heat, such as a fuel-fired
furnace. All-electric air source heat pumps come equipped with their own
supplementary heating system in the form of electric-resistance heaters.
Bivalent heat pumps are a special type, developed in Canada, that use a
gas- or propane-fired burner to increase the temperature of the air entering
the outdoor coil. This allows these units to operate efficiently at somewhat
lower outdoor temperatures. A problem with most air source heat pumps is
that the heat output (and efficiency) drops with colder outside temperatures,
exactly the opposite of what the house requires.
Ground Source. A ground source heat pump
uses the relatively constant temperature of the earth or groundwater or
both as a source of heat in the winter (see Figure 8).
This allows it to maintain its output in cold weather and makes it more
efficient than an air source heat pump, which must work harder as the air
temperature drops. In ground source pumps, heat is removed from the earth
through a liquid, such as groundwater or an antifreeze solution, upgraded
by the heat pump, and transferred to indoor air. During the summer months,
the process is reversed: heat is extracted from indoor air and transferred
to the earth through the groundwater or antifreeze solution.
Heat Pump Efficiency. Heat pump efficiency
is measured separately for the cooling and heating cycles. For cooling,
the Seasonal Energy Efficiency Ratio (SEER) of an air source heat pump
ranges from a minimum of 9 to a maximum of about 16. The Heating Seasonal
Performance Factor (HSPF) for the same units ranges from a minimum of 5.9
to a maximum of 8.8. The SEER of a ground source heat pump ranges from
11 to 17, and the HSPF ranges from 8.3 to 11.6.
At the lower end of the product range, both air
and ground source heat pumps have single-speed reciprocating compressors.
Heat pumps with the highest SEERs and HSPFs invariably use variable or
two-speed scroll compressors.
A homeowner who has an electric furnace and wants
to stay with electricity as an energy source may be able to reduce heating
costs by up to 50% by converting to an air source heat pump, and by 65%
by converting to a ground source heat pump. Actual dollar savings will
vary depending on factors such as local climate, the efficiency of the
current heating system, the cost of electricity, and the size and HSPF
of the heat pump installed.
Figure 8. Piping for a ground source heat pump is
usually run vertically in a deep trench (shown here) or horizontally in
a shallow trench if there is enough space on the property.
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Integrated Systems for Space and Water Heating
In many houses that have been well insulated and air sealed, the space-heating
load is so low that it is difficult to justify the expense of a high-efficiency
furnace. If it's time to replace the water heater as well as the space-heating
system, integrating space and water heating in a single appliance can result
in high efficiency and reduced costs.
An integrated condensing gas-fired space- and water-heating system can
have an efficiency of over 90% for both applications. Space heating can
be hydronic or forced-air. The overall purchase and installation cost may
even be lower than for individual appliances, and the efficiency is high.
Mid-efficiency gas-fired combined systems also exist, but their overall
efficiency potential is lower than that of the condensing systems. Some
integrated systems have even used a conventional gas-fired water heater
as the basic energy generator, connecting it to a fan coil for warm air
heating; however the result is a system with quite low overall efficiencies.
Mid-efficiency oil-fired integrated systems, based on a low-mass boiler,
high static burner, and external water storage tanks, offer high seasonal
efficiencies--83%-84% for both space and tap water heating.
sidebar
Room-by-Room Heating
Many people use small space heaters to heat their most-used rooms, so that
electric baseboard heaters or an inefficient central heating system can
be used sparingly.
Some space heaters can also be very effective radiant heat sources,
warming solid bodies (like people) in their line-of-sight without necessarily
having to heat up all the air. Good examples are the new direct-vent gas
fireplaces, advanced combustion wood fireplaces and portable electric infrared
radiant heaters. (Never use an unvented combustion appliance in a tight
house.) If properly located in a major living space, a radiant space heater
can lower the overall heat demands of the house, while making the occupants
feel more comfortable.
Direct-Vent Wall Furnaces
Direct-vent gas wall furnaces are self-contained heating appliances that
draw in combustion air and discharge by-products through a vent to the
outside. They are permanently attached to the structure of a building and
are not connected to ductwork. These units circulate heated air by gravity
or with the help of a circulating fan. Furnaces with circulating fans are
usually more efficient.
Wall furnaces are compact and less expensive than central furnaces,
but they are generally less efficient. They come in a variety of heating
capacities and with efficiencies that range from 70% for a standard efficiency
unit with a pilot light to 82% for a mid-efficiency unit with electric
ignition and induced draft.
Freestanding Room Heaters
Room heaters are self-contained and have heat outputs much lower than those
of central furnaces. Often they resemble new, freestanding wood stoves
and are fired by wood, wood pellets, oil, or gas. They are not connected
to ductwork. A vent pipe allows combustion by-products to escape to the
outdoors. Heat is circulated by radiation, natural convection, or with
the aid of a circulating fan. Standard and mid-efficiency units are available
with AFUE ratings between 60% and 82%.
If electricity rates are low, an electric space heater can minimize
use of an inefficient central system. Portable electric room heaters are
available as convection, radiant, and fan-assisted units, ranging from
500W to 1500W capacity. These can be very expensive to run at typical electricity
rates.
Advanced Efficient Fireplaces
While conventional fireplaces are extremely inefficient and can cause serious
indoor air quality problems, new advanced wood- and gas-fired designs offer
safe, efficient, attractive alternatives (see "Fireplaces: Studies in Contrasts,"
HE Sept/Oct '94, p. 27). Gas fireplaces have simulated fire logs and flames,
which are visible through glass doors. They are similar to room heaters,
except that they include a number of decorative touches. They have the
potential to provide good, efficient heating. However, many models don't
live up to the sales literature. Look for direct-vent gas fireplaces with
radiation-transparent pyro-ceramic glass, intermittent ignition, good heat
transfer to the house, an insulated outer casing, and an effective venting
system to ensure safe removal of the combustion products.
Ductless Minisplit Heat Pumps
A new type of heat pump, called a ductless minisplit, is ideal for retrofit
to homes with hydronic or electric-resistance baseboard heating in areas
where electricity costs are reasonable. Inside, wall-mounted units can
be installed in individual rooms, all served by one outdoor section. This
is less expensive than installing a central heat pump with ducts, or putting
individual wall or window heat pumps in each room.
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