Overview
Conventional heat pumps have
been heating and cooling homes
for decades. In fact, about one in
three homes in the United States already
uses one. However, there aren't many
north of the Mason-Dixon Line because
they don't work efficiently in subfreezing
temperatures. These heat pumps are great
at pumping heat in or out of a house in
moderate temperature swings like those
found in the Sun Belt. But they're notoriously
inefficient and expensive to run in
cold Northern winters.
“Dual-fuel” heat pumps are different.
Attached to your existing furnace, this system
looks (and works, during the summer)
like a high-efficiency central air conditioner.
However, in those mild spring
and fall months in the snowbelt, they provide
cheap heat just as well as they do in
the South. As the temperatures drop, the
pump shuts off and tells your furnace to
take over.
A dual-fuel heat pump, such as
Lennox's XP-14, will cost about 20 to 25
percent more (including installation)
than an A/C. Depending on your region
and fuel costs, however, it can pay for itself
in five or six years.
Heating With a Dual-Fuel Heat Pump
As long as the temperature is
above 35 degrees F or so, a heat
pump can pull heat from the outside
air for less than it costs to fire
up the furnace. The furnace kicks
in for only the coldest months.
Heat pumps save energy
because transferring heat is easier
than making it. Surprisingly, even
when it feels cold outside, there is
still a decent amount of heat waiting
to be pumped. Under ideal
conditions, a heat pump can
transfer 300 percent more energy
than it consumes. In contrast, a
high-efficiency gas furnace is
about 90 percent efficient.
Check Into Tax Credits
In addition to saving money in the
long run, a dual-fuel heat pump
might pay you back right away.
This upgrade may qualify for an
energy-savings tax credit plus
rebates. Ask your utility company
and HVAC installer about available
incentives in your area.
Pumping cheap heat out of thin air
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Figure A: Cool-Period Operation
During cooler seasons like fall and spring, the heat pump handles the heating duties.
The dual-fuel system still incorporates the furnace, but without using any burners,
just its air distribution features. The heat pump sends hot “refrigerant” through the
air conditioning coil within the furnace. The furnace fan draws air from the home's
cold-air returns and blows that air over the warm coil and then sends the warmed
air throughout the house. In the summer, the heat pump reverses the refrigerant
flow so cold liquid flows over the coil and cools your home just like a conventional
air conditioner.
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Figure B: Cold-Period Operation
During cold periods, the heat pump slumbers while the furnace takes over, supplying
the heat by operating like the conventional furnace it is. The burners ignite the
fuel (oil, propane or natural gas) and the furnace fan blows air drawn from the
cold-air returns and warms it by blowing it over the heat exchanger and on
throughout the home.
As shown in Figure A, an air-source heat
pump is basically a hybrid air conditioner.
Both have a compressor (a high-pressure
pump) that circulates refrigerant (a volatile
gas) through indoor and outdoor coils, a
network of tubes designed to facilitate the
capture and release of heat. But while an air
conditioner can move refrigerant in only
one direction, a heat pump can force
refrigerant in either direction, for heating
one way and cooling the other. The pump
does this by means of an extra diverting
device called a switchover valve.
Run the numbers
To figure out whether a heat pump is
practical for your home, you'll need to
contact a heating contractor and ask a
few questions:
1. Start with a Heating and Cooling Load
Analysis. Don't trust the label on the old
furnace; ask your installer to show you
the math. According to some reports,
there's a good chance that your system
may not have been sized correctly
in the first place. Even if it
was sized properly originally, subsequent home
improvement projects (new
insulation, new windows or an
addition) can change your
heating and cooling needs.
2. Check the numbers. Manufacturers
use different technologies,
but one number can
provide an apples-to-apples
comparison. For cooling efficiency,
check the Seasonal
Efficiency Ratio (SEER). The
higher the SEER, the more efficient
the unit is. Of course,
units with higher ratios cost
more, but every two points
can reduce cooling costs by
about 15 percent. (Energy
Star–rated pumps are at least 8
percent more efficient than
standard models.)
3. Conduct a comparative cost
analysis. If you live in an area
with lower-priced natural gas
and sky-high electrical rates,
a heat pump will not pay itself
off as quickly. Your installer
can factor in local energy rates
(including peak and off-peak
electrical rates) to calculate
your potential savings and
payback.
4. Ask about compatibility.
Dual-fuel heat pumps are
designed to work as a straightforward
A/C replacement, but
older furnaces probably won't
work with a new switch-hitting
system. You'll probably have to
upgrade to a brand new furnace
to have this system—adding a sizable chunk to the
cost of the project.
