How to Save on Fuel Bills: Combining a Heat Pump with an Existing Furnace
By Don Steeby, author of Alternative Energy: Sources and Systems
With ever-increasing fuel costs, it only makes sense to investigate new ways to
keep our houses warm every winter. One method is to combine an air-to-air heat pump
with an existing furnace. This concept is not a new one, but lately it has become more
popular — especially if the existing furnace is using propane or fuel oil.
Air-to-Air Heat Pumps: The Basics
Air-to-air heat pumps have been around for a long time and became popular in the
1970s as a way to save on fossil fuel during the first Arab Oil Embargo. The way a heat
pump works is relatively simple — a refrigeration system transfers heat from the outside
air into the house. This is the same way a residential air conditioner works, only in
reverse. The problem with early heat pumps was that they were not very effective in
colder climates. The point of diminishing return was generally somewhere between 40
and 50 degrees F outside air temperature. In other words, when the outside air dropped
below 50 degrees F, the heat pump did not have enough heating capacity to transfer as
many BTUs (British thermal units) from outside to sustain a warm-enough indoor air
temperature. At these lower temperatures, the heat pump would rely on an auxiliary heat
source to keep the house warm. This was usually electric resistance heat, and the
homeowner’s heating bill went sky high.
Fortunately the residential heat pump has drastically improved since those early
days. Today’s heat pump utilizes a revolutionary new type of refrigerant known as R410A. This refrigerant contains a much higher heating capacity per pound than the earlier
refrigerants used in older model heat pumps. In addition, R-410A is friendlier to the
environment because it does not deplete the ozone layer and has a much lower global
warming potential than its predecessor.
Equipment Selection
Before we go any further, it should be noted that any heating appliance is only as
good as the structure it is heating. Many times the equipment is blamed for poor heating
when in reality the integrity of the structure is to blame. In other words, make sure the
home is properly insulated and doors and windows are properly sealed before proceeding
with equipment selection. To properly size the equipment, it is imperative that a
comprehensive heat load analysis is performed on the home. This will ensure that not
only is the equipment correctly sized, but it will also alert the homeowner to any
deficiencies that may exist in the structure, such as inadequate insulation. There are a
number of quality software programs available that will produce accurate residential heat
load calculations.
Once the proper load requirements are determined, there are several rules that
should be followed when selecting equipment. Typically a heat pump is sized for the
cooling load. This is done to ensure that the system is not oversized, which will result in
short-cycling and poor cooling in the summertime. However, consider selecting
equipment that has two-stage capabilities: this feature will allow the equipment to be
slightly oversized for cooling, and allow for greater capacity during the heating season. A
two-stage unit will operate at a lower BTU output when there is only a partial demand for
heating, such as when the outdoor air temperature is milder but the home still requires
some heat to stay comfortable. In addition, two-stage heat pumps will keep the home
more comfortable while saving energy, because the heat pump won’t have to work as
hard when it’s operating at the lower stage. In the summertime, the two-stage unit will
adjust to the cooling demand without short-cycling. A number of manufacturers offer not
only two-stage equipment, but also heat pumps that operate with variable speed
components. Look for units that carry a higher SEER (seasonal energy efficiency ratio)
value for cooling and a higher COP (coefficient of performance) rating for heating. This
will ensure that your heat pump will operate more efficiently than units with lower
ratings, thus saving money.
Heat Pump Installation
In many cases, the operation of the heat pump is only as good as its installation.
As with any type of refrigeration equipment, proper installation practices are a must.
These include following the correct methods for brazing tubing, proper evacuation
techniques, and by following the manufacturer’s requirements for system startup. In
addition to these practices, there are other items that should be noted. For instance, it is
recommended that the outdoor unit be elevated above the ground. Remember that the
heat pump will be running during the winter months when there is snow on the ground.
Mounting the outdoor unit several inches above the ground will help prevent snow from
gathering around the coils which can limit the amount of air flow, thus reducing its
efficiency. If the project involves replacing an existing R-22 air conditioning system,
another consideration is the replacement of the refrigeration line set. If possible, the old
line set should be replaced to prevent contamination of the new system by oil remaining
in the lines of the old system. If replacing the existing line set is impossible, there are kits
available that will chemically flush out the old lines and make them suitable for R-410A
refrigerant.
Another R-22 retrofitting issue to consider is that the indoor coil also must be
replaced. When this happens, modifications to the existing ductwork will more than
likely be required. Sometimes the best practice is to simply replace the existing furnace
plenum with a new one that matches the dimensions of the new indoor coil. Also, check
to see that the existing ductwork is capable of handling the airflow requirements of the
new heat pump. The rule of thumb for adequate air flow through the existing duct is 450
cubic feet per minute per ton of refrigeration. It will be easier to modify the ductwork
during installation of the new heat pump rather than several months after the new system
has been running. Also, be sure to insulate and seal any ductwork that passes though
crawl spaces or attics.
The controls portion of the system also needs to be consider during heat pump
installation. When retrofitting systems, it may become necessary to interface the
thermostat of one manufacturer with the heat pump of another and with a furnace made
by someone else. This balancing act sometimes requires that additional control
components are installed in order to make the individual units function as one system.
One such control issue to take into consideration is the defrost cycle. During the defrost
mode, the furnace needs to be energized for supplemental heat so the house does not cool
down too drastically. To accomplish this feat, the installer may need to find an available
electrical terminal on the control board of the outdoor unit that will only be energized
during the defrost mode. This terminal can be interlocked with a standard relay that can
in turn energize the furnace’s heating circuit. In addition to defrost controls, an outside air
sensor will need to be installed and wired to the thermostat. This informs the heat pump
to shut off if it gets too cold outside. This same sensor will automatically tell the
thermostat to change over to furnace heat and shut out the heat pump circuit until it
warms up again outside.
Overall Performance and Savings
So how well can a heat pump perform? This depends upon a number of factors,
including correct sizing and installation and how well the building is insulated. If the heat
pump is sized correctly for heating, there is no reason why the homeowner can’t run the
equipment until the outside air temperature drops to 20 degrees F. This feature can result
in an annual fossil fuel cost savings of up to 30 to 40%. To do a quick calculation on the
equipment’s payback, divide the total equipment cost (including installation) by the
dollar amount of fuel saved for one year. This figure will equal the number of years that it
will take to recover the cost. Of course, the savings will vary based on how cold the
winter is and whether the cost of fuel continues to go up.
One last item that should be mentioned is that heat pumps generally deliver a
heated air temperature that is cooler than a conventional furnace. This becomes an initial
concern to the homeowner who traditionally enjoys that blast of hot air every time the
conventional furnace kicks on. This cooler heat pump airflow can sometimes be
associated with drafty conditions. Therefore it is important that the air delivery system is
properly balanced after the new heat pump has been installed and started up in order to
maximize comfort levels within the home. It should be noted that this lower-than-normal
discharge air temperature can actually benefit the home’s overall wintertime comfort.
This is because of two reasons. The first reason is that the air delivered by the heat pump
can mix more readily with the room temperature, because there is little difference
between the two temperatures. This mixing effect eliminates any cold spots. Secondly,
heat pumps have longer run cycles than conventional furnaces. This may at first seem
like a waste of energy, but remember that the greatest cost in electricity occurs when the
equipment starts up. Less starting and stopping means less energy consumed. Also, these
longer run cycles will reduce the amount of temperature swing that can occur between
heating cycles with a conventional furnace. By following these guidelines, an air-to-air
heat pump combined with a traditional fossil fuel furnace can offer greater energy
savings than conventional systems while maintaining a comfortable home environment.