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Geothermal Heat Pumps
What are they and how do they work?
Q: What is a geothermal heat
pump?
A
geothermal heat pump is an electrically powered device that uses the natural
heat storage ability of the earth and/or the earth’s groundwater to heat
and cool your home or business.
Q. How does it work?
Like
any type of heat pump, it simply moves heat energy from one place to another.
Your refrigerator works using the same scientific principle. By using the
refrigeration process, geothermal heat pumps remove heat energy stored in the
earth and/or the earth's groundwater and transfer it to the home.
Q. How is the heat transferred
between the earth and home?
The
earth has the ability to absorb and store heat energy from the sun. To use that
stored energy, heat is extracted from the earth through a liquid medium
(groundwater or an anti-freeze solution) and is pumped to the heat pump or heat
exchanger. There, the heat is used to heat your home. In summer the process is
reversed and indoor heat is extracted from your home and transferred to the
earth through the liquid.
Q. You mentioned heating and
cooling - does it do both?
One of
the things that makes a heat pump so versatile is it's
ability to be both a heating and cooling system in one. You can change from one
mode to another with a simple flick of a switch on your indoor thermostats. In
the cooling mode, a geothermal heat pump takes heat from indoors and transfers
it to the cooler earth through either groundwater or an underground loop system.
Q. Do I need separate ground
loops for heating and cooling?
No.
The same loop works for both. All that happens when changing from heating to
cooling, or vise versa, is that the flow of heat is reversed.
Q. What types of loops are
available?
There
are two main types: open and closed. The next two sections will give you
specifics about each.
Q. Does the underground pipe
system really work?
The
buried pipe or "ground loop" is the most recent technical advancement
in heat pump technology. The idea to bury pipe in the ground to gather heat
energy began in the 1940's but its only been in the last few years that new
heat pump designs and improved pipe materials have been combined to make
geothermal heat pumps the most efficient heating and cooling systems available.
Geothermal Heat Pumps: Closed
loop systems
Q. What is a closed loop
system?
The
term "closed-loop" is used to describe a geothermal heat pump system
that uses a continuous loop of special buried plastic pipe as a heat exchanger.
The pipe is connected to the indoor heat pump to form a sealed, underground
loop through which an antifreeze solution is circulated. Unlike an open-loop
system that consumes water from a well, a closed-loop system re-circulates its
heat transferring solution within a pressurized piping system.
Q: Where can this loop be
located?
That
depends on land availability and terrain.
Most closed loops are trenched horizontally in yards adjacent to the
home. But any area near a home or business with appropriate soil conditions and
adequate square footage will work.
Q: How deep and long will my
trenches be?
Trenches
are normally four to six feet deep and up to 400 feet long, depending on how
many pipes are in a trench. One of the advantages of a horizontal loop system
is being able to lay the trenches according to the shape of the land. As a rule
of thumb, 500-600 feet of pipe is required per ton of system capacity. A well
insulated, 2,000 square foot home would need about a three-ton system with
1,500-1,800 feet of pipe.
Q: How many pipes are in a
trench?
Normally,
a run of pipe is laid at five feet then looped back over itself at three feet
once the bottom pipe is covered with soil. This allows more length of pipe to
be put in one trench and has no adverse affect on system efficiency. Other loop
designs use four or six pipes and allow for shorter trenches if land area is
limited.
Q: What if I don't have enough
room for a horizontal loop?
Closed-loop
systems can also be vertical. Holes are bored to about 120 - 150 feet per ton
of heat pump capacity. U-shaped loops of pipe are inserted in the holes. The
holes are then backfilled with a sealing solution.
Q. How long will the loop pipe
last?
Closed-loop
systems should only be installed using high density polyethylene or polybutylene
pipe. Properly installed, these pipes will last 50-75 years. They are inert to
chemicals normally found in soil and have good heat conducting properties. PVC
pipe should not be used under any circumstances.
Q: How are
the pipe sections of the loop joined?
The
only acceptable method to connect pipe sections is by thermal fusion. Pipe
connections are heated and fused together to form a joint stronger than the
original pipe. Mechanical joining of pipe for an earth loop is never an accepted
practice. The use of barbed fittings, clamps and glued joints is certain to
result in loop failure due to leaks.
Q: Will an earth loop affect
my lawn or landscape?
No.
Research has proven that loops have no adverse effect on grass, trees or shrubs.
Most horizontal loop installations use trenches about six inches wide. This, of
course, will leave temporary bare areas that can be restored with grass seed or
sod. Vertical loops require little space and result in minimal lawn damage.
Q: Can I reclaim heat from my
septic system disposal field?
No. An
earth loop will reach temperatures below freezing during extreme conditions and
may freeze your septic system. Such usage is banned in many areas.
Q: If the loop falls below
freezing, will it hurt the system?
No.
The antifreeze solution in the loop will keep it from freezing down to about 10
degrees F. Three types of antifreeze solution are acceptable: propylene glycol,
calcium chloride and methyl alcohol.
Q: Can I install an earth loop
myself?
It's
not recommended. In addition to thermal fusion of the pipe, good earth coil
contact is very important for successful loop operation. Non-professional
installations may result in less than optimum system performance.
Q: I have a pond near my home.
Can I put a loop in it?
Yes,
if it's deep enough and large enough. A minimum of six feet in depth at its
lowest level during the year is needed for a pond to be considered. In pond
loops, copper, polyethylene or polybutylene pipe can be used.
Geothermal heat pumps: Open
loop systems:
Q: What is an open-loop
system?
The
term "open-loop" is commonly used to describe a geothermal heat pump
system that uses groundwater from a conventional well as a heat source. The
groundwater is pumped into the heat pump unit where heat is extracted, and then
the water is disposed of in an appropriate manner. Since groundwater is a
relatively constant temperature year-round, it is an excellent heat source.
Q: What do I do with the
discharge water?
There
are a number of ways to dispose of water after it has passed through the heat
pump. The open discharge method is the easiest and least expensive. Open
discharge simply involves releasing the water into a stream, river, lake, pond,
ditch or drainage tile. Obviously, one of these alternatives must be readily
available and must possess the capacity to accept the amount of water used by
the heat pump before open discharge is feasible.
A
second means of water discharge is the return well. A return well is a second
well bore that returns the water to the ground aquifer. A return well must have
enough capacity to dispose of the water passed through the heat pump. A new
return well should be installed by a qualified well driller. Likewise, a
professional should test the capacity of an existing well before it is used as
a return.
Q: How much groundwater does
an open-loop system need?
Geothermal
heat pumps used in open-loop systems need differing amounts of water depending
on the size of the unit and the manufacturer's specifications. The water requirement
of a specific model is usually expressed in gallons per minute (g.p.m.) and is listed in the specifications for that unit.
Your heating contractor should be able to provide this information. Generally,
the average system will use ~10 g.p.m. while operating.
Your
well and pump combination should be large enough to supply the water needed by
the heat pump in addition to your domestic water requirements. You will
probably need to enlarge your pressure tank or modify your plumbing to supply
adequate water to the heat pump.
Q: What problems can be caused
by poor water quality?
Poor
water quality can cause serious problems in open-loop systems. Your water
should be tested for hardness, acidity and iron content before a heat pump is
installed. Your contractor or equipment manufacturer can tell you what level of
water quality is acceptable. Mineral deposits can build up inside the heat
pump's heat exchanger. Sometimes a periodic cleaning with a mild acid solution
is all that's needed to remove the build-up.
Impurities,
particularly iron, can eventually clog a return well. If your water has high
iron content you should be sure that the discharge water is not aerated before
it's injected into a return well.
Finally,
you should opt against using water from a spring, pond, lake or river as a
source for your heat pump system unless it's proven to be free of excessive
particles and organic matter. They can clog a heat pump system and make it
inoperable in a short time.
Q: Does an open-loop system
cause environmental damage?
No.
They are pollution free. The heat pump merely removes heat from or adds heat to
the water. No pollutants are added whatsoever. The only change in the water
returned to the environment is a slight increase or decrease in temperature.
Some
people are concerned that open-loop systems contribute to the depletion of our
ground water resources. This issue is not critical in some parts of
Q: Are there any laws that apply
to open-loop Installations?
In
some localities, all or parts of the installation may be subject to local
ordinances, codes, covenants or licensing requirements. Check with local
authorities to determine if any restrictions apply in your area.
Geothermal heat pumps: Parts of
the system
Q: What are the components of
a geothermal heat-pump system?
The
three main parts are:
1.
The heat-pump unit.
2.
The liquid heat exchanger medium (open or closed
loop).
3.
The air-delivery system (ductwork).
Q: Are all geothermal heat
pumps alike?
No.
There are different kinds of geothermal heat pumps designed for specific
applications. Many geothermal heat pumps, for example, are intended for use
only with higher temperature ground water encountered in open-loop systems. Others
will operate at entering water temperatures as low as 25 degrees F which are
possible in closed-loop systems.
Most
geothermal heat pumps provide summer air conditioning, but a few brands are
designed only for winter heating. Sometimes these heating-only systems
incorporate a groundwater cooled coil that can provide cooling in moderate
climates. Geothermal heat pumps can also differ in the way they are designed.
Self contained units combine the blower compressor, heat exchanger and coil in
a single cabinet.
Q: Will I have to add
insulation to my home if I install one of these systems?
Geothermal
heat pumps will reduce your heating and cooling costs regardless of how well
your home is insulated. However, insulating and weatherizing are key factors in
realizing the most savings from any type of heating and cooling system.
Q: Can a geothermal heat pump
also heat water for my home?
Yes. Using
what's called a desuperheater, some types of geothermal heat pumps can save you
up to 50 percent on your water heating bill by preheating tank water. Desuperheaters are standard on some units, optional on
others. Some geothermal models can provide all of your water needs on demand at
the same high efficiencies as the heating cooling cycles.
Q: Is a geothermal heat pump
difficult to install?
Most
units are easy to install, especially when they are replacing another
forced-air system. They can be installed in areas unsuitable for fossil fuel
furnaces because there is no combustion, thus, no need to vent exhaust gases.
Ductwork must be installed in homes that don't have an existing air
distribution system. The difficulty of installing ductwork will vary and should
be assessed by a contractor.
Q: Can a geothermal heat pump
be added to my fossil fuel furnace?
Q: I have ductwork, but will
it work with this system?
In all probability, yes. Your installing contractor should be
able to determine ductwork requirements and any minor modifications, if needed.
Q: Do I need to increase the
size of my electric service?
Geothermal
heat pumps don't use large amounts of resistance heat, so your existing service
may be adequate. Generally, a 200-amp service will have enough capacity, and
smaller amp services may be large enough in some cases. Your electric utility
or contractor can determine your service needs.
Q: Should I buy a heat pump
large enough to heat my home with no supplemental heat?
Your contractor
should provide a heating and cooling load calculation (heat loss, heat gain) to
guide your equipment selection. Geothermal heat pumps are sized to meet your
cooling requirements. Depending on your heating needs, a geothermal heat pump
will supply 80 to 100 percent of your design heating load. Sizing the heat pump
to handle your entire heating needs may result in slightly lower heating costs,
but the savings may not offset the added cost of the larger heat-pump unit.
Also, an oversized unit can cause dehumidification problems in the cooling
mode, resulting in a loss of summer comfort.
Q: Do geothermal heat pumps
have outdoor units?
No.
The equipment goes inside your home, usually in the basement, garage or crawl
space. Because it's indoors, the lifespan of the compressor and major
components are greatly extended, most having a lifespan of 20 years or more.
Geothermal heat pumps: What are
the major benefits?
Q: How efficient is a
geothermal heat pump?
They
are more than three times as efficient as the most efficient fossil fuel
furnace. Instead of burning a combustible fuel to make heat, they simply move
heat that already exists. By doing that, they provide three
units of energy for every one unit used to power the heat-pump system.
Q: What does a system like
this cost?
A
system for the typical home will cost more than if you bought a separate
furnace and central air-conditioning system. But you wouldn't really be
comparing apples to apples. To get an accurate comparison of costs you need to
consider the following:
•
Payback or how long it takes to recover the difference in costs between the two
systems using energy savings. Payback for most geothermal heat-pump systems
runs two to six years.
•
Energy efficiency of the two Systems. To get an accurate picture, make sure
efficiency claims are substantiated. Your lifestyle and how well your home is
insulated affect how economical a system will be.
•Total
operating savings from heating, cooling and domestic hot water must be combined
to get an accurate picture of total energy savings.
•Energy
costs and availability, both present and future.
•
Maintenance costs and system reliability • System lifespan.
Q: What about comfort?
A
geothermal heat pump system moves warm air (90 degrees - 105 degrees) throughout
your home via a standard duct network. Typically, a very even comfort level is
found throughout the home. This is because the warm air is moved in slightly
higher volumes and, therefore, saturates the home with warmth more evenly. This
helps even out hot or cold spots and eliminates the cold air blast common with
fossil fuel furnaces. It's also a great comfort to know that you've reduced
your energy consumption while using an inexhaustible energy source - the earth.
Q: Can I get a tax credit for
installing this system?
It
depends on where you live. Some states and provinces do have tax credits for
installing geothermal systems. Check with your electric utility, Department of
Commerce or Ministry of Revenue for further details.
Q: Which system is best, open
- or closed loop?
The
net results in operating cost and efficiency are virtually the same. Which
system to choose depends mainly on whether you have an adequate groundwater
supply and means of disposal. If you do, an open loop
can be used very effectively. If not, either a horizontal or vertical
closed-loop system is your best choice. Over a period of years, a closed-loop
system will require less maintenance because it's sealed and pressurized,
eliminating the possible build-up of minerals or iron deposits.
Questions you should ask about
a new heating system
Regardless
of the type of heating system you may be considering for your home or business,
there are specific questions you should ask the dealer installer. These
questions deal with finding out the actual efficiency of the system, any
operating limitations it may have, and the bottom line of operating costs. The
answers here are meant as a guide for what you should try to find out with your
questions.
Q: What is the Btu size of the
furnace that's being proposed?
Furnaces
are designed to provide specific amounts of heat energy per hour. The term 'Btuh’ refers to how much heat can be produced by the unit
in an hour. Before you can know what size furnace you'll need, you must have a
heat loss/heat gain calculation done on your home. From that, an accurate
determination can be made on the size of the heating system you'll need. Most
fossil fuel furnaces are substantially oversized for home heating requirements,
resulting in increased operating costs.
Q: Is the efficiency rating
actual or just a manufacturer's average?
All
types of heating and cooling systems have a rated efficiency. Fossil fuel
furnaces have a percentage efficiency rating. Natural gas, propane and fuel oil
furnaces have efficiency ratings based on laboratory conditions. To get an
accurate installed efficiency rating, factors such as flue gas heat losses,
cycling losses caused by over sizing, blower fan electrical usage, etc., must
be included.
Geothermal
heat pumps, as well as all other types of heat pumps, have efficiencies rated
according to their coefficient of performance or COP. It's a scientific way of
determining how much energy the system produces versus how much it uses.
Most
geothermal heat pump systems have COP's of 2.5 - 3.5.
That means for every one unit of energy used to power the system; two and
one-half to three and one-half units are supplied as heat.
Where
a fossil fuel furnace may be 50-90 percent efficient, a geothermal heat pump is
about 300 percent efficient. Some geothermal heat pump manufacturers and
electric utilities use computers to accurately determine the operating
efficiency of a system for your home.
Q: Will the minimum entering
water temperature have an affect on which heat pump I buy?
Yes. If
you have an open-loop system, your entering water temperatures may range from
the 70s in the southern
Q: Are the dealer and loop
installers qualified?
Don't be
afraid to ask for references from dealers. A reputable dealer won't hesitate to
give you names and numbers to call to confirm his capabilities. The same with the loop installer.
Q: Will open or closed-loop be
best for you?
That
depends on several factors, as stated earlier. A dealer should be willing to
install what's best for you, not for him.
Q: Will the loop joints be
heat fused?
The
only acceptable method for joining sections of the special pipe used for
closed-loop systems is heat fusion. Any other method will eventually result in
failure of the loop.
Q: How long is the payback
period for your geothermal heat pump system?
To
figure this accurately, you must know how much per year you'll save in energy
costs with a geothermal system and the difference in costs between it and a
conventional heating system and central air conditioner. As an example: If
you'll save $700 per year with a geothermal system and the cost difference is
$2,000, your payback will be less than three years.
Q: If a home has ceiling cable
heat or baseboard heat, do air ducts need to be installed?
Not
always. It may be desirable to install geothermal heat pump room units. For
some small homes, one room unit would provide most of the heating and cooling
needs. Ceiling cable or baseboard units could then be used for supplemental
heat.
Q: If I want to know more
about geothermal heat pump systems, whom should I contact?
YOUR ELECTRIC UTILITY.
Most electric utilities have information about these systems. I you have
a question they can't answer, they will put you in contact with someone who
can.
Remember!
Your Electric Utility Wants You To Be An Informed, Efficient User Of
Electricity. Contact Them Any Time You Have A Question About Heating And
Cooling Systems.
Reprinted with permission from the
Electric Heating Institute of Indiana