The push is on to decarbonize buildings and that means code changes and incentives to move toward carbon- free electric homes accomplished by using heat pumps for: 1) Clothes-drying, 2) Water-heating, and 3) Air-conditioning/heating via either “Air to Air” (Heat
from outside air) or “Ground to Air” heat pumps (heat from underground)
WHAT IS A “HEAT-PUMP” A Heat-pump moves heat via refrigerant pipes from one area to another. It
DOES NOT CREATE heat as does as a space heater. The magic happens via high pressure pipes and low-
pressure pipes and also the type of refrigerant chosen for optimum phase change from liquid to gas in
both the high and low environments.
WHAT IS A “Mini-Split” A mini split is a descriptive term for its “small” unit size versus a whole house
central HVAC system. It is “split” into two sections, 1) the cassette unit inside and 2) the
compressor/condenser unit on the outside of the building. The term “ductless” means not integrated
into a central heating system, The potential to install the inside cassette unit anywhere makes it great
for remodeling existing homes/apartments because no ductwork is required. But 240VAC power is
required to the outside unit.
Using an electric (resistive) floor heater is 100% efficient because for every watt used, we get one watt
of heat out. The power companies love these because an electric heater appears as a direct short to
their wires. (High current at 1200-1400 watts near the maximum of your breaker).
A heat pump is 3 to 5 times more efficient than an electric resistive heater because we are grabbing
heat from the other side of an insulated wall and sending it to the other side, that is, using available heat rather than creating heat.
HEAT-PUMP DETAILS: A house has two different temperature environments: In winter we have a: 1)
warm indoors via insulation and: 2) cold outdoors. The heat-pump works because of the physics of gas to
liquid phase change. We can heat and cool gas-to-liquid in the warm environment and heat and cool
the same gas-to-liquid in the cold environment. The magic is the pressure developed inside the
refrigerant pipes by the compressor/condenser, and ALSO, the factory engineers decide on what
refrigerant they want to use I.E. the chosen trigger temperature of the refrigerant best suited to
naturally return to a liquid.
There are many different refrigerants that have a wide range of phase-change temperatures. CO2
changes to liquid at minus 87f for example and R-410A (most common) changes to gas at minus 55F. We
can change phase from liquid to gas in both the cold outside and warm inside environments due to what
pressure the gas is under and heat or cool the house. (Think how a spray-can will get cold while holding
your finger on the button) A cold winter day at -10F seems pretty warm to a -55F liquid. The Winter
limiting factor: If the outside temperature were to get colder than -55 than the R-410A gas would
naturally change state to liquid and the heat pump would no longer work.
The liquid state will lose its heat and turn very cold when it vaporizes. We pump the heat by
vaporization either throwing that heat to inside (heating mode) or to the outside (Air conditioning
mode) via reversing the electric Heat-pump pressure zones. I.E. Spay the can inside the house or spay
the can outside the house.
Summary of Phase Change: The “heating side” is under high pressure to confine the gas phase-
change at high temperatures and the “cooling side” is under low pressure to confine the gas phase-
change in cold temperatures. The metal air -fins located at both the inside unit and outside unit do the
physical transfer of heat/cold.
ADVANTAGES: -Air-to-air Mini-Splits are much quieter than a window hanging air conditioner because
all the noise making parts are outside,
-Much faster cooling and heating than a window air conditioner, -
-Much quieter than a central Air system,
-Enables Zoned heating/cooling via their placement in thehouse,
-No fire safety issues that electric floor heaters have,
-No harmful gases from a furnace burning fossil fuels.
-If there are enough PV solar electric panels feeding your house, the Heat-pump energy
used is free.
A Smart Home option: You can engage Voice control of your temperature on any system using “Sensibo
Sky” for $100. It will connect to Alexa, Google, and Siri.
COST Mini-Split and Ground Source: Mini-Split air-to-air heat pumps are great. Their cost is relatively
low at about $1200 for a 9000 BTU unit plus the cost/labor of a 240-volt supply that can be piped from
the breaker box around the outside of the house to where the outside mini-split compressor is located.
“Ground” to Air heat pumps require very high Up-Front costs for drilling underground in the yard. A
typical 2000 sq ft house requires three drillings at 300 feat deep costing about $18000 plus the cost of
the main air handler/compressor in the house at $7000 plus labor. The average Colorado underground
temp is about 53F good for both cooling and heating.
RATING EFFICIENCY of Heat-pumps:
SEER = Seasonal Energy Efficiency Rating, used to rate the Air-conditioning mode in the USA where 13 is
poor and 42 is state of the art.
HSPF = Heating Efficiency Rating - where 7 is extremely poor and 15 is state of the art. Europe only uses
COP = Coefficient of Performance (Energy used vs Performance) Cheaper Mini Splits under $1000 will be
older technology with SEER = 20. Any HVAC installer who recommends this, send him out of the house.
State of the art efficient Mini-Splits have SEER=33 up to SEER=42.
THE HOUSE QUALITY: Of course, the quality of your external wall and attic insulation is a key factor for
how much heat is lost out of the house. Also the insulation affects the RUN-TIME required to keep warm
or cool. Sealing up leaks in the house and getting the external walls stuffed with Dense-pack Cellulose is
highly recommended as a first step. I have worked with 4 of these insulation companies and “Bestway
Insulation” is the best I found (303) 469-0808. They have a radio show on Saturday mornings:
CAPACITY FOR THE WHOLE HOUSE: Mini-Splits come many sizes: 9000 (3/4 ton and most efficient),
12,000BTU (one ton), 18,000BTU (1.5 ton) and each one is more expensive. To heat a whole house
would require three 9000BTU units, one for the main living room, one for the master bedroom, and one
for the basement. If the kitchen is closed off from the main hall area, it may also require a mini split.
The basement unit is important if your basement is usually very cold because when standing in bare feet
on the carpeted floor of your living room (above the basement), the cold floor will suck heat out of your
feet even if sitting on the sofa. The basement concept is not to totally heat the basement but just to
bring it up above ~60+. The basement heat that rises to the basement ceiling will be noticeable
especially for elder folks’ feet with shoes on. Another advantage of multiple mini-splits is that in the
future if one goes bad, the other units are still running
My Carrier model 38MPRAQ09AA301 (Heating and cooling my house since Jan 2019) has an excellent
SEER (cooling) rating of 42 and an HSPF (heating) of 15. It is a 9000BTU unit and will usually run-in low
mode putting out a gentle ~100F at 200 watts. (Remember that a Hair Dryer uses 1200 -1500 watts for
comparison). When the outside temperature gets colder, the unit will jump to a second stage of about
700 – 800 watts putting out about 120F degrees. If I set the remote control to max at 86 degrees, it will
jump up to 1900 watts and try to put out about 150F. It uses a 20-amp circuit breaker. Larger Mini-Splits
require a 30amp breaker. Mini-splits must pull heat out of the outside air, and when the outside
temperature is below zero, mine will use 800 watts and put out 110 to 120F but at minus 0 it is not able
to go to the max heating at 150F and apparently the sensors tell it not to try. This is when multiple mini-
splits all add together for a comfortable house.
The inside cassette of the Mini-Split usually hangs high on a wall and is very quiet using the eco settings
when running in low mode. Even in high mode it is quieter than a furnace running. There are also ceiling
recessed units and floor standing units similar to under a hotel window.
Outside: Note that the highly efficient “outside” compressors will change their RPM fan speed based on
the load asked for from inside the house. This RPM can create a vibration that is conducted into the
house through the house wood framing. So mount those outside compressors on the ground if possible
or on a wall of the garage that is not near the living quarters. A compressor mounted on the roof may
also conduct vibration into the room ceiling below depending on the framing.
Your old Furnace died: If you want to stay with a whole house gas Furnace with Central Air conditioning,
there are Hybrid furnaces available which may cost less than multiple mini-splits. This Hybrid means
that the furnace will NOT use gas to heat the house but use the outside air conditioning compressor in
reverse mode (a Heat pump) to supply heat. It will not switch over to gas until the outside temperature
is below 20F. I have witnessed these successfully used and still running since 2004. The compressor
housing outside is very large due to the physics of physical surface-area required to exchange heat