What
is an Air Source Heat Pump? and what does an Air to Air heat pump mean?
We
have talked about how the heat pump works and by now you
understand (much like an air conditioner) it transfers the
heat from one place to another.The
only difference between an air conditioner and a heat pump is that the
it can reverse the cycle of the refrigerant using the reversing valve.This means that the it can cool or
heat your house.
But
where is that heat that it moves?The
heat is in the AIR of your home.
So
basically an Air To Air heat pump or Air source heat pump means it
absorbs the heat from the air inside your home and rejects it the air
outside your home.In the
summer time it moves heat from inside- that’s why when you put your
hand to feel the air coming out of your outdoor unit it feels hot.
And in the winter the it
reverses the cycle and absorbs the heat from the outside air and moves
(or rejects) this heat to the inside air of your home.Again a picture is worth a thousand words:
Got the picture? The Air to Air Heat Pump or Air Source Heat Pump
transfers the heat back and forth between the home air and the outside
air…
Having said that, I bet you have some questions so I
will try to answer them:
First Question - In the winter time the air is cold outside
so how can we absorb heat from a very cold air already?
But in order to do that the refrigerant going to the outdoor coil
has to be colder than the outside air for it to absorb any heat.That requirement creates a limit on how
low the outdoor temperature can be for the heat pump to work.As a rule of thumb if the outdoor
temperature is any colder than 400 F, the Air to Air heat pump will be very
inefficient and will fail to give enough heat for
comfort.
The reason is that the refrigerant going to the outdoor is colder
than the outside air by about 20 degrees.That’s means the coil temperature will go below 32 degrees- the freezing
point of water.In a humid climate
or rainy weather the outdoor coil will freeze over.
Yup that’s ICE, frozen over an outdoor Air source heat pump coil, and that ice
prevents the coil from working.That’s why all Air to Air heat pumps have what we call it a de-ice or defrost cycle
to melt iced up coil.So if you
live in a very cold climate like Canada
an Air to Air Heat Pump or Air Source Heat Pump will have a hard time meeting
your heating requirements.
Most heat pumps have an electric coil heater to heat up your home
when it is too cold for the normal Air to Air heat pump cycle.We call this the Emergency Heater.Some units also have a similar Auxiliary
Heater.These units will still
provide enough heat for comfort when it is below 40 degrees outside.However these electric resistance
heaters are very inefficient and expensive to run (if they were cheaper you
wouldn’t have a heat pump in the first place).So you have effectively lost the energy
saving benefits of a heat pump if the weather is below 40F for extended periods
of time.
Please view the electric coil heater
diagram:
Second
Question- How does can a or Air source heat pump (or an air conditioner
for that matter) reject heat from the inside of the house to the
outdoors when it is already so hot outside?
Well
the answer is almost the same.You
still need the refrigerant to have higher temperature than the outside
air, for it to be able to move or reject heat.So there is still a practical limit for the outdoor
temperature to be.But this
limit is about 120F and it is very unusual to find 120F outdoor
temperature anywhere.
Don’t
get me wrong the hotter the outside temperature the harder your heat
pump or your air-conditioner will have to work to keep up and the more
hurt you will feel in your wallet when you pay your electric bill.It helps when you choose the
location of the outdoor equipment to make sure there is no additional
heat load on it- for example, don’t put it on a flat black tar roof.Put it in the shade with sufficient
ventilation if you live in a very hot area and make sure the
temperature where you locate the equipment is not more than the ambient
outdoor temperature.
Conclusion:
·Air
to Air Heat Pumps can be very efficient and cost effective systems.Since they are by far the most
efficient electric HVAC systems they are especially cost effective if
you live in area where the price of natural gas is more expensive than
the electricity.
·They
also make sense if you live in a rural area where it is difficult to
get natural gas or propane.
·In
a cold climate areas Air to Air Heat Pump or Air source Heat Pump will
have a very hard time & sometimes may fail when the outdoor
temperature falls below 35o F.
·The
Heat
Pump thermostat is completely different from and more complex
than the air conditioner thermostat, because the Heat Pump thermostat
controls the heat pump itself as well as the Auxiliary Heater, the
Emergency Heater and the Reversing Valve.
Have
your say about what you just read! Leave me a comment in the box below.
