Heat Pumps
By Jim Wheeler
Heat pumps have
suffered from bad publicity over the years. Early designs weren’t always the
most reliable, and they were often very drafty, since heat pumps require more
airflow than gas furnaces do.
But simply put, heat pumps are just air conditioners that can also run
backwards. To understand the principle, feel the air that blows from the
condensing unit of an air conditioner (the outside part). You’ll notice that it
is about 20 degrees warmer that the outside ambient air temperature. Yet, even
when the outdoor temperature is above 100 degrees, the air conditioner makes the
indoor air cooler.
This cooling happens because a warm, high-pressure gas is sent to a coil and fan
outdoors where the heat is released, and a cool, low-pressure gas is cycled
through an indoor coil and fan, where indoor heat is absorbed. As a result, the
air passing through the indoor coil leaves it about 20 degrees cooler than the
indoor ambient temperature.
In the winter, though, a reversing valve inside the heat pump sends the warm gas
to the indoor coil and the cool gas to the outdoor coil, where (regardless of
the outdoor temperature) some heat is absorbed from the air and transferred to
inside the house. However, the colder the outdoor temperature, the less
efficiently the heat pumps works. In fact, a heat pump works best when the
outdoor temperature is above freezing.
Heat
from a winter Minnesota night
Does this mean that heat pumps shouldn’t be used where temperatures normally
drop well below 32 degrees during the winter? No. In fact, heat pumps may
operate at about the same cost as gas furnaces even in northern climates,
depending on the costs of natural gas and electricity. If you check the average
daily winter temperature in most areas, you’ll find they seldom run below the
mid-30s range. (Ask the marketing people for your local utilities; they'll be
able to give you an accurate record of average temperatures.)
True, during cold nights, some supplemental heating is usually necessary. That
can come from electric resistance heaters, or for much greater efficiency, from
specially adapted gas furnaces that can help heat pumps through periods of
extreme cold.
So try to forget the bad reputation heat pumps gained during their early
history. Modern versions use tougher compressors; that means they are even more
reliable during the cooling season. And models with variable-speed indoor fans
are especially good at preventing cold drafts in the winter.
Up to this point, we have been discussing what we call air-air heat pumps—those
that take heat from the outdoor air and transfer it to the indoor air. There are
other types of heat pumps that draw heat from water and from the ground, which
provide efficiency that is more consistent, require no supplemental heat, and
may provide the most cost-effective form of heating and cooling. We will discuss
them in a future article.
Jim Wheeler is an award-winning writer and teacher with more that twenty-five
years in the field of HVAC.
