Why is my heat pump smoking?
  • Posted on February 12, 2018
  • By Rod Miller

There are few things scarier when it comes to your home than the sight of fire or smoke anywhere near it. So, when any machinery that is related to heating your home is visibly smoking, it makes sense that your first reaction is to get the fire department to your house right away.

The next time you see, “smoke” coming from your heat pump however, take a deep breath
before dialing 911. Why? Well, it’s for the same reason quotes appeared around smoke in the
previous sentence; what you’re seeing come from your heat pump is probably not smoke at all.

What is it then?

What you are seeing is most likely steam. It appears any time the heat pump goes into “defrost
mode.” As a heat pump runs, it will create water condensation along its coil, and if it is cold
enough outside that water will freeze. Once it does, the coil has trouble doing its job.
Fortunately, heat pumps are built in anticipation of this sort of thing happening and that is when
it will go into defrost mode.

When a heat pump defrosts, it switches back to air conditioning for a minimal amount of time to
release heat form the coil so the ice can melt off of it. During the switch to air conditioning, you
should still remain warm however; there’s an auxiliary strip heater inside the unit that will turn
on and offset the cold air that would normally blow into your home.

While it may seem odd for the air conditioning and heat to be running at the same time, it’s only
temporary in both the amount of time it takes and how often it happens. Once the heat pump
has finished defrosting, it will automatically go back to being a heating unit again.
If you are looking outside during this process, you might see “smoke”, but what you’re really
seeing is just the melting ice turning into steam.

Why does this happen exactly?

Think of it this way, during the winter months, a heat pump operates how an air conditioner
runs during the summer, but in reverse. The refrigerant that circulates through a heat pump
system takes heat from outside and releases it into your home as it goes through evaporation
and condensation. The previously mentioned trouble a heat pump can run into happens during
this stage; when it’s trying to absorb the outdoor heat. The longer a heat pump struggles to grab
heat from outside the home, the less effective it becomes at keeping you and anyone else inside
warm. The defrost mode exists so you never have to experience lapses in heating.

What else happens when my heat pump defrosts?

The appearance of smoke is not the only thing that might alarm a new heat pump owner. The
first thing that happens when the heat pump defrosts is the outdoor fan motor stops running.

Don’t worry; this is a standard occurrence. Secondly, when the refrigerant flow changes
direction, a loud sound will come from the heat pump, but this is common as well. Once the
frost has melted off of the coil, the refrigerant flow will have to reverse back to the heating
cycle, which will result in another loud “swooshing” sound, but this just means the unit is up and
running again.

How will I know if something’s actually wrong with my heat pump?

While the steam and noises that come with your heat pump defrosting are normal, that
doesn’t mean the unit is immune to damage. The heat pump should defrost itself as
needed, so if there’s consistently a sheet of ice on it, something is probably wrong with the
defrost cycle. If the defrost cycle is not working right, you could see increased utility
bills and the heat pump could wind up damaged.

Also, be mindful of how long the defrosting sequence takes; it should not take much
longer than a few minutes, unless it’s built up a large amount of ice, which could take 30
to 45 minutes to defrost. If the heat up seems to be defrosting non-stop, there could be
a larger issue than just the coil having excess ice on it. If the unit is going into defrost
mode more than periodically, that’s another indicator you might have a problem. A bad
defrost control, defrost sensor or thermostat, outdoor fan motor or low charge or
restriction are a few reasons why your heat pump is not functioning properly.
If any of these issues arise, be sure to call your heating and cooling company to mitigate
any further damage.

What preventive measures can I take?

There is not much that you can do to keep your heat pump from needing to go into defrost
mode. That is nature taking its course; when the unit starts to absorb heat, condensation is
inevitable, and when it gets cold enough out, that water will freeze.

However, there are some other steps you can take to ensure your heat pump runs efficiently for
years. If there’s frost on your unit’s outdoor cabinet, don’t worry about it, but remove any
snowdrifts that have collected against the cabinet. This build up could generate a block that
defrost mode will not be able to remedy. Also be sure to schedule heat pump maintenance
every year before the cold weather comes to address any issues ahead of time.


Remember, the next time you look outside and see “smoke” coming from your heat pump, do
not panic. It’s actually steam you are seeing and when steam is coming from the heat pump,
that is a good thing! It means the unit recognizes there is too much ice on its coil, and it needs to
defrost to run more efficiently. When your heat pump runs efficiently, your house will remain
warm and your utility bill costs will be kept under control.

For more information about your heat pump or any other HVAC-related needs, contact Rod
Miller HVAC at 301-569- 7993 (Gaithersburg) or 301-587- 1517 (Silver Spring).


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