If you have a heat pump, the answer to this question is very simple: NO. You should not cover your heat pump because it runs all year long, and as long as it’s running, it needs to be able to freely take in air along the sides and release exhaust out the top. If a storm causes snow or ice to accumulate on the heat pump, you should only remove the snow or ice from the top. Attempting to remove snow or ice from the sides could damage the heat pump, and it’s unnecessary because the pump itself should automatically melt the frozen precipitation when it goes into defrost mode. If this doesn’t happen, call for HVAC service immediately.
Obviously if you have a furnace, the answer is the same. Don’t cover it.
Now, on the other hand, if you have a separate air conditioning unit that is NOT a heat pump, things get slightly more complicated. As long as it isn’t running during the winter (not always a sure thing here in the Tennessee Valley!), you can cover it if you really, really want to. However, it’s generally not necessary and it can lead to some problems.
First of all, covering your air conditioning unit isn’t necessary under most weather conditions. Outdoor HVAC units are designed to withstand harsh weather, so most major manufacturers do not recommend covering them. However, there are two conditions under which covering your AC unit might be a good idea—major hailstorms and blizzards. A cover can help to prevent large hailstones from damaging your AC unit. During a blizzard, a cover can prevent large amounts of snow from getting inside the unit, where it could cause damage if it melts and refreezes before the water can drain out. In both of these cases, the cover should be removed as soon as the hail or snow stops falling.
Leaving a cover on your AC unit throughout the winter can lead to a couple of potentially serious problems. A covered AC unit can be an attractive shelter for rodents like mice and chipmunks, who may decide to nest in it and/or chew on the wiring. Furthermore, even with a cover on, some moisture will get into the unit due to humidity in the air or the evaporation of water from the ground around it. The cover blocks airflow, preventing this moisture from drying and leading to the growth of mold inside the unit.
If you do decide to cover your air conditioning unit during the winter, the most important thing is to remove it before you turn the AC back on. Running the unit with a cover on can cause serious damage, so if there’s a chance you (or your kids) might forget and turn the unit on while it’s still covered, it’s probably best to leave the cover off.