The outdoor unit of a heat pump (that metal noisemaker that disturbs the peace on so many otherwise nice summer afternoons) will occasionally grow frost around the outside of the unit when it's in heating mode. If so, how much is OK? What about the amount of frost on the unit in this photo? Yes, a little bit of frost on the heat pump's outdoor unit is normal. No, the amount of frost you see above is not OK. Why does it happen? It's just the science of phase changes. Outdoor air—yes, even cold, dry outdoor air—has some water vapor in it. Air with water vapor in it has a dew point, the temperature at which the water vapor changes phase and becomes a liquid. When that outdoor air finds a surface that's at a temperature below its dew point, the water vapor condense out of it. If that surface and the surrounding air are below the freezing point as well, the condensed water freezes. All that depends on the outdoor air and the temperature of the heat pump condenser.

The refrigerant in a heat pump typically runs 10 to 20 degrees Fahrenheit colder than the outdoor air. If that temperatures happens to be below the dew point of the air and below the freezing point, you get frost. The typical conditions for this to occur are when the outdoor air temperature is below 40° F and the relative humidity is about 70% or higher. As the temperature drops below 20° F, the formation of frost diminishes. As I write this, heat pumps in Atlanta may well be frosting up a bit. The temperature is 39° F and the RH is 65%. How much frost is too much? If you see a little bit of frost forming, check back in a while and see if it's still there. Heat pumps have a defrost cycle that melts frost that builds up. The defrost cycle simply changes the direction of flow so that the hot refrigerant doesn't take its heat indoors but instead goes back through the outdoor coil. The fan in the outdoor unit stops to keep cold air from hindering the defrosting process. A little bit of frost is OK as long as it disappears after a while.

If you see so much frost on the unit that air cannot pass through the fins on the outdoor unit, as in the photo above, or if frost stays on the coil for a two hours or more, then something is wrong. york residential hvac unitsIt could be the defrost controls, the reversing valve (shown in photo at right), or improper refrigerant level. wall ac unit not coolingIf frost forms on the outdoor unit and stays there, it's time to call your HVAC company and have them repair the system.portable ac better than window unit The defrost cycle on a heat pump can use a lot of energy. When the refrigerant flows backwards to defrost the outdoor coil, it's not heating the house. In fact, without supplemental heat, your system will blow cold air into your home.

Most heat pumps have supplemental heat, though, and most of the time it's electric resistance heat. In the defrost cycle, your supplemental heat kicks in and you pay more. But how smart is the defrost system on your heat pump? Does it go into defrost mode only when necessary? Or does it cycle on and off even when it doesn't need to? I'll have another article about heat pumps, the defrost cycle, and energy consumption soon! How the Heck Does a Heat Pump Get Heat from Cold?! Dew Point — A More Meaningful Measure of Humidity? The Shocking Truth About Heat Pumps Photo courtesy of Josh Littrell of the East Kentucky Power Cooperative. † Unfortunately, many people refer to the outdoor unit as the condensing unit. Yes, that's what it is when it's running in cooling mode, but in heating mode, that's where the evaporation happens. In winter, that metal noisemaker should be called the evaporating unit. Calling it the outdoor unit works in all seasons. For an introductory explanation of the refrigeration cycle, which moves heat between indoors and outdoors, see my article, The Magic of Cold.

How to Keep a Window Air Conditioner from Freezing Up A common complaint for window air conditioners, aside from not getting the room cool enough, is the complaint of ice forming on the air conditioner. Most people believe this is a symptom of too much refrigerant. It may be, but that's not the only reason why window air conditioners freeze up. Here are a few reasons why window air conditioners form ice. Check your air conditioner's filter. A dirty filter can cause an air conditioner to freeze up. Clean your filter every couple of weeks. If there are extenuating circumstances like household construction where there's a lot of dust in the air, clean it more frequently. Clean the cooling coil. This is something you many need a professional to do. If the cooling coil is blocked or dirty, it can cause the air conditioner to freeze up. Clean it every two or three years. Determine if there is an inadequate amount of refrigerant. Too much or too little refrigerant contributes to air conditioner icing.

If you suspect an incorrect amount of refrigerant, contact your service professional. Make sure you have the right size air conditioner. If you have purchased an air conditioner that is too large for the room, freezing may result. The air conditioner on-off cycle is short because of the room size. The constant short cycle can cause the air conditioner to freeze. Keep an eye on the outside temperature. If the temperature outside is too low, the air conditioners cooling coil also drops in temperature. As a result, the cooling coil may form ice. Fall is a prime time for air conditions to start freezing up. The dropping nighttime temperatures can contribute to the cooling coil freezing up. Several factors can cause a window air conditioner to freeze up and stop working. These range from low refrigerant to extreme outdoor... Why Does My Air Conditioner Freeze Up All the Time? X. Must See: Slide Shows. How to Keep a Window Air... How to Tell If a Window Air Conditioning Unit Is Frozen.