While a small amount of frost collecting on your heat pump's outside unit is normal, a unit that has iced over signifies that you have a serious problem on your hands. You're most likely to notice this in the morning, since low overnight temperatures make your heat pump's outdoor unit more likely to freeze. When a heat pump is operating in heating mode, the coils in the outdoor unit produce small amounts of condensation. When temperatures dip below freezing, the condensation turns to ice and causes your unit to freeze up.

To prevent this, heat pumps have a defrost cycle that's designed to periodically melt ice off of the outdoor unit. When the defrost cycle turns on, your heat pump momentarily switches back to air conditioning mode and its compressor begins to condense refrigerant in the outdoor unit. Condensing the refrigerant generates a significant amount of heat, and the fan in the outdoor unit will be turned off during the defrost cycle in order to trap all of the generated heat. This allows your heat pump to quickly melt any built-up ice. When your heat pump freezes over, it typically means that your defrost cycle is not working correctly. To help you get your heat pump working again without freezing over, here are a few causes of a failing defrost cycle and what you can do about them.

Malfunctioning Sensors or Timer

Older heat pumps used timer-based defrost systems that would periodically switch the heat pump to air conditioning mode based on an internal timer. In newer heat pumps, the defrost cycle is often controlled by a temperature sensor that allows the heat pump to only engage its defrost cycle when necessary. Regardless of which type your heat pump has, a failing timer or sensor can cause the defrost cycle to never trigger — your heat pump continues operating in heating mode until it eventually ices over. You'll need to call professional heating repair services to test all of the electrical components of your outdoor unit, which can help pinpoint the fault.

Stuck Reversing Valve

Heat pumps rely on a reversing valve to switch from heating to air conditioning modes. This valve reverses the flow of refrigerant in the your heat pump. When your heat pump is cooling the air inside your home, refrigerant expands inside the indoor unit in order to cool the air around the blower fan. When it switches to heating, refrigerant expands in the outdoor unit in order to transfer thermal energy from the outside air into your home.

When your reversing valve becomes stuck, your heat pump can't switch the direction of the refrigerant in the system — it will never be able to switch to operating as an air conditioner when it tries to engage its defrost cycle. Call a heating repair company to have your reversing valve inspected and replaced — not only will your heat pump continue to ice over, but you'll later be unable to switch it to air conditioning mode during the summer.

Insufficient Refrigerant

Low refrigerant levels are caused by a leak somewhere in your heat pump's coils. When refrigerant is low, the heat pump's compressor will be unable to generate heat by condensing the refrigerant in the outdoor unit — there's simply not enough refrigerant in the coils for the compressor to adequately convert it from a vapor to a liquid. Only licensed HVAC technicians should work with refrigerant, so you'll need to schedule an appointment with professional heating repair services to have the level of refrigerant in your heat pump inspected, your leak fixed and your heat pump recharged with additional refrigerant.

When your heat pump's outdoor unit freezes over, the weight of the ice can cause considerable damage to it. Your fan may be knocked out of alignment, the fins on the unit can break off and the compressor motor can overheat due to the insulating effect of the ice. It's important to call heating repair services as soon as possible to have your heat pump inspected and repaired in order to avoid serious damage.