Heat pumps are one of the best heating options in places like North Carolina that rarely get extremely cold during the winter. The main reason heat pumps are such a great choice is that they’re typically at least three times more efficient than any type of furnace. That means a heat pump will allow you to heat your home for far less while also being able to provide effective cooling during the hotter months. The only thing to note is that heat pumps do need to occasionally defrost during the winter and temporarily stop heating during the defrost cycle. In this article, we’ll provide a full overview of heat pump defrost cycles and also discuss why heat pump systems usually require some form of auxiliary heating.
Why Heat Pumps Need to Defrost
The way a heat pump works is that it uses extremely cold refrigerant to absorb heat from the outdoor air the condenser fan blows over its coil. It then compresses the refrigerant, which causes it to instantly boil and transform into a “superheated” gas. When the refrigerant then gets pumped to the coil inside the building, all of the heat it contains naturally flows out into the air being drawn through the air handler by the HVAC blower.
Refrigerants have an extremely low boiling point, usually somewhere between -15 and -22 degrees Fahrenheit. That means the refrigerant starts off below this temperature when it first enters the coil outside. The fact that it is so cold means that frost and ice continually develop on the coil as a result of it being much colder than the air.
The issue if a heat pump didn’t defrost is that the ice eventually starts insulating the coil and preventing air from reaching it. The result is that the refrigerant can no longer capture heat and the heat pump stops heating. This is why the defrost cycle kicks in to melt all of the ice so that the system can quickly start heating again.
How Often Does the Defrost Cycle Run?
Some heat pump systems are just on a timer and automatically switch over to the defrost cycle at set intervals, usually sometime between every 30 and 90 minutes. This can be an issue at times since it can lead to the defrost cycle not running often enough. That means the heat pump may occasionally perform poorly or not heat at all as a result of all of the ice on the coil. This is why many heat pumps instead have a temperature sensor that monitors either how cold the coil is or the temperature of the refrigerant when it exits the coil. This ensures that the system will switch over to the defrost cycle as needed to prevent too much ice from forming on the coil. The sensor also ensures that the defrost cycle will continue running until the coil is fully thawed out and only run as long as needed instead of just running for a set amount of time.
How the Defrost Cycle Works
The only way to effectively melt all of the ice on a heat pump’s coil is for the system to start pumping hot refrigerant into it instead of cold. This is done by activating the system’s reversing valve so that the refrigerant starts flowing through the system in the opposite direction, which is similar to what happens when the system switches from heating mode to cooling. The difference is that the fan in the heat pump doesn’t run during defrost mode, which is important since cold air blowing over the coil would cause it to thaw more slowly. When the reversing valve activates, it results in cold refrigerant starting to flow through the coil inside the building and absorbing heat from the air moving across it. The refrigerant then gets compressed before entering the outdoor coil so that it becomes hot enough to quickly melt all of the ice.
How a Heat Pump System’s Auxiliary Heating Works
Some heat pumps are part of a dual-fuel HVAC system that also has a gas furnace. More commonly, a heat pump system has several electric heat strips located within the air handler. In either case, the furnace or the heat strips provide auxiliary heating and will sometimes come on when the defrost cycle starts and continue running until it finishes and the system switches back to normal heating mode. The auxiliary heating doesn’t always come on in defrost mode, but it will if the temperature in the house drops by more than a degree or two.
There are also situations when the heat pump and the auxiliary heating will both run at the same time. The one drawback to heat pumps is that they heat quite slowly compared to furnaces and other electric heating options. In most cases, a heat pump can only raise the temperature by around one degree per hour. It will generally never have issues with keeping a house warm and maintaining the desired temperature. However, it could take hours for a heat pump alone to bring the temperature up by even a few degrees, such as when you turn the temperature back up in the morning after turning it down at night. This is where the auxiliary heating comes in handy since it ensures the house gets fully heated more quickly.
The last reason auxiliary heating is necessary is that the furnace or heat strips provide a backup source of heat in case it gets too cold outside for the heat pump. Having a backup source also ensures you can still heat your house should the heat pump break down or die.
Your thermostat will always display that your HVAC system is Aux Heat or AUX mode whenever the auxiliary heating is currently on. This is generally nothing to worry about since the system should switch back to normal heating mode fairly shortly unless it’s so cold outside that the heat pump can’t work. The time you do need to be concerned is if the system stays in Aux Heat mode for several hours since this generally indicates your heat pump needs some type of repair. In most cases, it means your heat pump won’t turn on due to something like an electrical issue or a failed capacitor or that it’s stuck in defrost mode. The latter sometimes happens as a result of the reversing valve breaking when it activates or getting stuck.
If you’re searching for an experienced, local HVAC company in Wilmington, NC, look no further than Green Dot Heating & Air. We have more than two decades of expert experience in the full range of heating, cooling and electrical services. If you’re experiencing any issues with a heat pump, air conditioner or furnace, you can turn to us for prompt, reliable repairs. We can work on any make and model and will ensure that the issue gets repaired in the most effective way possible. We’ll also take the time to explain everything clearly so you understand what the issue is and why whatever repairs the system needs are necessary. Whether you’re dealing with a heat pump that won’t defrost or won’t turn on or need any other heating repair, contact the pros at Green Dot Heating & Air today.
