Why Is Your Auxiliary Heat ON But No Heat?

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It happens sometimes. Though not often, you may occasionally run into situations where you think the aux heat function is ON. Maybe you’ve even conformed on the thermostat that it’s ON. However, you can’t feel the heat.

What could be wrong? Is the heat pump damaged? Or maybe it’s a problem with the auxiliary heating strips? Let’s find out.

How Aux Heat Works

To understand what could go wrong with your auxiliary heat function, it’s important first to understand how aux heat works.

Auxiliary heating functions on most heat pump systems to provide supplemental heat when the heat pump system is overstretched. It’s typically needed in the winter season when the extremely cold conditions outside can cause the heat pump to underperform.

Heat pumps extract heat from the air outside the house and ump it inside the house to raise indoor temperatures. It’s a significantly more complex process. However, fundamentally, that’s what happens. As such, the effectiveness of the heat pump depends heavily on the amount of heat contained in the outside air.

When it gets extremely cold outside, the amount of heat contained in the air diminishes. Therefore, the heat pump may no longer supply enough heat inside the house. Moreover, heat pumps defrost more often during extreme winter weather. So, a significant portion of the heat that would have gone into the house ends up in the defrost process, further reducing the heat that gets into the house.

Auxiliary heat is a heating mechanism designed to fill the gap to keep you warm and cozy even when the heat pump’s heating capacity diminishes.

There are two main types of auxiliary heating solutions;

Electric resistance coils

The majority of heat pumps use electric resistance coils built into the mini split to provide auxiliary heat. When it gets extremely cold outside, the thermostat automatically switches on the resistance coils, which function just like any electric heater.

The wires used for heating are high-resistance wires that produce heat when current is forced through them. The higher the current, the higher the heat output. Of course, the amount of heat generated also depends on the type of resistance wire.

Dual-fuel systems

Since electric heating is the most expensive form of heating, some auxiliary heat systems employ dual-fuel systems to provide supplemental heating. When temperatures outdoors drop below a critical balance point, the thermostat sends a signal for the secondary heat source to come on.

Most such systems are wired to a gas furnace. When the heat pump is overwhelmed, the gas heater will switch on automatically and provide supplemental heat to keep you warm.

4 Reasons Why Your Auxiliary Heat is ON But No Heat

As we’ve mentioned, auxiliary heating should come on automatically. When extremely cold conditions are detected, the heat pump thermostat automatically requests “help,” and the supplemental heat source starts running automatically. No further effort is required on your part.

If the auxiliary heat source is engaged, but there’s no heat coming from it, then there’s a problem. Here’s what could be wrong;

1. Aux heat isn’t ON

The first thing you should consider is that perhaps the auxiliary heat source isn’t ON. Maybe you’re misled.

This can happen for many reasons. For instance, if you have a malfunctioned thermostat, it may display “aux ON” even when auxiliary heating is not on. Remember that the display on the thermostat an LED system that can malfunction.

Maybe there’s a short circuit. Or perhaps the thermostat battery is depleted. All these can cause the thermostat to display misleading readings.

Solution: Diagnose the thermostat. Is it working? Is it working appropriately? In particular, is the aux heat section working well? You also need to check the wiring to make sure that everything is working currently.

Additionally, check any other signs on the heat pump designed to alert you when the heat pump is on. For instance, some Daikin models have a blinking light on the indoor air handler that only lights up when aux heat is ON.

2. One of the conditions for aux heating isn’t met

Auxiliary heating is dependent on a couple of factors, one of them being that indoor temperatures must be higher than a specific setting – usually 40˚F. The exact value varies from one heat pump to the next. However, for most heat pumps, auxiliary heating can only come on if indoor temperatures dip below 31˚F.

Given that aux heat is also dependent on several other factors, sometimes the sensors can send a message to the thermostat requesting aux heat when indoor temperatures are still high enough. In this case, aux heat can come on without necessarily producing heat.

The same may happen if indoor temperatures are higher than the thermostat setting, but outdoor sensors call for aux heating. The confusion can cause the unit to display “aux ON” without actually engaging auxiliary heating.

Solution: The first thing you should do here is to check your thermostat. What’s the indoor temperature? If it’s higher than 40˚F, you don’t need heating, and the “aux ON” display was probably a false alarm.

Remember that you can change the aux heat setting so that auxiliary heating only when a set of conditions are met and not just one condition or the other. This can help prevent false alarms in the future.

3. The aux heat source has malfunctioned

As we’ve mentioned, auxiliary heating is a separate mechanism from the heat pump. Although some auxiliary heaters are built into the heat pump, they have separate wiring and function differently. Indeed, the majority, as we’ve said before, are electric heaters and gas heaters.

It means that the auxiliary heater can malfunction without necessarily affecting the heat pump. For instance, perhaps the electric resistance coils are damaged. Or, maybe, the wiring to the coils is compromised. It could also be a malfunctioned fan. If it’s a gas furnace, there could be an issue with the gas supply.

These issues are difficult to catch early because the heat pump runs separately. It won’t stop working because the electric resistance coils have blown or because the gas supply to the supplementary gas furnace is blocked. Often, you only realize the problem when you try to engage the aux heat function.

Solution: There’s only one solution here – regular maintenance. Always make sure that the heat pump and auxiliary heating mechanisms are cleaned and maintained for optimal function. If it’s an electric resistance coil heater, have an HVAC technician check and test it before the heating season begins. The same applies to gas furnaces.

However, if the issue has already happened, the only option is to call an HVAC technician. It’s not advisable to attempt DIY on auxiliary heat systems, especially gas heaters.

4. Compromised in the airways/ducts

Finally, you may also not be getting heat because the airways designed to deliver auxiliary heat to your home have failed. Remember that gas heaters, for example, are primarily ducted systems that rely on the home’s main ductwork to deliver heat to different areas of the home.

If the unit connects to an auxiliary electric heater, it may also rely on the home’s ductwork. If the ductwork is compromised in any way, the delivery of warm air may be impeded.

Airflow in the ducts can be compromised for several reasons. First off, dirty filters can constrict air passage into the duct system, meaning that very little warm air will come out into your rooms. Another possibility is a leak in the ducts. A substantial leak may divert the warm air elsewhere, perhaps outdoors, leaving you in the cold. Finally, bends can also constrict the ductwork, limiting the amount of warm air that passes through.

Solution: Here, too, the first step is maintenance. Proper and regular maintenance is vital because it helps you catch and address potential issues early. If the filters are dirty, you can wash them or replace them. You can also replace damaged filters. You can also identify dents, bends, and leaks in the ductwork early and address them before the heating season begins.

However, if the problem has already happened, the only solution is to call an HVAC professional to diagnose and recommend a fix. Make sure to shut off the heat before you call the HVAC technician.

Key Takeaway

Nearly all heat pumps have a unique feature known as auxiliary heat, or aux heat, designed to supplement the heat pump when the going gets tough. Aux heat typically comes on in extremely cold winters when outdoor temperature dips below 40˚F. The auxiliary heat source can be an electric resistance coil built into the heat pump’s indoor air handler or a gas heater somewhere within the home.

Although it’s not common, it’s also not unusual for auxiliary heat to misbehave or fail. This can happen for many reasons, including a faulty thermostat, a failed auxiliary heat source, or compromised ductwork.

You can prevent most of the issues with regular maintenance. Scheduled professional checkup, at least once per heating season, is especially crucial. However, if the issue has already happened, your only option may be to switch off the heating system and call an HVAC technician.  

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