Few heating complaints confuse homeowners more than inconsistent air temperature. One minute the vents are pushing out warm air, the next minute it feels cool. I hear variations of the same question every winter: “Why does my furnace blow hot air then cold?” Others describe it as a furnace blows cold air then hot, or a heater blows cold air then hot during every heating cycle.
The truth is, this symptom can be completely normal—or it can be an early warning sign of a real problem. The key is understanding how modern systems operate, especially electric systems like the Goodman MBVK electric furnace. In this article, I’m going to explain why temperature swings happen, how electric furnaces behave differently from gas systems, and when fluctuating air temperature means it’s time to investigate further.
Understanding What You’re Actually Feeling at the Vents
Before we talk about equipment, we need to clarify perception versus performance. Homeowners often describe temperature changes based on how the air feels, not on its actual temperature.
-
Cold air usually means room-temperature air being circulated
-
Cool air often means air that’s warming but hasn’t reached peak temperature yet
-
Warm air may feel cooler if airflow is higher than expected
When someone tells me their furnace blows hot then cold, the first thing I evaluate is whether the system is failing—or simply transitioning between normal operating stages.
The Goodman MBVK Electric Furnace: How It Heats
The Goodman MBVK is a variable-speed air handler that becomes a full electric furnace when paired with electric heat strips. Unlike gas furnaces, it does not produce heat instantly. Instead, it relies on electric resistance elements that energize in stages.
The MBVK’s ECM variable-speed blower motor is designed to ramp up gradually. That ramp-up is intentional. It improves efficiency, reduces noise, and helps distribute heat evenly throughout the home—but it also changes how airflow feels during a heating cycle.
If you’ve noticed your furnace blows cold air then hot, the MBVK’s startup sequence is often the reason.
Goodman outlines the MBVK’s design philosophy and performance characteristics in its product documentation (Goodman MBVK air handler series), and those features play directly into temperature fluctuations at the vents.
Furnace Blows Cold Air Then Hot: Common and Normal Causes
Let’s start with the scenarios where this behavior is not a malfunction.
Blower Starts Before Full Heat Output
Electric furnaces do not generate heat instantly. When the thermostat calls for heat, the blower may start moving air before the heating elements are fully energized. During that short window, the air coming out of the vents may feel cool.
This explains why homeowners say their heater blows cold air then hot, especially at the beginning of a heating cycle.
Once the elements reach full temperature, the air warms quickly and consistently.
Variable-Speed Airflow Feels Cooler at First
Older furnaces used single-speed blowers that slammed on at full power. The MBVK’s variable-speed motor increases airflow gradually. That gradual increase can make early airflow feel cooler, even though the system is heating normally.
This design improves comfort overall but changes expectations for people accustomed to older equipment.
Ductwork Temperature Lag
Cold ductwork can absorb heat at the start of a cycle. If the ducts run through an unconditioned space like an attic or crawlspace, the first air delivered may lose heat quickly.
Once the ducts warm up, the air temperature stabilizes. This is a common reason a furnace blows cold air then hot without any equipment failure.
Furnace Blows Hot Then Cold: When the Pattern Reverses
Now let’s talk about the opposite complaint: a furnace blows hot then cold during the same cycle. This pattern deserves closer attention.
Heating Elements Cycling Off Due to Overheating
Electric furnaces include high-limit switches that shut off the heating elements if temperatures rise too high. When airflow is restricted—often due to dirty filters or blocked ducts—the elements may overheat.
When this happens, the heat shuts off but the blower keeps running. The result feels like warm air followed by cool air.
This is one of the most common reasons homeowners ask, “why does my furnace blow hot air then cold?”
Staged Heat Behavior
The Goodman MBVK often uses staged electric heat. If the thermostat calls for a small temperature increase, only one stage may activate. Once the demand is satisfied, that stage shuts off, but the blower continues briefly.
That brief continuation of airflow can feel like a temperature drop, even though the system is operating correctly.
Thermostat Fan Settings
If the thermostat fan is set to “ON” instead of “AUTO,” the blower runs continuously. After the heating elements shut off, the system continues circulating room-temperature air.
This is a simple but extremely common reason for a furnace blows hot then cold complaint.
Gas Furnaces: Similar Symptoms, Different Causes
Although this article focuses on the Goodman MBVK electric furnace, many homeowners experience similar issues with gas systems and search for answers across furnace types.
In gas furnaces, heat comes from combustion. If burners shut down unexpectedly while the blower continues running, the result feels identical to an electric furnace cycling off heat.
Common gas furnace causes include:
-
Dirty flame sensors
-
Ignition failures
-
Venting or pressure switch issues
Homeowner-friendly explanations of these problems are covered in gas furnace troubleshooting resources such as HVAC.com’s guide on fluctuating heat output (HVAC.com furnace airflow and temperature issues).
Airflow: The Root Cause of Most Temperature Swings
Across electric and gas systems alike, airflow is the most important factor in stable heating.
Restricted airflow can cause:
-
Overheating and safety shutdowns
-
Heat cycling on and off
-
Uneven temperatures throughout the home
The U.S. Department of Energy consistently emphasizes airflow and filter maintenance as core requirements for reliable heating performance (DOE heating system maintenance guidance).
For the Goodman MBVK, proper airflow is especially critical because variable-speed systems react quickly to pressure and temperature changes.
Why the Goodman MBVK Makes These Issues More Noticeable
Modern systems like the MBVK are more responsive than older furnaces. They monitor temperature, airflow, and electrical load continuously. When something isn’t right, the system adjusts immediately.
That responsiveness is a benefit—but it also means symptoms appear sooner. A dirty filter that an older furnace might tolerate can cause noticeable cycling in a high-efficiency electric system.
In other words, if your heater blows cold air then hot, the MBVK is often doing exactly what it’s designed to do: protect itself while maintaining airflow.
When Temperature Swings Mean You Should Call for Service
While many cases of fluctuating air temperature are normal, there are times when professional evaluation is necessary.
Call for service if:
-
The furnace never delivers consistently warm air
-
Temperature swings happen repeatedly during every cycle
-
The system shuts down unexpectedly or trips safety limits
-
You smell burning odors or hear unusual noises
Troubleshooting charts and diagnostic frameworks, such as those published by Energy Star for heating systems (ENERGY STAR heating system guidance), can help homeowners understand when a symptom crosses the line from normal to problematic.
Preventing Furnace Temperature Fluctuations
Most temperature swing complaints can be reduced or eliminated with basic maintenance:
-
Change air filters regularly
-
Keep supply and return vents open and unobstructed
-
Ensure ductwork is properly sealed and insulated
-
Verify thermostat configuration for electric heat
-
Schedule annual professional inspections
These steps not only stabilize airflow and temperature but also extend the life of the system.
Final Thoughts from the Field
If your furnace blows cold air then hot, or your furnace blows hot then cold, don’t assume the worst. In many cases—especially with systems like the Goodman MBVK electric furnace—this behavior is part of normal, efficient operation.
The key is consistency. Brief temperature changes during startup or shutdown are expected. Ongoing swings, frequent heat loss, or repeated safety shutdowns are not.
Understanding how your system is designed to operate turns confusion into confidence and helps you know exactly when it’s time to take action.
