Hvac question
Thread Starter
Junior Member
Joined: Nov 2024
Posts: 24
Likes: 7
Hvac question
In my 2025 F150 work truck, the HVAC system is manual with no automatic functions. On these fall mornings I am using the heater, and like to use the recirculation system, as it helps warm up the cab quicker, and then maintain the temp at a lower fan speed. When I push the button to activate the recirculation mode, it works, but just for a while. Just a few minutes later, I can hear a change in airflow noise, and notice that the indicator light is off. I can immediately reset it, but the cycle continues.
First question, is this normal? Is the system designed to drop out of recirculation mode after a set period, and if so, why? Or is there a malfunction in my system? If so, it is still in warranty, and I will get it corrected. But I am coming here first to see if others with manual systems have experienced this, and to read other replies.
First question, is this normal? Is the system designed to drop out of recirculation mode after a set period, and if so, why? Or is there a malfunction in my system? If so, it is still in warranty, and I will get it corrected. But I am coming here first to see if others with manual systems have experienced this, and to read other replies.
Senior Member
Joined: Dec 2020
Posts: 859
Likes: 470
From: Pittsburgh, PA
Yes. There are a few algorithms in there that calculate the risk of window fogging and of CO2 buildup and will automatically set it back to partial or full intake to make sure humidity levels and CO2 levels stay under control for safety reasons.
There is actually also an automatic partial recirculation function that engages with high blower speed on heat and with an engine that's not fully warmed up. Basically 50% recirculation and 50% fresh air. This is fully automatic and we have no control over it, but you can hear it sometimes. Around 35-40 mph it always switches to full intake, but if the engine isn't fully warmed up, it will switch right back to partial recirculation once you fall below that speed again. You can usually hear that change. You can also tell it's not full recirculation because if you press the recirculation button, it gets louder still.
Letting the partial recirculation do its thing is really the way to go. It has enough fresh air to keep CO2 and humidity levels reasonable, but also means a faster cabin warm-up and faster engine warm-up than running full intake, which is good for engine life, fuel economy, and emissions.
There is actually also an automatic partial recirculation function that engages with high blower speed on heat and with an engine that's not fully warmed up. Basically 50% recirculation and 50% fresh air. This is fully automatic and we have no control over it, but you can hear it sometimes. Around 35-40 mph it always switches to full intake, but if the engine isn't fully warmed up, it will switch right back to partial recirculation once you fall below that speed again. You can usually hear that change. You can also tell it's not full recirculation because if you press the recirculation button, it gets louder still.
Letting the partial recirculation do its thing is really the way to go. It has enough fresh air to keep CO2 and humidity levels reasonable, but also means a faster cabin warm-up and faster engine warm-up than running full intake, which is good for engine life, fuel economy, and emissions.
