Theory on heater and mpg

[MavMeister25]

There have been some recent threads here about how the heater significantly drops mpg, even more than AC.

In my fiddling around and experimenting with my new 2025, it seems that whenever the windshield defroster is on, the drop in mpg is the greatest. Even if you keep the fan speed and temp setting the same, just turning off the windshield defroster seems to keep my mpg higher.

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[bgn]

I think the AC is on when the defrost is on, so that would make sense.

 

[MavMeister25]

I think the AC is on when the defrost is on, so that would make sense.

Defrost doesn’t use AC. It uses heat.

 

[HeyBales]

I think you'll find that this truck just like every car I've ever owned or been in - if defrost is on, even defrost & foot setting as the option many times was - AC is on.
Many car's you can hear, or see the RPM dip, as the compressor comes on.

The temp that comes out only matters to what you have the dial set to.

AC removes humidity no matter the temp.

So when you get in with snow on shoes, and it melts, and evaporates, it doesn't frost up the inside of the windshield as the heat carries it up but freezes there.

So makes sense just like every car - defrost off saves some engine power.
In this case with hybrid, saves some HVB power, which ultimately saves some engine running.

That's why I don't run with defrost on, never have, unless I needed it.
Sometimes just to keep ice off the windshield, indeed defrost is only way to get the heat there - but AC is on.

 

[OleFordGuy]

Defrost doesn’t use AC. It uses heat.

Sorry your incorrect. The defroster mode utilizes the AC System/Compressor to dehumidify the air while heating at the same time. Heating via the floor or dash vents does not use the AC System/Compressor.

 

[boatman]

Hmm??? I wonder how the defrost systems worked before A/C or vehicles that didnt come with that feature?

 

[OleFordGuy]

It would have blown hot air same as the other vent locations without dehumidifying on non a/c vehicles

 

[Darnon]

Hmm??? I wonder how the defrost systems worked before A/C or vehicles that didnt come with that feature?

Obviously it still worked, but windows without defroster blowing on them were more prone to fogging, and the next trip might be even worse frosted because of the extra humidity condensing on the interior and freezing. Blowing hot/dry air and lowering the cabin humidity just makes it all work better.

 

[Waterick]

Hmm??? I wonder how the defrost systems worked before A/C or vehicles that didnt come with that feature?

Older systems without A/C, simply used the warm (fresh) air, as usually those systems did not have a recirculate mode that builds ups moisture and contributes to the need to defog as has been noted.

 

[slashsnake]

There have been some recent threads here about how the heater significantly drops mpg, even more than AC.

In my fiddling around and experimenting with my new 2025, it seems that whenever the windshield defroster is on, the drop in mpg is the greatest. Even if you keep the fan speed and temp setting the same, just turning off the windshield defroster seems to keep my mpg higher.

Sometimes there are other factors than the one you are looking at. For example my pickup gets 10mpg less in driving a mile with my son than without him. Does his 60lb weight impact my vehicle that much? It's not because of my son, it's because my mom picks him up from school. I drive down a hill to her house, up a hill back to my own. It's the hill, not the 60lbs of weight there.

Wondering if this is more because people use the heater in cold weather only? You have denser air in winter (more friction/drag on the vehicle). You have higher friction on the engine (colder oil and fluids). You have potentially winter fuel blends with less energy per gallon. Basically expect to lose about 15% of your efficiency (MPG) when it's 20 out vs. when it's 75 out. And Hybrids could be up to 30%.

So if a heater is decreasing efficiency by 3%, and A/C by 7% for example, and you run a heater in winter and A/C in summer, you are down 18% in mpg when running the heat vs. 7% when running A/C.

 

[Glen Baker LLC]

Hmm??? I wonder how the defrost systems worked before A/C or vehicles that didnt come with that feature?

'In the olden days'
The windshield fogged up until it got hot enough to clear an area. Then you peered through that little area to drive until the whole windshield cleared. OR you wiped it with your hand or sleeve. Then tried to look through the streaks in the evening on your way home.
I won't talk about ice on the windshield

 

[Gray Goose]

Hmm??? I wonder how the defrost systems worked before A/C or vehicles that didnt come with that feature?

Roll down a window. Driving with a window open at -20F is not pleasant but it gets the humidity out quickly.
Edit: It's one of the reasons the old vehicles had quarter windows.

 

[BrianA]

Defrost doesn’t use AC. It uses heat.

Not sure about the maverick, but I think he is right. Historically Ford typically runs the ac compressor when defrost is on to further dehumidify the air and provide better defrosting. Of course re-heating air conditioned air is not efficient, so makes perfect sense.

 

[YazYaz]

The meaning of A/C has changed. People relate Air Conditioning with cooling however many companies now use the term to describe any type of "conditioning" of the air, such as cooling, heating, dehumidifying etc.

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