Theory on heater and mpg

[Carlitos_92]

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.

True, but I wish they wouldn't. The first two letters in "HVAC" still have distinctive meanings, and in the design/construction industry, we have to be specific about what we are talking about.

For the context of this discussion, A/C refers correctly to a refrigerant system with compressor, condenser, evaporator, and expansion valve. As most of the posts have stated, this system is used to cool the air, but also operates in the defrost cycle along with the heating function to dehumidify.

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[The Real Maverick]

A) the "chilling" air conditioner compressor only runs at 35°F or warmer.

B) when in "defog" above 35°F it is a double-whammy hit to MPG:

1. You spend energy to chill the air to get the water to drop out (the condensate drips under cars)​

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2. Then you spend energy to heat that dried air up to desired setting​

C) Thus defog / defrost setting is worse than heat alone, or air conditioning alone.

D) Heating usually takes more energy than cooling. And it makes sense when you look at the delta T.

​

Winter 20°F to 70°F = 50° of heating​

Summer 90°F to 70°F = 20° of cooling​

 

[The Real Maverick]

Sorry your incorrect. The defroster mode utilizes the AC

You are on the right track.

 

[Ozarkbeard]

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

Not very well. I had a work truck with no A/C. Had to keep a towel handy to wipe off the windshield on very humid/rainy days at certain temp levels.

 

[Ozarkbeard]

...whenever the windshield defroster is on, the drop in mpg is the greatest...

Makes perfect sense, bc the HVAC system is using both heat and A/C compressor to accomplish de-humidification and heating.

1. The electric A/C compressor in the Hybrid runs off the HV battery. When the battery charge gets too low, the engine runs more to help charge the battery.

2. The heater uses waste heat from the gasoline engine to heat the cabin. So oftentimes, even if your Hybrid might normally be running in electric-only mode, the gasoline engine has to run more to provide waste heat, if you have the heater on.

A double mileage-hit in the Hybrid.
A single hit (from the A/C) in the ecoboost.

 

[icegradner]

Defrost doesn’t use AC. It uses heat.

Please read your owners manual, you will learn a lot. As others have said, the AC compressor is used when humidity values exceeds the programmed value.

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.

Now the answer this question. I assume you have a hybrid, since that is in your user tag.

The heat generated by the ICE is used not just to heat the cabin, but also to keep the high voltage battery at optimal operating temperature. It does this using the exhaust gas heat exchanger, which uses the heat from the exhaust to warm up the engine/battery coolant when needed. When you run the heat in the cabin the ICE needs to come on more often to maintain the desired temperature for the coolant. Since the hybrid ICE uses what is called the Atkinson cycle, it runs cooler over all than a traditional ICE vehicle. Therefore it needs to run more to generate the same amount of heat. That is why we often see a big drop in MPG in hybrids during winter. Combine that with the less efficient burning winter grade gas, and you could see a large MPG drop compared to the warmer months of the year.

 

[mikellmikell]

MAX runs the motor all the time.

 

[Red454]

I've been using the engine block heater on my 2.5 Hybrid engine to get a head start on the warming cycle in the mornings. Hopefully starting off "warm" helps to reduce engine wear.

Vehicle is in the garage and I get the heater going for roughly 1.5 hrs ahead of departure. 1/4 mile down the road I can start to feel heat. Temp gauge comes up faster. I checked the power draw on the block heater and it looks like between 300 / 400 Watts.

8.6 mile trip to work - all city streets, top speed of 40mph. Also I should mention that I run in Sport mode for the first few miles to get the engine up to temp after seeing the pudding in the oil cap. After using this method (sport mode), the pudding has disappeared. Mileage takes a hit of course, but I expect that in the winter. Summer is in the 54mpg range, now 36 to 40mpg.

 

[Sugarbush Bob]

Defrost doesn’t use AC. It uses heat.

The AC is on during defrost in order to remove humidity.

 

[Sugarbush Bob]

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.

You can feel the engine slow down from drag when the AC or Defrost is on

 

[MavMeister25]

Thanks for the info, everybody.

I just assumed that since the defroster was on the same screen menu as the other 2 vents (dashboard and feet), that it was just 3 different ways of directing the air from the same source and with the same settings. Now I know that clicking the defrost one on top ALSO kicks in the AC automatically.

 

[YazYaz]

I've been using the engine block heater on my 2.5 Hybrid engine to get a head start on the warming cycle in the mornings. Hopefully starting off "warm" helps to reduce engine wear.

Vehicle is in the garage and I get the heater going for roughly 1.5 hrs ahead of departure. 1/4 mile down the road I can start to feel heat. Temp gauge comes up faster. I checked the power draw on the block heater and it looks like between 300 / 400 Watts.

8.6 mile trip to work - all city streets, top speed of 40mph. Also I should mention that I run in Sport mode for the first few miles to get the engine up to temp after seeing the pudding in the oil cap. After using this method (sport mode), the pudding has disappeared. Mileage takes a hit of course, but I expect that in the winter. Summer is in the 54mpg range, now 36 to 40mpg.

When plugged in, the block heater will not turn on until the temp goes below 0 degrees. Has the temp been going below 0 degrees in Indy?

 

[YazYaz]

SWITCHING MAXIMUM
DEFROST ON AND OFF
Press the button.
Air flows through the windshield air vents,
and the blower motor adjusts to the
highest speed.
You can also use this setting to defog and
clear the windshield of a thin covering of
ice.
Note: To prevent window fogging, you
cannot select recirculated air when
maximum defrost is on.
Note: The heated wiper park also turns on
when you select maximum defrost.

There is a difference between "defrost" and "defog". You do not "defrost" the outside of the glass with cold air. You must use hot air which is simply a matter of changing the "flow" of the air.

 

[olderthanyourdirt]

Defrost doesn’t use AC. It uses heat.

Heat and the AC to dry the air, been that way for over 20 years

 

[Cherokee]

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

A dirty T-Shirt and some elbow grease.

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