best way to defrost?

Water is water even frozen A/C is a dehumidifier works in all weather that is why the A/C comes on in defrost there is no where near enough A/C to over come the heater so full blast heat will melt the ice on the outside and dehumidify at the same time.

 

How do you figure a frozen A/C system will do anything? If the evaporator freezes up, and the condensation freezes, air cannot pass. If air cannot pass, then it wont reach the windshield. And while the heat will melt the outside ice, it won't dehumidify the outside, only the inside. Or did I misunderstand your post?

 

Frozen water on the glass not the evaporator. Water is water even frozen(,) A/C is---- that better?

 

Has no one ever figured out what the function of the thermistor on the evaporator is?

It's a temp sensor that the controller uses to regulate the AC system. When it gets above a certain temperature, it turns on, and when it gets down to a lower temperature, it turns off. Most vehicles have that range in the 36-45F area to prevent ice buildup. (Some older vehicles had a 'desert' setting that would let it get down into the 20s for hot dry climates, but drivers had to be wary of blockage. Hydrolocking the compressor will happen if the refrigerant doesn't have enough air moving across the evaporator to make it boil.)

That means if it's below freezing feel free to put the air on defrost all you want - if that thermistor is reading anything below 35F (and that should be outside air going across it as the heater core is 2nd) the system will not turn on, period.

Why does this work for us?

Whenever the air is below freezing, water content in the air is typically very very low. That's why you can see your breath, why frost forms so easily, etc. The air can't hold much water since it just tends to frost out. Since there isn't much water to remove, and the heater core bringing temp above freezing increases how much water the air can hold so much, the AC system isn't even needed at that point.

When ambient temp is above freezing there's potentially a lot more water available to condense out of the air and make our windshields fog up. Coincidentally this is only a few degrees away from the temps that the AC system can run so most of the time it can help us out.

 

A/C only dehumidifies if it is cooling warmer humid air that can condense water on the evaporator . A/C does not cause condensation to form if it is warming air like if the outside air is colder than 35F.

So no, A/C does not dehumidify in all weather.

 

worthy - the AC system cannot push heat into the cabin because to do so the flow of refrigerant would have to be reversed. It would need the heat from the compression to do so and it can't get past both the condensor and the expansion valve, as going through either causes drastic drops in temperature.

That would be pretty cool if we could do that though. Dual switchable TXVs, a reversible compressor, and bam instant heat!

 

A/C is heat if the outside temperature is below the temperature of the evaporator. You don't have to reverse the flow.

 

I put it on full windshield defrost , crank the heat and fan

 

Install an engine block heater with a thermostat cube. Set your defrost control to windshield, heat on full. When you go out in the morning, windshield defrosted and truck warm.

 

Just park in a heated garage.

 

I'd like to know your reasoning for this if you'd be so kind. I'm not understanding how a heat pump can become a heat generator.

My confusion is as follows:

Assumed typical operation: the condensor gives the compressed gaseous refrigerant's heat to the outside air which produces a high pressure liquid; this liquid refrigerant passes through thermal expansion valve into the low pressure evaporator, the rapid pressure loss creating a very low temperature; the evaporator allows the cold liquid refrigerant to absorb heat from the cabin air and boil before returning the compressor.

On a mild day the temperature produced in the cabin by the system is potentially colder than on a hot day (typically resulting in a lower duty cycle) because the transfer of heat from the condensor to the outside air is more efficient. If the outside air is hot enough, and airflow through the evaporator fast enough, it may never shut off due to this effect.

So my question is how during the cold weather when the condensor is even more efficient can heat be produced from the evaporator, providing on startup the condensor and evaporator are at the same temperature, or whether their temperatures matter at all, and assuming the thermistor cannot measure the below-range temperature on the evaporator surface?

Another situation I would like to test "A/C is heat if the outside temperature is below the temperature of the evaporator" is when the truck has been sitting outside on a warm sunny day. The driver gets in, starts the truck, and turns the knob to max AC. This will recirculate 150F+ degree air through the evaporator when there's 90F+ air going through the condensor.

 

I use spray on de-icer & set the air outlet selection dial to windshield only, outside air mode, with maximum heat. When the windshield clears up I switch to floor only. I leave the air outlets alone. I don't think the A/C comes on if the outside air temperature is below 32 degrees F.
On cool fall mornings I use the A/C with heat to dehumidify & stay warm.

 

Doesn't the AC come on automatically with either defrost setting ?

 

Yes.

Frank

 

So where is all the confusion here ? Stick it on Defrost , crank the heat and fan

Works for me everytime

 

Not complicated enough in order to post a question.

Frank

 

It can only be a heat generator if it cycles and brings the evaporator to 35-40F. If you are correct that the thermistor won't even let the A/C cycle if it's colder than 36-45F ambient then it won't be a heat generator.

I was assuming that it is set to below 36F for winter use when it is already cold dry air and not likely to freeze the evaporator.

The second 150F example wouldn't hold for very long if I'm following.

 

An air conditioner is a dehumidifier by design and cooling is a by-product of the refrigerant cycle. In the winter the A/C coming on with defrost serves two purposes, one to help dry the cabin air on the windshield facilitating defrost, and two, to lubricate the compressor seals. A compressor sitting for long periods without running will cause the seals to deteriorate. Prior to the a/c automatically coming on with the defrost cycle it was recommended to run your a/c periodically to ensure the seals stayed lubricated. A car a/c compressor is nothing like a sealed hermetic compressor such as your home unit or refrigerator. A car compressor is a direct-drive unit consisting of a compressor, shaft and seal, along with the external clutch drive.

 

82 and sunny today at my house with a low in the 60s tonight (I'm still running my A/C on most days). We may have honey boo boo down this way but, ice (and even frost) is not too much of a problem.

My suggestion, move south.

 

Thank you for clarifying what you meant. I thought you were talking about condensation on the evaporator, not the windshield. A frozen evaporator would not work.

To clarify and correct some of the technicality here in your post:

"Hydro-locking the compressor will happen if the refrigerant doesn't have enough air moving across the evaporator to make it boil"

The R134a (refrigerant) is already in gas form before it enters the evaporator. It converts from liquid to gas when it passes either the orifice tube, or the TXV, depending on the system. The compressor will not hydro-lock like that. It takes a significant amount (considering percentages) of the R134a in liquid form to cause problems. You would need an orifice that has failed (broken) or a TXV stuck wide open to even have a chance to hydro-lock a compressor.

Also, not every AC system has a thermistor. Some use the pressure sensor to calculate temperature of the R134a. The controller does not care about AMBIENT temperature. Instead it is concerned with the evaporator's core temperature. The difference it this: Stand outside in 32 degree weather. you feel cold, yes. Now, in that same 32 degree whether, hold an ice cube in your hand. The ice is also at 32 degrees, but it will FEEL colder because CONTACT will transfer heat faster than ambient. It is the CONTACT temperature of the R143a that is the biggest concern, for that will allow the condensation on the core to freeze.

I ask you this, if you think that there is so little humidity in the air when it is freezing, try running your defroster without the compressor running and see how long it takes. You will find that your windshield will fog up quite quickly.

You can actually do this with a properly set up TXV valve. You don't need a reversible compressor. Home heat pumps work exactly the same, using a switchable TXV system, and the compressor runs normally.

Again, a bit of clarification:

The R134a is already in liquid form when it enters the condenser. The COMPRESSOR is what changes the R134a into a liquid, not the condenser. The job of the condenser is to remove the heat caused by compressing a gas into a liquid form. As the liquid passes through the orifice, it is not the "low pressure" evaporator that causes the low temperature. The TXV (or orifice) is the part that does this job. What happens is that when the R134a passes through the orifice, it is coming into an area allowing the liquid to evaporate or "boil" (hence the name evaporator core) this rapid expansion of R134a into it's natural gas state causes it to become very cold.

Also: "Another situation I would like to test "A/C is heat if the outside temperature is below the temperature of the evaporator" is when the truck has been sitting outside on a warm sunny day. The driver gets in, starts the truck, and turns the knob to max AC. This will recirculate 150F+ degree air through the evaporator when there's 90F+ air going through the condensor"

You are not comparing the same information. You are referring to a truck sitting outside on a warm day. This discussion is about cold days. His comment is accurate. If the outside temp is say -15 deg, and the evaporator temp is at 32 deg, then simple physics says that when the -15 deg air passes through the 32 deg evaporator, it will actually warm up.