A/C hot on startup, then cold. Normal?

[Li'l Work Truck]

The above question. My A/C starts out super hot, then about 30s later it gets cooler. The air is capable of getting cold, but on the really hot days it just manages to keep the cab from being warm. All this has come to light within the last week or so.

Maybe a recharge is necessary?

 

[Uncle Gump]

Honestly... you should put a set of gauges on it.

But... moving hot air until things settle in is normal. MAX A/C setting will recirculate the inside air and get colder faster.

Does the A/C clutch have quick cycling times... usually the first indication system is low.

On a hot humid day... the compressor clutch may never cycle

 

[Li'l Work Truck]

I'll check it out! I'm not sure about the clutch frequency, but I'll rent some gauges and watch for cycling at start and after a drive.

 

[Lefty]

I'll check it out! I'm not sure about the clutch frequency, but I'll rent some gauges and watch for cycling at start and after a drive.

I have a regular cab gen II. The air comes on hot at first but cools in less than a few minutes. And after that it can even run chilly on the hottest of days.

It wasn't that way when I first got it so I charged it. No problems ever since. II did borrow a guage from a buddy.

 

[Curious Hound]

It is natural to have hot air at first. The sun has been heating up the cab. But the AC system hasn't been running. You start the truck and turn on the AC. That starts the cycle. The compressor gets moving and starts building pressure and moving refrigerant. The refrigerant isn't naturally cold. It has to move through the system and do the scientific magic. It takes time. Once the system is running, it can only do "so much". It has a capacity. Think of it as being able to change the air temperature by a certain number of degrees. Let's say 30 degrees (this is just an example). So, if the air in the cab is 110 degrees, and the system can drop it by 30, then it will initially put out 80 degree air. Of course, that 80 degree air is mixing with 110 degree air. So you don't feel much relief yet. As more air is changed to 80 degrees and mixes, eventually, the air in the cab is only 100 degrees. 100 degrees minus 30 degrees means the system starts putting out 70 degree air. Then that mixes with the 100 degree air. And so on and so forth. Eventually, the air in the cab is 80 degrees. But, remember, the sun is still trying to heat up the cab. It's a continuous fight. Eventually, you hit and equilibrium where the system can't get any cooler. This is because it has reached its maximum capacity. The sun is putting heat in as fast as the system can remove it because the system is trying to put that heat outside where the air is already hot. When you get gauges and check pressures, you have to use a chart that is based on the ambient outside temperature. We generally think of AC systems making cold air. But what the really do is move, or transfer heat. There has to be a place to put that heat. And when it's blistering hot outside, it is more difficult to move heat from inside to outside.

It's like a little guy digging a hole. Eventually, he gets to a point where the hole is too deep and he can't throw the dirt any higher to get it out of the hole and it keeps falling back in on him as fast as he shovels it out.

The moral of the story? Make sure your system has the right amount of refrigerant and make sure the condenser coil (out in front of your radiator) is clean and not full of bugs and mud. And make sure the evaporator coil (in the AC duct) is clean and not full of leaves, dust, McDonalds receipts, etc.

 

[Li'l Work Truck]

I'll check those things, too. Thanks, @Curious Hound.

 

[Li'l Work Truck]

I have a regular cab gen II. The air comes on hot at first but cools in less than a few minutes. And after that it can even run chilly on the hottest of days.

It wasn't that way when I first got it so I charged it. No problems ever since. II did borrow a guage from a buddy.

I'll take a look and see what the gauges say. I'll either rent a set from a parts store or grab an HF set.

 

[Curious Hound]

I'll add 2 more things, in case you care.
1. There is such a thing as "too much" refrigerant in the system. So, if you check pressures and find that it needs to be topped off, it's best to follow the chart and add until the pressures are right. More does not equal better. You will need a thermometer to check ambient temperature where you are working. Do not go by what the wrather app says. You need to know what the temperature is right where you are - in the sun, in the shade, in a garage with door open or shut, whatever. Not 10 miles away at the airport.

2. Here is a diagram of the Scientific magic I mentioned earlier. That refrigerant is "changing state" between hot low pressure gas, hot high pressure gas, cold high pressure liquid and cold low pressure liquid in different parts of the system. That is what allows it to take heat out of the cabin air via the evaporator coil and move it to the outside air via the condensor coil.

 

[Uncle Gump]

Just a bit of a correction....

There is NO hot low pressure gas or cold high pressure liquid in any properly functioning A/C system.

Hot is always high pressure liquid or gas

Cold is always low pressure liquid or gas

If the system is full... refrigerant will change states at the same pressure. It will evaporate in the evaporator (liquid to gas)... and condense in the condenser (gas to liquid).

I think Eric might have been typing slower then he was thinking...

 

[Li'l Work Truck]

I'll add 2 more things, in case you care.
1. There is such a thing as "too much" refrigerant in the system. So, if you check pressures and find that it needs to be topped off, it's best to follow the chart and add until the pressures are right. More does not equal better. You will need a thermometer to check ambient temperature where you are working. Do not go by what the weather app says. You need to know what the temperature is right where you are - in the sun, in the shade, in a garage with door open or shut, whatever. Not 10 miles away at the airport.

2. Here is a diagram of the Scientific magic I mentioned earlier. That refrigerant is "changing state" between hot low pressure gas, hot high pressure gas, cold high pressure liquid and cold low pressure liquid in different parts of the system. That is what allows it to take heat out of the cabin air via the evaporator coil and move it to the outside air via the condensor coil.

View attachment 145691

Ok, that's good info. I'll get a thermometer, too. I've been meaning to get one anyway.

 

[Curious Hound]

Just a bit of a correction....

There is NO hot low pressure gas or cold high pressure liquid in any properly functioning A/C system.

Hot is always high pressure liquid or gas

Cold is always low pressure liquid or gas

If the system is full... refrigerant will change states at the same pressure. It will evaporate in the evaporator (liquid to gas)... and condense in the condenser (gas to liquid).

I think Eric might have been typing slower then he was thinking...

I disagree. Looking at the diagram;
1. The low pressure gas has just picked up heat in the evaporator. So, it's hot.
2. Compressor increases pressure and the gas is still hot.
3. Gas condenses to liquid and gives up its heat in the condensor. Still high pressure. = cool high pressure liquid
4. Goes through orifice/expansion valve and changes to cool low pressure liquid entering the evaporator where it picks up heat during that phase change.

Start all over again. 4 phases of the cycle.

 

[Uncle Gump]

You forget the pressure temperature relationship.

Even the picture you posted agrees with what I said.

If the pressure is low... it's cold... liquid or gas.

What absorbs the heat from the air is the change in state of the refrigerant. There is no change in pressure. So low is cold.

Vice versa for the high side.

Touch the low side hose close to the compressor while running. It will be cold...