I didn’t have an inspection today so I was watching an Indian guy talk about refrigeration all day. I even watched a video from 1944 about refrigeration. I have a question for all of you HVAC pros. In normal operation the refrigerant enters the compressor as a gas (vapor) the compressor compresses it. Since its under pressure, the boiling point lowers so it becomes a liquid. (Right?) when it goes through the expansion valve it becomes a vapor (decreased pressure=decreased boiling point)

My question is…

When the unit is off the refrigerant loses heat so does it all go back to liquid? Is that why the start up of the AC draws more amps? Because the compressor and motor are under a higher load?

Increased pressure=higher boiling point
Decreased pressure=lower boiling point

That’s why the refrigerant condenses to liquid (exothermic reaction releases heat) in the compressor then flashes to gas in the coil when the pressure drops (endothermic reaction absorbs heat). That’s part of the reason Felix Baumgartner had to wear a special pressure suit when he did his world record jump - to keep his blood from boiling due to the extreme low pressure.

I’ll let the HVAC pros answer the rest

An increased mechanical load maybe part of the reason for increased current at start up. The motor itself is a large reason for the high current at start up. When voltage is first applied to a stationary motor the motor is generating no counter electromotive force (CEMF). Once it starts moving and generating CEMF the current decreases dramatically. The CEMF actually produces a voltage opposite that applied to the motor increasing the apparent impedance of the motor hence reducing the current demand.

Get the HVAC book called REFRIGERATION and AIR-CONDITIONING (ARI) , used at refrigeration schools, will answer all your questions.

You only have two heat transfers within a normal cycle in the cooling mode. 1 being at the indoor A-coil and 2 within the outdoor condensing coils.

The discharge from the compressor is a high pressure vapor discharging into the condenser of the outdoor unit. R-22 refrigerant at 250 PSI becoming a high pressure liquid due to the exterior fan and heat transfer the pressure remains at or near 250 PSI all the way to the expansion valve located at the A-coil where the high pressure liquid is flashed into a low pressure vapor within the A-coil where another heat transfer takes place by the indoor fan across the coil.

Low pressure vapor at 65 PSI returns to the suction side of the compressor and restarts the cycle.

When the unit is off don’t think about a heat loss think pressure loss because the system equalizes through the expansion valve returning to a temp equal to the exterior ambient. When the compressor is off and the refrigerant equalizes it normally stays in a vapor state equal to its surrounding temp, no significant liquid in the compressor during the off cycle

Gotta go inspect see ya tonight

Thanks Chuck. I knew that. I am not sure why my brain failed me while writing the post.

Thanks James. I am ordering now.

Charley! Thanks! That makes so much sense. I get it now.

Gerald…you are speaking another language. Thanks for trying though :slight_smile:

BTW Juan while you are thinking pressures you might already know but just incase you don’t once the unit is turned off one should wait for the pressure to equalize between high and low sides before restarting roughly 4 to 5 minutes this prevents the compressor from starting against a high head pressure and possibily tripping the breaker.

Some higher end units have a start capacitor to allow easier start up of the compressor, how to tell the difference between run and start capacitors run are oval shaped with metal and start are round with bakeolite (plastic)

Think awhile about this, residential A/C units are normally 240 volts single phase which requires at least two hot wires to the compressor why do all compressors of this type have 3 wires attached to the compressor. Try not to over think this:p:D


Not even in the same ball park;-)

To the capacitor?

The terminals on the compressor are labled CSR for Common, Start and Run

Yes the compressor has a internal start winding from the capacitor