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# HVAC High pressure switch

## HVAC High pressure switch

(OP)
I've searched high and low, pun intended, to find out, at what actual pressure, an HVAC pressure switch might kick in, (or I supposeI should say kick out, since it is normally closed).

Or let me put it another way.

If a high pressure switch is continually interupting the circuit, how can one tell if it is simply doing its job, in protecting the system, or whether it is faulty?

Obviously to know if it is faulty or not, it would be good to know the supposed designed switching pressure.

The internet is full of dumb answers, saying "if the switch is constantly breaking the circuit then you need a new switch".

How dumb is that!

Does anyone know an actual switching pressure?

Even just a general value 300psi, 400 psi ??

### RE: HVAC High pressure switch

To start, there are often two high pressure switches and almost always a low pressure switch.

The lower high pressure switch is used to simply turn on/off the condenser fan(s).
The higher high pressure switch is to save the system from disaster. Since the load is basically uncontrollable sometimes it can get too high. That would toast the compressor, so the high high pressure switch stops everything. Often called "high pressure lock-out". Some require human intervention to reset others just require the pressure to drop considerably before "lock-in" occurs. Usually about a 100PSI of drop.

The low pressure switch is the one that controls the actual refrigeration. Whenever the suction side pressure exceeds some number the compressor is started and runs until the pressure drops to some low amount. There is usually 40~50PSI of hysteresis in the low pressure switch.

There is sometimes a second lower pressure switch that's there to protect the system from lost gas. It's called the "low pressure lock out". If the pressure ever get too low it means there is no refrigerant left - a very common refrigeration failure. Because the compressor motor is cooled by the refrigerant, running without it is a pending disaster.

The above description is for a quality commercial refrigeration system. There are lots of systems that are completely cheaped-out and forgo lots of the safety and protection available with a quality system.

ALL THE PRESSURES are completely contingent on the particular refrigerant in use so without more detail no one can tell you what pressures. I will say it's likely a couple of hundred PSI.

On cheap-crap systems they often forgo everything but the high pressure switch. They also just switch the compressor on/off with temperature and nothing else. In all cases the high pressure switches should really never cycle at all. If it does there is something wrong. Usual problems that would cause this are an excessive ambient, clogged condenser fins, or a dead condenser fan.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: HVAC High pressure switch

The system high pressure is set by the condenser temperature. An upper limit to the condenser temperature, or the temperature of the liquid leaving the condenser, is about 150F. A higher temperature would indicate a problem such as a failed cooling fan. So, to answer your question, the high pressure switch would be set higher than the refrigerant vapor pressure at 150F.

### RE: HVAC High pressure switch

(OP)
"On cheap-crap systems they often forgo everything but the high pressure switch."

Oh dear. That's how mine is.

It's a Honda Accord.

I have a set of gauges coming so soon I will be able to determine the actual pressure that the switch is cutting out at.

But lets say it's 300psi, that doesn't really let me know if the switch is scrap or not.

The official Honda Factory Service Manual doesn't even mention pressures limits.

### RE: HVAC High pressure switch

(OP)
All sorted.

Too much freon was my problem.

Just for reference, the switch opens the circuit at 200 psi

Thanks

### RE: HVAC High pressure switch

Interesting.

That's a lot lower than I'd have ever guessed.

Is this an R12 look alike?

Thanks for the feedback.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: HVAC High pressure switch

(OP)
R134a I believe, but I don't know for sure.

I didn't pay much attention.

I didn't realise the fluon man had a choice.

I just presumed that's what it had to be.

### RE: HVAC High pressure switch

At 200 psig R134a is 130F.

### RE: HVAC High pressure switch

Thanks for that temp angle Comp. Makes sense if one thinks about it.

Keith Cress
kcress - http://www.flaminsystems.com

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