We have a fairly old central A/C system that has required very little repair
over its 25-year life. It is a dual-zone system. Recently, one compressor
started running only intermittently. Today it would not start at all (just
the inside fan came on, but no compressor running, no cooling).
In trying to trouble-shoot this problem myself (at least to see if I could
find the root cause), I confirmed that the normal voltage existed across the
wires from the thermostat, but discovered that at least one of the two
"pressure switches" has an open circuit -- these "pressure switches" are
wired in series, between one of the thermostat wires and the compressor
control unit. I think that each of these "pressure switches" monitors the
refrigerant pressure (one pressure switch on each of the refrigerant lines
leading back to the house), and is supposed to interrupt the thermostat
voltage if the refrigerant pressure gets too high.
I seem to remember, for example, that if you run the A/C, then turn it off
(by raising the thermostat setting), then immediately turn the A/C back on
(by lowering the setting), there is a chance of "blowing out the
compressor," since when it comes back on, the pressure it's pumping against
is still very high. So, probably a main reason for the "pressure switches"
is to prevent the compressor from starting up if the refrigerant pressure is
still too high from recent pumping.
Anyway, to see if the problem really was just one of these "pressure
switches," I shorted the two of them out. The compressor started like a
charm, and has been running perfectly, cooling the house.
My question: If I continue to run the A/C system this way, with the pressure
switched shorted out, until I can get them replaced, am I running a major
risk? Should everything be OK as long as I'm careful not to re-start up the
A/C right after it has been turned off? I.e., if I'm careful to wait 5-10
min before re-starting the A/C (or just let it cycle normally via a fixed
thermostat setting) is there no danger of "blowing out the compressor?"
You wont like this, and its a sad and simple fact..
First, no EPA card, you have no right to be working on the system...nor, do
you obviously have the training to properly diagnose the issue.
No, you overload the start windings, the thermal overload trips, and you
have to wait until the pressues equalize, with no power to the unit, and the
The pressure switch operation is to prevent you from doing what you just
overiding a safety device that is there for a reason.
Yay..now..why is the switch out of the loop?
IF and since no one can tell from here, IF you have a problem with a low
charge condition..say, its a LP, or low pressure switch that is bad, you
just sent the compressor to hell.
IF its a high pressure switch, and the head pressure goes sky high for
ANOTHER problem, then you can among other things, send the compressor to
If the head pressures go high enough, you will find out what a blown out
compressor looks like....its got a hole where the terminal leads used to be,
and then its over..
Bottom line...you need to call out a properly trained person, one that can
tell you if its ok like it is, properly check the system out, and at 25
years old, the best advice might be to replace it.....
However, you need to tell the person that is looking at it what you
Then, when you find out if something is wrong, and what the repair is, you
can make an informed buy if needed.
I know absolutely nothing about HVAC, so I can't comment on the
reasonableness of your proposal.
However... A 25 year old system is aching to be replaced; the payback
should be very good. If your patch job destroys it, then it will just be
forcing to do what you should be doing anyhow.
I am assuming, without knowing any better, that your patch does not threaten
your house or you, just the A/C.
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