Compressor Pump Down Pressure

riverrunJuly 15, 2008

While a tech was checking out my system with his gauges , he told me my brand new compressor was bad. When I asked him to explain he told me that it would not pump down into a vacum. It pumped down to about 5 psi but he insisted all compressors should pump down to at least -20psi. Is this correct?

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If I am understanding you, you are saying with his guages attatched, he shut off the liquid side valve on the condenser and ran the compressor attempting to pump all the gas into the condensor coil?

When done this is to capture the gas only when performing service on the coil and lineset and should never be done below 0-1 PSI on suction.

This is a way for an inexperienced tech to see if valves are seating but not the best and only way to do it.

AC Compressors are not vacuum units and shouldnÂt be made to run that way.

    Bookmark   July 15, 2008 at 10:31AM
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Yes he had his gauges attatched and I believe he blocked in the suction using his manifold valves. I agree with your comments about inexperienced techs because he did say it looked as if a vale was bad and the compressor should be changed.

It sounds like this tech is wrong and the unit is fine because the suction and discharge pressures were fine.

I wanted to verify these were not vacum compressors which he said they were.

    Bookmark   July 15, 2008 at 11:01AM
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I might add that new compressors wear in and perform better in time

No different than car engines after rings and valves get seated.

    Bookmark   July 15, 2008 at 11:05AM
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A filter dryer was not installed, could this lead to a problem?

    Bookmark   July 15, 2008 at 11:52AM
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New system or compressor replacement or repair?

If brand new and evacuated properly, should be no problem, although always recommended

Open system to repair? - Always

Compressor burnout? - Oversized one with suction line filter also should be used

    Bookmark   July 15, 2008 at 3:07PM
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Wow... I still havent figured out why he did a pumpdown to begin with. Was he repairing something? Installing a filter dryer? If thats the case, most condensers come with a liquid line dryer inside the condenser..most of them... but its kinda late now to be installing a dryer.

U know what you should do... If he thinks your brand new unit has a bad compressor...INSIST he replace the entire outdoor unit...not fix it... see what he does...


    Bookmark   July 15, 2008 at 9:16PM
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I called this tech because I had a majorly huge electric bill and this ac system was the only addition/change made. So he comes out and slaps the gauges on and says," You got problems!"

I asked what normal pressures were and he said70 suction and 250 discharge.

The room temp at this time was 90 degf and outside temp was 95 degf. I now know the temperatures had alot to do with the pressures. He made no repairs.

He fooled around with the gauge valves and I guess thats when he pumped down. The gauge on the suction side went to 5 psi and he immediately told me it should pull at least negative 20 psi.

    Bookmark   July 15, 2008 at 10:47PM
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He canÂt know the proper PSI's (more important the vapor temps) without checking all the other items Darrell posts on occasion for readings required to understand system operation/performance.

Here's a good one, ask him the last time he calibrated his guage set, and how he did it. Get ready for the deer in the headlights look.

    Bookmark   July 15, 2008 at 10:55PM
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you need to call another tech. i wouldnt consider 5psi not punmping down. there are more than one reason why a oompressor wont pump down....
gross overcharge, overcharge due to long lineset and probably the one reason he was thinking of is a bad suction valve on the compressor.
things that are indicative of a bad suction valve are low amp draw, low compression ratio (system pressures almost equal). ie low head pressure and high suction pressure and compressor not pumping down.

what were the pressures before he tried to pump it down?

what was the subcooling temp? (if he didnt check this he's a moron)

what was the superheat temp? (if he didnt check this he's a moron)

are both coils clean?

is the filter clean?

what is the temperature split across the inside coil?
(temp difference between the air going in and the air coming out)

if there isnt a drier inside the unit and they didnt install one they also probably didnt pull a vacuum on the system.

if this is the same company that installed the unit i would call them back and ask for a senior tech. this guy needs to zero out his gauges and get a clue.

also just a fyi.... compressors arent supposed to run in a vacuum even if they are capable of doing so.

"theyre supposed to pump down to negative 20psi" lol thats a crock

    Bookmark   July 16, 2008 at 6:41PM
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"If this is the same company that installed the unit I would call them back and ask for a senior tech. this guy needs to zero out his gauges and get a clue."

I really hope you didnÂt really mean it when you said to dial it to zero?

Zeroing out a gauge set will only make sure they are uncalibrated.

There are a few ways to calibrate but the easiest is to hook them up to a jug of Freon that has been in a stable atmosphere, take it's temperature with an accurate thermister thermometer and dial the gauge(s) to the corresponding pressure from the appropriate vapor pressure temperature relation chart of the specific gas used.

Guys that zero their gauges are usually 5-25 PSI out of whack, which means all the difference when reading a unit. Even new gauges can be 5-10 off, trust me.

    Bookmark   July 16, 2008 at 7:09PM
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