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AC basics

Hey folks!

I am really new to the AC world, and took my first readings today on a unit I posted about a few weeks back, and now have a few newbie questions. As I set up the gauges, I found valves for both the ref & vac connections, but the ports were 'live' when I screwed my gauges on. Should these be turned off or what is procedure here? I found the cond unit to be totally clogged, and used NuBlast to clean the coils, and can now see right through them. After running about 40 min, I got 124psi on the ref line, and 33 on the vac. Does this sound like I got a real reading on the two pressures? Ref line was 88* and vac was 64.5*. The issue is the coil in the furnace freezes up, but the cond unit has been totally clogged for some time.My gauges have 4 knobs, (blue, red, 2 yellow hoses) all were closed when reading pressure. I ordered 2 very large books from Dan to dive in, but missed the install & troubleshoot 1&2 classes offered this summer.

Thanks,  Tim
Just a guy running some pipes.


  • jim lockard
    jim lockard Member Posts: 1,059

    Sounds like you need to ride my truck for a week or 2 that would be the easy way to teach you most of (not all) the things about charging A/Cs. 
  • Empire_2
    Empire_2 Member Posts: 2,343
    edited August 2009
    Hey Tim:

    Your 1st Q: The valve you connected to are simply schrader core style and are for access purposes such as to connect your gauges.  The valve as a whole can be front seated for various purposes such as pump down and isolation of the condenser to the inside coil and it's inter connecting pipping.  Usually the valves are front seated or (closed) because they hold a precharge, or nitrogen charge depending on manufacturer when new, then opened or back seated to operate the system.  As you get to know different systems, this will make a lot more sense.  The only way to get the education is to either ride along or read on the topic.

       A quick glance at your pressures sounds like the system possibly has a leak or was charged incorrectly.  MANY factors are required such as:

    OAT= outdoor temp

    IWB: indoor wet bulb.

    super heat, sub cooling, type of Ref. Etc...

    Type of expansion device and a whold host of other things.  I wish I could tell you all you need to know, but that my friend would take alot of time.  Just remember measure everything so your replies can be bassed on cause and effect of the system you are working on.

    Sorry if that didn't help much


    Mike T.
  • Timco
    Timco Member Posts: 3,040

    got my books a few days back and will just start working through them. a lot to learn. too bad I am not east coast or I would pay to ride along!


    Just a guy running some pipes.
  • Techman
    Techman Member Posts: 2,144
    AC basics

    Hi Tim,what books did you buy?Here's a" ballpark" explination of your gauges.Typically you use AC  from 70*-95**od air temp,here at L.I.N.Y..At 70*od the head press should be about equal to 95* condensing temp.So on you hi side gauge{or the friendlyP/T chart} find 95* on the 22 scale ,then follow a straight line out to the pressure scale,which should be 181.8 psig.Then on a 95* day the cond temp  is around 125* .So for r22,125* condtemp =277.9 psig Now these numbers  are for a 10 seer system.The low side gauge{back press or suction, but, NOT vac unless the reading is actually below 0 psig}should never be below 32* or 57.5psig for r22.,and that 32* would be on a 70* od  day. I suggest you contact Testo Corp and ask for their"Testo AC Application Guide &Reference" .It is really good reading,and pages #31-37 will explain the higher seer units Good Luck!
  • MarkPFalade
    MarkPFalade Member Posts: 68
    edited September 2009

    Usually what happens is the condenser is clogged, service tech finds high head/discharge pressures but instead of carefully checking the condenser coil he assumes someone has overcharged it and lets some gas out to reduce the pressure. Later you come along, clean the condenser and suddenly you're looking at an undercharge, which is what your readings are saying. Problem is sometimes, because the unit has been running in a distressed condition for probably years, when you charge it up to spec the poor compressor's little heart can't take being increased back to full load and it goes kablooie. Same thing for the fan motor because it too has been running under stress.


    Best/quickest way to determine a clogged condenser is to grab the liqid line while it's running. If it feels hot to the touch check for a clogged condenser first. Sometimes you have to take your screwdriver, gently spread some fins apart and look right down in there. That's when you'll see the clog. The surface being clean means nothing. Also, while we're on the subject, if the unit has a "split" condenser coil which is two or more single row coils sandwiched together it MUST be dismantled, carefully, coils separated and washed down in between them as well as through. If you don't separate them you'll only blow the gunk into the next layer and never get it out which means you haven't solved the problem.

    Good luck! 
  • M Downey_2
    M Downey_2 Member Posts: 21
    Don't forget airflow !

    Another thing to check is airflow inside. I f you have an extremely dirty filter or a bad motor or belt, or if it was run without a filter for a long time, this will also manifest it self with similar symptoms.
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