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A/C Pressures

say to much airflow or humidity.I would check suction line temp at the coil and then entering return temps in see what
numbers I come up with.


  • Paul Fredricks_3
    Paul Fredricks_3 Member Posts: 1,549
    What's Up?

    We have a customer with a 20 y.o. condenser and A-coil. Our tech called me to say the pressures were 92 and 235. It's 80 degrees plus outside. He says the evap is warm on the top and cold on the bottom, there is a 10 degree TD, and the a/c doesn't work well. My thought is a bad valve in the compressor.

    Any thoughts? It's been quite some time, but will pumping it down and seeing if the low side holds a vacuum test the valves?
  • lee_7
    lee_7 Member Posts: 458

    possble obstruction in coil. Is this a cap tube or txv system?
  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083

    it's a bad valve, Paul, you wouldn't see the suction that high. I would be looking for a clogged metering device or airflow issues. Maybe even exessive dust build up on fan paddles or a clogged coil.

    Years ago, I had this quirky job with pressures not making sense. Tpride. Something made me look closer at the blower (belt). Turns out the bearings were replaced a while ago, and the tech installed the squirrel wheel facing the wrong direction. Wasn't scooping air like it should.

    Turned the thing around and guess what? Now the motor overheats from being too small hp/drawing more amps, and then, the condenser compressor craps the bed from the increased, but proper load on it. It had been running for so long with screwed up pressures that it was fragile.

    Had to put in a 3/4 hp motor and replace the condenser. That was a tough one to explain to the HO~!
  • Paul A.
    Paul A. Member Posts: 7
    Are you kidding me?

    Bad suction valves would indeed cause higher than normal suction pressures due to compressor discharge pressure getting back into crankcase. Any shortage of airflow be it a dirty coil or blower wheel, as well as a restricted metering device, will cause lower than normal suction pressures. Why does a n evap. freeze-up when you have dirty filters? Answer, because the coil temp falls below 32 degr. Fall in pressure, fall in temp.
  • Paul A.
    Paul A. Member Posts: 7
    Are you kidding me?

    Bad suction valves would indeed cause higher than normal suction pressures due to compressor discharge pressure getting back into crankcase. Any shortage of airflow be it a dirty coil or blower wheel, as well as a restricted metering device, will cause lower than normal suction pressures. Why does a n evap. freeze-up when you have dirty filters? Answer, because the coil temp falls below 32 degr. Fall in pressure, fall in temp.
  • Tom Quatroni
    Tom Quatroni Member Posts: 6

    Hello, 20 year old unit. Replace it already with a new coil. The HO will save at least 35% on bills. 1987 it was probably 8 seer to go to 13. Why put any money into it?
    GREG LAUER Member Posts: 103

    Wow, I can't believe this post. There is no mention of what is important! What is the superheat? What is the subcooling? what is the ampdraw in relationship to the nameplate rla? what is the discharge temp? Do we know what superheat and subcooling is or must Paul and I instruct on how to get these readings? If my techs tried to fix everything like the above posts we would do nothing but callbacks! This guy needs help lets give it to him. stop the madness and guessing! blower wheel backwards? air flow issues? come on the low side wouldnt be in the 90's unless it was 410a. cap tube restricted? again low suction would be present. If I have guess and I may as well I would recover the unit and weigh in the proper charge it MAY BE overcharged. But what do I know.
  • Techman
    Techman Member Posts: 2,144
    My 2 1/2 cents

    Hi Paul F.With that high of a suction press I say BAD suction reeds.If the discharge reeds were leaking then the head would be low and the suction hi.By trying the "pull down"test and waiting for the low side press to rise ,that would show leaking discharge reeds.Lets say you have a 2-3-4 piston comp with broken or even missing suction reeds in all but one piston ,you should still be able to "pull down"the low side press ,shut the unit off ,and the low side press wouldn't budge an inch!The hi side press of 235 is hi for bad suction reeds ,unless the cond coil is really dirty.Any restrictions in the liquid line, metering device ,evap coil, fan,fan barrel ,belts,filters would give low low side readings.If this is a TXV and the t-bulb is "air born" then MAYBE the readings are normal for that ! An 80* day should have about a 226 head but an 80+ * day would be higher. 80+ means 85*? Which means about a 260-265 head!Thats what I think!
  • Empire_2
    Empire_2 Member Posts: 2,343

    EASY buddy!!!! Did you have a bad day? You could have handled your opinion a little differently. You are right, but Geese.................... Chill out. More info would have been nice from the Q: asker, but that doesnt always happen.

    Mike T.
  • don_185
    don_185 Member Posts: 312
    So why

    did not Paul give the readings to began with if he knew what they were?Then maybe we could have given a better answer.

    Are you saying that a 10 degree drop could not be to much airflow? What about the bypass factor of a evaporator coil?

    What about high heat load and humidity?

    If you ever seen a comp with weak valve you would know that the pressure differential between suction and discharge decays.The suction pressure will be high and the head pressure would be low.This is not the case.

    Just for the record I've seen lowside pressure in the 100 untill the load comes down.And I've seen them hover way above that and never come down on system that are over loaded...aka to small of a system.

    I always thought without all the info everything posted here was a guess.Whats change?

  • Mitch_4
    Mitch_4 Member Posts: 955
    the high side @235

    does not seem unreasonable, maybe a mite high, but not too bad.

    I lean to either non-condensibles, restricted airflow, or a problem with the TXV if you have one.

    Has there been other service techs there? what was done?
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Let's Play Nice

    Let's play nice and get back to basics.

    Here's what we know

    - The outside ambient temperature is 80+ degrees

    - The low side pressure is 92 psig

    - The high side pressure is 235 psig

    Here are some assumptions:

    1 - The system is operating with R-22.

    2 - If the sytsem is not cooling at all, the indoor air temperature will also be 80+ degrees

    3 - If assumption 2 is correct, the evaporator saturation temperatuture should be in the range from 45 degrees to 55 degrees (not knowing what the + in 80+ actually is)

    4 - If assumption 3 is correct, the low side pressure will be in the range from 76 psig to 93 psig.

    5 - If assumption 4 is correct, the system's low side pressure is acceptable.

    Since the outside ambient temperature is 80+ degrees, the condenser saturation temperature that the system is operating with is just fine.

    WHen a system is operating at "design" conditions, you can expect a delta-t of 17 - 20 degrees across the evaporator coil. Design conditions indicate a room temperature of 75 degrees at a relative humidity of 50%.

    If the r.h. is lower than 50%, the delta-t across the evaporator coil will be higher.

    If the r.h. is greater than 50%, the delta-t across the coil will be lower.

    My guess is that the system is just fine... Just let it run and things will fall into line.

    Here are some useful pieces of information to have your technician supplty you with that will enable you to paint a more complete picture of the system:

    - Evaporator outlet temperature (suction line)

    - COndenser outlet temperature (liquid line)

    - More accurate temperature readings (ambient)

    - Accurate space temperature

    - Relative humidity in the occupied space

    Hope this helps!

  • Paul Fredricks_3
    Paul Fredricks_3 Member Posts: 1,549

    OK, I going to go out there myself and get more info. Of course, today it will be in the 70's. Yesterdays high was 88.

    More info. The unit is in the basement and the house wasn't really hot, but I don't know what that means. I suppose the airflow could have been to high, but a 10 degree TD sounds like it would be a wind tunnel.

    I'll keep you all posted.

    And I love that people are passionate about what they do, but please read your posts before you send them. Some read like you're screaming at us. More flys with sugar, ya know?
    GREG LAUER Member Posts: 103

    Sorry I am not screaming. It just upsets me that with the very little amount of info anyone would give you an answer. you could be there for days using info that is not useful.
  • Empire_2
    Empire_2 Member Posts: 2,343
    Greg, I apologize....:=)

    I to was having a bad day and I to hate when not enough info is given, but we try our best to "Guesstimate" if you want to call it that. I'm sure we will all agree when it's all said and done.....(Peace)

    Mike T.
  • Paul Fredricks_3
    Paul Fredricks_3 Member Posts: 1,549

    GROUP HUG!!!
  • Empire_2
    Empire_2 Member Posts: 2,343


    Mike T.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    That's better

    Now don't we all feel better?
  • Paul A.
    Paul A. Member Posts: 7
    I'm surprised

    that after all of this, the one thing that nobody has even touched on is a thing I like to call a psycometric chart. Given dry/wet bulb temps on and dry/wet bulb temps off the evap. one could easily have checked sensible load, latent load and best of all coil performance. The coil TD my indeed be 11 deg. but, that's an awful lot of assumptions.

    What is coil bypass factor? If you are refering to air getting around the coil, then you are taking air off the coil and therefor suction pressure would be lower than normal. You can't re-write the laws of physics.
  • don_185
    don_185 Member Posts: 312
    Yes Paul

    you are correct.I was talking about the air going around the coil.I may not be able to rewrite the laws of physics however, I sure can question them when in the field.

    Example..heavyload on the evaporator.Lets say someone was
    inside the coil cabinet on a acoil to clean it.
    Lets say this person forgot to reinstall the panel.Lets also say that you have a furnace with a 5 ton drive and a 3 ton condenser.No one made adjustment to the fan speed and you were getting 1600 cfm from the motor.

    Lets say we were getting 1200 cfm thru only 3/4 of the coil
    and the other what 400 cfm was bypassing the coil and going
    thru the missing plate.Would law of physic still tell me my suction pressure would be low? Would superheat be high tellin me my coil was starve or would it be right on the money? Would I not then ponder why my delta t is still only
    10 degree?

    Yes it made be a awful lot of assumption but in the real world it happen.

  • Paul A.
    Paul A. Member Posts: 7

    don't you have to assume, without any further info, that the unit has been there for 20 yrs. working properly? Let's remember the original description of the problem. "The coil was warm on the top and cold on the bottom." Is the TD low because of conditions at the evap. or is it lazy due to decreased capacity from a failing compressor. dry/wet bulb temps. on and off along with discharge, liquid, superheat and subcooling temps and a psych. chart should point you to the problem.
  • lee_7
    lee_7 Member Posts: 458

    WHat's a reed?
  • don_185
    don_185 Member Posts: 312

    I would assume.Everything I posted is an assumption.LOL.

    Welcome to the site Paul...Its good to have you here.

    I hope they return to let us know what they found.So we all
    can say yep, that what I thought too.

    Most times they never return.

  • Techman
    Techman Member Posts: 2,144

    Morning!The internal suction and discharge valves of the comp are thin ,oblong,"spring steel" ,flat metal "reeds". To-ma-to,Tom-at-o,
  • Rich W
    Rich W Member Posts: 175

    txv system? What's the required subcooling? What's the liquid line temp.? If subcooling is 15deg then LL temp should be near 98deg. at 235 lbs.

    Checking superheat? What's the indoor WB temp? DB temp entering condenser? What does your superheat slide-rule show for superheat? What's the vapor line temp?

  • Tom_99
    Tom_99 Member Posts: 9

    On Friday, a mechanical engineer I hired sent me a load calculation for an AC system for 1800 square feet of office space in the first floor of a two-story building. (I posted about this space about a month ago.) The office is for an auto repair garage that occupies the rest of the first floor. The office has one window-less masonry exterior wall. On the other side of two of the other three walls is unconditioned garage space. The last wall adjoins conditioned space of the adjacent building. Above the office is conditioned space on the second floor. Only 3 people in the office. One desktop computer. No plants. So the sensible heat load calculation for the walls and people came out to 12,350 BTUH.

    The combined sensible and latent load is calculated at 60,800 BTUH. The biggest part of this is the sensible load and latent load of the 880 CFM fresh air ventilated from the outside: sensible load of 19,400 BTUH ((1.1)x880CFMx20dt) and latent load of 28,960 BTUH (880CFMx4840x.0068).

    First Question: So the latent heat load is nearly 400 percent of the sensible heat load. Put another way, the latent load represents 80 percent of the total load. Does this seem “bout right” or way off?

    Second Question: The 60,800 BTUH load seems to call for a 5 ton compressor. Could the compressor be sized just to deal with 30,000 BTUH of total sensible heat load? On days where the outside design temperature is exceeded, would the system focus first on maintaining the indoor temperature at 75 degrees and allow the indoor humidity to rise above 50 percent? If so, this would allow a 3 ton system.

    Third Question: Product descriptions of Heat Recovery Ventilators (HRV) say that they can capture up to 70 percent of the sensible and latent energy of ventilated air as it is exhausted. For my situation, would this mean that an HRV could reduce the 19,400 BTUH of sensible load of the ventilated air by 70 percent down to 5,820 BTUH? Would it reduce the 28,900 BTUH of latent load of the ventilated air 70 percent down to 8,688? If this were so, the total load would appear to drop to 26,868 BTUH, allowing a 2 ton or 3 ton system, wouldn’t it?

    I am trying very hard not to end of with an oversized system that cycles on and off alot without removing humidity. I am going to bring up these questions with the engineer when I talk to him on Monday. But I would really appreciate any reaction from people reading this beforehand. Thanks.

  • Paul Fredricks_3
    Paul Fredricks_3 Member Posts: 1,549

    I'll be back!!

    I'm always here!
  • mtfallsmikey
    mtfallsmikey Member Posts: 765
    Yes, we sometimes overlook the obvious..

    Like some of my mis-dignoses: not capping off suction line when sliding Armaflex over it (mentioned in a previous thread, done long ago before the advent of line sets), compressor tripping out on high head (old A/S Husher,remember these?...sucked up a fertilizer bag into condenser fan intake). But, we share these experiences like the backward blower wheel because we think the poster knows the basic troubleshooting drill, and we relish in the weirder things we have found. Happy Posting!!...mfm
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