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  1. #1
    Join Date
    Jan 2009
    Location
    St Peters, Mo
    Posts
    56

    High head pressure has me worried!

    Changed evap coil on 30 year old 5 ton commercial split yesterday. Temperatures were extreme here in St. Louis to say the least, but 375# head pressure has me worried. Any feed back would be appreciated, all data listed below.


    Indoor entering drybulb: 95.5 deg
    Indoor entering wetbulb: 80.4 deg
    outside ambient: 110 deg
    Low side pressure: 80#
    High side pressure: 375#
    Suction line temp: 84.2 deg
    Condenser leaving temp: 142.7
    Cond delta T : 32.7 deg

    Charged to target superheat of 34deg ( 84-50). Could the high head be due to extreme conditions or could there be something else?
    Call for Heat means get more firewood

  2. #2
    Join Date
    Jul 2010
    Location
    nebraska
    Posts
    1,629
    Condensing temp 40 degrees over ambient along with 37 superheat,which is way too high(actual target 25). Looks like that condensor needs a good cleaning,split the coils if it's double row. Probably needs more gas too once it's cleaned up good.

    Had one thursday doing the same thing. Was thinking overcharged with restriction at first but then saw double coils. Had already washed them but needed to split, was a blanket of crap between not coming out. Once clean it took 38 oz,head ran 290 with superheat within 1 degree of target.

  3. #3
    Join Date
    Jan 2009
    Location
    St Peters, Mo
    Posts
    56
    The superheat table I have in front of me (generic) calls for target superheat of 37deg with oat of 110deg, indoor wb temp of 80deg and indoor db of 95deg. condenser split is 32deg, 142-110. I'm just worried I might have noncondensibles since I opend the system up. Will go back monday on more normal conditions and reexamine.
    Call for Heat means get more firewood

  4. #4
    Join Date
    Apr 2010
    Location
    Minnesota
    Posts
    517
    Tough when it's 110 Oat. ouch. On a 30 year old unit you have a relatively small condensor coil so you get higher temp splits across the condensor. Your liq line temp should be around what.... 10-15 degrees over the oat max? Much higher and you have a dirty condensor/undercharged. Is there a pressure tap on the compressor discharge so you can pump down and check for non condensibles?

  5. #5
    Join Date
    Jan 2009
    Location
    St Peters, Mo
    Posts
    56
    The outdoor unit has king valves, so yes it can be pumped down. I did not measure liquid line temps but it was smokin hot and is exposed to direct sun on the roof. Coils dont look too dirty, but cleaning may be beneficial. Didn't think undercharged with 50deg sattemp and 84deg slt.
    Call for Heat means get more firewood

  6. #6
    Join Date
    Apr 2010
    Location
    Minnesota
    Posts
    517
    I didn't mean that you were necessarily undercharged in your case. Just sharing general knowledge. You can tell what your condensor efficiency is by the liq line temp-oat (approach temp)

    Could it be that you have some hail damage, or bent fins, slower rpm cond fan motor than original???

    Thankfully we never see 110 degrees here, maybe some of the southerners can chime in.

  7. #7
    Join Date
    Jan 2009
    Location
    St Peters, Mo
    Posts
    56
    Thanks for your input. We've had 10 straight days of 100deg plus with 0 rain, setting records that go back to dustbowl days of the 30's. It is killing equipment, people, and wearing out techs. I think more normal temps may give a better representation of equipment health. Not looking forward to putting a 5 ton condenser on the roof of a 2 story builing if/when it does expire.
    Call for Heat means get more firewood

  8. #8
    Join Date
    Apr 2012
    Posts
    44
    This high heat has exposed many dirty condenser coils in my area. Units that can't keep up but once cleaned are running just great.

  9. #9
    Join Date
    Apr 2001
    Location
    Over Here
    Posts
    1,105
    Quote Originally Posted by wingman65 View Post
    I did not measure liquid line temps but it was smokin hot and is exposed to direct sun on the roof.
    Was going to ask about your liquid line temp but you answered my question.

    Liquid temp is always important to help you determine if your high head is either a lack of heat rejection or a lack of refrigerant flow or compromised refrigerant.

    Even with extreme conditions as yours, knowing your subcooling temp is just as important as your superheat in determining the state or condition of any refrigerant charge.

  10. #10
    Join Date
    Jan 2009
    Location
    St Peters, Mo
    Posts
    56
    Thanks Thermojohn, at the end of a long hot day I neglected to measure ll temps, was hastily trying to get system running. Will revisit monday and post up to date info, everything seemed ok but high head. Are those numbers too far off track for the conditions at hand, if so, what should I look for???
    Call for Heat means get more firewood

  11. #11
    Join Date
    Apr 2001
    Location
    Over Here
    Posts
    1,105
    Quote Originally Posted by wingman65 View Post
    Thanks Thermojohn, at the end of a long hot day I neglected to measure ll temps, was hastily trying to get system running. Will revisit monday and post up to date info, everything seemed ok but high head. Are those numbers too far off track for the conditions at hand, if so, what should I look for???
    375psi for R-22 is going to be a little rough on a compressor and is not close to what I like to leave - even for a weekend. Next thing to determine is your subcooling to define why your head is so out of kilter.

    Your high head most likely is going to be one of the three things I posted earlier.

  12. #12
    Join Date
    Jan 2009
    Location
    St Peters, Mo
    Posts
    56
    The 375 head is the concern that lead me to post. Luckily it is a business and it will be shut down over the weekend. I just seek possible answers as too why it is so high.
    Call for Heat means get more firewood

  13. #13
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,069
    Your head will be high with that high of a heat load from the indoor conditions. Once the indoor drops below 90, your head will drop.
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