Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.
If our community has helped you, please consider making a contribution to support this website. Thanks!

Rooftop AC pressures.

Options
JMWHVAC
JMWHVAC Member Posts: 117
in A-C, Heat Pumps & Refrigeration

I was on a call for a DBUC-T072N100A rooftop. They reported the compressor was shutting down while the thermostat was calling and main blower still running, and the reporter is a common sense good guy.

So I find suction pressure at 33psi and head at 250ish, outdoor mid-upper 90’s. I thought the head seemed high for low refridgerant but I know older 9ser, piston X-valve systems run a lot higher than even old but higher seer units. I added 3+ pounds to arrive at 51sp and 280hp.

Then my superheat was still 40*, way above the superheat charging chart, the only charging info I could find. As a test I slowly opened the compressor access panel which allowed air to bypass the condenser. Once my HPressure reached about 350* the superheat came down within range. So if I continue charging till superheat is correct that means the afternoons the rest of this week when it is 100* it would be running near 400psi discharge pressure.

Is this to be expected? Seems awful high to me. Did not find anything else out of sync. I figured the compressor shut downs could have been from overheating or low pressure cutoff but don’t know.

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 27,591

    I hope someone who knows what they are talking about answers here, but… working from pure theory and no practical knowledge whatsoever (always risky!) with any refrigerant you are going to need higher pressures to condense (and hence reject heat) at higher condensor — that is, the outdoor unit) temperatures.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • pecmsg
    pecmsg Member Posts: 7,294

    1St check your air flow to rule that out.

    Any history on this system?

    As your SH goes down so does the head pressure.

    With the limited information your still undercharged.

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 12,515

    R-22 refrigerant needs to be at least about 59 PSI to keep the evaporator saturation temperature above 32°F. Otherwise, the evaporator coil may ice up.

    Operating at 51 PSI is way too cold. At that pressure, the evaporator saturation temperature is below freezing. I know that if you are above 60 PSI, you can usually operate without worrying about icing the evaporator coil, assuming the airflow is correct.

    I hope you did not leave it operating at 51 PSI on the low side.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • Ironman
    Ironman Member Posts: 7,928
    edited 1:35PM

    You’re still low on refrigerant, but the high side pressure should be much lower. Have you checked for a dirty condenser coil or a high side restriction like a clogged filter/drier? Restricted piston?

    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    EBEBRATT-Ed
  • JMWHVAC
    JMWHVAC Member Posts: 117

    Thank you all for comments. As far as evap freezing, when I came it was under 40psi with a stream of water out the drain and no build-up of ice. Seemed strange. But the whole “secondary” section of evap was barely even wet with condensation. Maybe 400psi at 100* isn’t crazy for a 9ser unit.?. First time I looked at this unit and first time anybody did for good many years.

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 20,515

    I agree with @Ironman high superheat low suction and high head point to a liquid line restriction. Filter drier, expansion valve plugged distributors.

    Could be as simple as a bad powerhead on the TXV.

  • ratio
    ratio Member Posts: 4,266

    Look for frost on the liquid line, that's a sure giveaway of a restriction. I had a no-cool on an RTU once, one of the two(!) liquid-line filter/dryers was plugged up. Apparently the unit had suffered a burnout some time in the past, and no one ever went back & took out the suction dryer & extra liquid line dryers. I also took twice the nameplate charge out when I removed all the extra parts. Worked a lot better after that!

Categories