Rooftop AC pressures.

JMWHVAC

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.

Jamie Hall

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.

pecmsg

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

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.

Ironman

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?

JMWHVAC

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

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

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!

EBEBRATT-Ed

@ratio

I had a split once and the call was it wouldn't cool. Someone had put a sight glass on it, and it was a orifice system.

They added charge to clear the glass and then added more. When I got there it was running (R-22) and had like a 150 head and a 100 suction. I am thinking a bad compressor that won't pump but the amps were high like it was trying to pump.

With nothing to lose I started taking charge out and the more I took out the better it got. Poor thing was choking itself to death. Nothing wrong but an overcharge,

mattmia2

https://forum.heatinghelp.com/discussion/comment/1899440#Comment_1899440

probably not low per se, it is jsut all stacked up on the high side becasue of the restricion or failed refrigerant control.

AllShorePlumbingNY

High superheat + low suction + relatively high head usually makes me think restriction more than simple undercharge. I’d check for frost on the liquid line, inspect the filter/drier, and verify the TXV isn’t starving the evap. The barely wet evap section is a pretty strong clue too.
I’d be careful adding more charge before ruling those out since a restriction can easily lead to overcharging symptoms later.

pecmsg

I’m not saying there’s not a restriction in the high side but highly unlikely

We still need more data to troubleshoot without guesses