A Little Test

All summer long we have talked about doing proper diagnostics on air conditioning.  I’m going to give you a scenario from a job site visit I was out on this morning that the contractor had been “playing” with all summer.  See if you can determine the problem and the correction before I tell you at the end of this posting.

Contractor called because all summer long, the unit has been locking out on the high pressure switch.  That is what he told me when we set up the site visit.  This was a 5 ton packaged heating cooling roof top unit, R-410A refrigerant, horizontal discharge duct work, 208/230 3-phase, with a TXV metering device, scroll compressor.  Unit was installed last fall. Dealer had cleaned the micro channel condenser coil, amped out the condenser fan motor, checked the run capacitor and all are good but the unit still trips high head pressure.  It doesn’t matter what the outdoor ambient temperature is — both cool and hot weather — it trips the safety. When he has his gauges on the unit, and resets the lock out, pressures are equalized at about 225 PSIG but as soon as the unit starts, the head pressure starts to climb and the suction pressure starts to fall.  After about 20 minutes the head pressure continues to climb and the suction pressure continues to fall, and eventually the unit trips high pressure.  (Any ideas yet? A few clues here!).

I get up on the roof this morning with the dealer, outdoor ambient is 63 degrees.  We get to the unit, put the gauges on it, unit is in a lock out, reset the lock out and start the unit.  Pressure start out equalized and then the head pressure start to climb and the suction pressure starts to fall.  After 15 minutes of run time, the head pressure is 450 -475 PSIG (and still climbing) and the suction pressure is down to 87 PSIG and 0 degrees super-heat (big clues here). Letting it run, the head pressure eventually keeps climbing and trips off on the safety.  Here’s the start of the test — where do you start looking? What other diagnostics do you need? Remember, this unit has a TXV for a metering device  – does that help in your diagnostic?

One of the “dead give-a-ways” on this job is the reaction of the pressures –  high head, low suction. First thing you think about is a restriction?  In a way, that is correct but in this case, the TXV was throttling down to the point that head pressure would rise and trip the safety.  So why would a TXV throttle down?

TXV’s close when the coil gets cold and open as it warms back up. What causes a coil to get cold?  AIR FLOW!

At the site, we had over a 30 degree temperature drop across the coil. We then measured the return static and it was 0.04 IWC (good number – very little restriction).  We then measured the supply static and it was 1.67 IWC. The fan chart for the unit only goes up to 0.8 ESP. Supply static was off the chart. To top it all off, the blower motor was only set to medium speed. The coil would start to freeze and the suction line would get so cold the TXV would shut almost completely down (restriction).  So what would cause the high duct supply static pressure even with the blower only on medium speed?

I then asked him how the heat worked last winter?  He said they had problems with that all winter cycling on limit control also. (any new ideas yet?)

We got inside the building and took off a register in the area the unit served and there was almost no air flow.  We got up into the ceiling area where the flex duct connector between the unit and the main trunk line came down, removed the flex duct and had lots of air flow out of the flex duct.  Went back up on the roof, measured the static pressure with the flex disconnected and only had 0.37 IWC static. At this point it was clear that there was a restriction in the main trunk.  The contractor crawled into the duct and found a fire damper had dropped down blocking off 3/4 of the trunk.  He got that raised back up, re-hooked up he flex, re-measured the static which was now 0.47 IWC.  We moved the blower speed to HIGH speed from medium per the blower chart for the unit to provide 2100 CFM (5-ton unit).  Our suction pressure was now 115 PSIG and head pressure was only 276 PSIG (it had warmed up a little outside) and the unit ran fine and was not cycling on the high pressure control.

Were you able to follow this and figure out the issue here?  All summer I have discussed PROPER AIR FLOW, using PRESSURES FOR DIAGNOSTICS,  checking SUPER-HEAT & SUB-COOLING.  Here is the perfect scenario of a contractor not doing proper diagnostics.  The fact that the unit cycled all winter on limit should have told him he had an air flow problem.  He did not seem to realize how the TXV worked (see my posting on TXV’s if needed) or he would also have had a clue to the problem, and the low suction pressure/ high head pressure should have been the first give-a-way as to his problem.

I can’t emphasise the importance of doing and using all the diagnostic tools you have.  Had this contractor performed all the necessary diagnostics, his customer would have gotten the heating and cooling they paid for.  The contractor would not have made all the trips back to the job to reset the unit because it was off on safeties.

Sometimes, it takes a little longer to do all the necessary checks, but in the long run, it will always be worth it.  I learned a little “word” a long time ago that has always stuck with me — DIRTF00TDo It Right The First Time!

So how did YOU do on my little test? Did you learn anything? Do you really see the importance of doing complete and proper diagnostics?  I hope so.

About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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6 Responses to A Little Test

  1. Bruce Porter says:


    • micah vanderheide says:

      great post, and something that most mechanics miss, low load can equal high head pressure. this is even more evident in Micro Channel coils vs copper tube and fin. I have now seen this situation several time now and a further explanation of Micro Channel and how it reacts to Low Load would be very useful.

  2. Jan says:

    This is an awsome post!!! Most people will go for the TXV and not look any further. I also have been in the industry as a commercial installer and did residential on the side for over 30 years and know that air flow is just as important as any other part of a system. I have been called on to fix residential furnaces that over heat just to find out the filter is plugged.

  3. Sergio says:

    Wery interesting post. But I didn’t catch, why head pressure was so high with restricted evap air flow? The low pressure wasn’t so high, and 0 degree superheat means that evaporator is full of refrigerant, so condensor can’t be flooded. Any ideas?

    • head pressure was high because the TXV was shutting down acting as a “restriction. This also accounted for low suction pressure. Low super-heat was because we were not moving any air across the coil so we were not boiling off any refrigerant even though we were under- feeding the coil.
      That help?

  4. Sergio says:

    And micro-channel coil wasn’t able to contain all refrigerant.. Yes, now it is clear.

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