Carrier XTC Alarm AL28

[dansharton]

Hello everyone. All suggestions and advices appreciated in advance.
I have carrier unit with AL28 alarm code coming on and unit shuts off. Under description of this code says - Low suction pressure. What should I check first? Is it low on ***** or expansion valve?
If it's low on ***** how can I check for leaks and recharge system? Can you point on posts for this matter please

[cadwaladr]

Are you an engineer? if not the evaporator could be iced up try putting it through a manual defrost ,to check for correct refrigerant gas level you need to find the sight glass on the receiver tank and see if the ball is floating while the unit is running,

[dansharton]

hello Thank you for reply. I'm not engineer but have some knowledge. We did have same issue on TK but with no alarm. There was low suction pressure and valve was bad. ON this Carrier XTC unit it happens 10 mins after unit starts. It shut downs with this alarm code. Thank you for suggestion will start checking today.

[cadwaladr]

Ten minutes it won't be iced up then,sounds like you may have lost gas or maybe a expansion valve issue,keep me posted

[Barberboyy]

Moisture I wouldn't mind betting run it in heat and see if it cuts out

[ozzie35]

When is unit last serviced? When it's cut out what's the pressures? It might be a blocked drier.

[IRMechanik]

Run it though a pretrip. It may be the liquid solenoid valve not opening. The pretrip will check everything electrical. Seen a ton of solenoid coils fail.

[dansharton]

Thanks in advance to everyone for all replies.
Figured it was low on *****. Charged with *****. Working fine. Cooling slowly not like regular unit. But at high RPM discharge pressure stays at 420PSIG. Then it switch to low RPM and little by little drops to 350-360PSIG. Was told need to clean coils.
What's the normal discharge pressure and suction pressure in high and low RPM?

[dansharton]

Also once charged with ***** and run pre-trip it gave me alarm - P175 - check high speed RPM. Nothing else. but it still very slow cooling down

[cadwaladr]

If it's charged with 404a head pressure should be 250 psi,did you add gas in liquid state? Cleaning the condensor coil is recommended to be done on a weekly basis if your ambient is high and the unit runs in a dusty environment.

[dansharton]

Yes its charged with 404a. Ambient is about 80 F. First unti starts with low RPM, then since set point set to 0 deg it increase RPM and run at high speed. When it runs at high speed discharge pressure showing 410PSIG then it starts dropping down. In about 5-7 mins unit drops RPM to low and running at low speed. At this point disacharge pressure showing 350PSIG. If you saying discharge should be 250PSIG then in my case its definitely too high.
Will clean coils first and then update post if someone interested.

[cadwaladr]

Was it charged in liquid? The pressure you have is too high chemically cleaning may be needed on the condenser coil I would think at 80f 300 psi should be the maximum? It should have tripped at the pressure you quoted?

[IRMechanik]

Wash the condenser out, if the head pressure drops it points to an air flow problem. If it doesn't, you may have non-condensables in the system. Check your fan shroud as well. The old XTC units have a plastic shroud that was bad for breaking apart.
Keep posting what you find, it helps everyone.

I'm guessing you have an airflow problem, the pretrip will compare pressure vs ambient and give you a high discharge pressure code for non-condenables. The high pressure cutout is 465 psi, you're very close to that. The unit is probable suffering in capacity due to high liquid temperature.

[gravy258]

on the 'keep it simple stupid' approach..
did you overcharge it ? As you've gone from one extreme to another.
and how did you charge it, using what process ?
On a XTC the bottom ball in the receiver tank should be up only, not the top one.