Sub cooing high/ suction pressure low

[book]

Today I was doing a maintenance check on a three year old Carrier Heat Pump M# 38YKCo36 321 S# 4202EO2360. The high sub-cooling puzzled me. You see the unitlable listed an indoor sub-cooling requirement of 15 degrees. I started with a sub-cooling of 27 degrees. So removed 1 lb. of R-22 and resulted in following conditions: OAT-78, WB-58, LLP-240psi, LLT-89, SLP-60, SLT-46, RA-70, SA-47. The compressor amp draw was 17A and system has a TXV on coil. I tried to remove more freon to reduce SC more but suction pressure would drop below 60 and SC remained right at 25 degrees. Any suggestions why SC remains high while suction pressure drops so low? I have experienced similar situations to this befor, on some units, when trying to charge a TXV by sub-cooling to the unit lable listing.

The condenser was washed clean and indoor air handler and coil is three years old with clean filter and no outward sign of dirty coil. That is, blower and return air compartment are clean.

[Edited by book on 07-19-2005 at 11:04 PM]

[bornriding]

You may have more than one problem.
low Suction press / low suct line temp indicates low air flow through evap. coil.
But, 245# head on 78 degree day seems a little high, especially with only 60# suct.
Of course 78 'f' outside is going to make yur condenser very efficient, so the increased subcooling could be attributable to the OAT. I feel that the 15 degree sucooling number may have been based on standard operating conditions. EG - OAT = 90+, Inside = 75 or so.
I think that I might start by cleaning the condenser coil and the evap coil ( at least checking them ) and filter.

[bama 101]

Clean both coils ,and fan blades And adjust charge as needed head press should be around 220psig, so it is overcharged most likely due to dirty evap and maybe even non condensables in system but that is only my thoughts

[docholiday]

Im with Born. those numbers indicate low load and with a return of 70, I believe it. Airflow was not measured and that also falls into low load. 23 degree drop accorss the coil suggests it as well.

Verify Airflow by CFM not feel and post the numbers. Believe it or not, you may still be overcharged.

[udarrell]

Possible plugged refrigerant metering device screen, or somewhere in liquid line (dryer, etc.) Test for temperature differences (even frosting) along liquid line to evaporator.
You could also check the temperature of the E-coils return ends and distributor tubes, that is less likely as cause.

Check the condenser delta-T to see if it falls within acceptable parameters. (Look up condenser's CFM.)

http://www.udarrell.com/ac-trouble-shooting-chart.html

[Chill]

You probably will get a better reading when the OAT rises and puts a little load on the system. The air flow seems low, could be undersized duct work. You could also have a LL restriction. follow udarrell's advice and measure the ll temp on both ends.

[sigma]

With 58WB and 70 DB your evaporator DT should be 18.7.
Definitely not enough CFMs.
Undarel has a good suggestion. Condenser DT would tell you
how clean is condenser coil or how are a condenser CFMs.
Maybe this is double row coil that has to be split for cleaning?
You said this was a PM service. Was there a complaint of no
cooling before? You measured 17 Amp draw on a compressor.
What was RLA? System is 3 years old. How did it work so far?

[udarrell]

Originally posted by book
Today I was doing a maintenance check on a three year old Carrier "Heat Pump" M# 38YKCo36 321 S# 4202EO2360. The high sub-cooling puzzled me. You see the unit lable listed an indoor sub-cooling requirement of 15 degrees. I started with a sub-cooling of 27 degrees. So removed 1 lb. of R-22 and resulted in following conditions: OAT-78, WB-58, LLP-240psi, LLT-89, SLP-60, SLT-46, Return Air- 70, Supply Air- 47 delta-T 23. The compressor amp draw was 17A and system has a TXV on coil. I tried to remove more freon to reduce SC more but suction pressure would drop below 60 and Sub Cooling remained right at 25 degrees. Any suggestions why SC remains high while suction pressure drops so low? I have experienced similar situations to this before on some units, when trying to charge a TXV by sub-cooling to the unit label listing.

The condenser was washed clean and indoor air handler and coil is three years old with clean filter and no outward sign of dirty coil. That is, blower and return air compartment are clean.

[Edited by book on 07-19-2005 at 11:04 PM]

Since it is a heatpump there is a possibility that the outside coil's check valve has stuck in a "partially open position." It would also could force some refrigerant through the outdoor expansion valve to the condenser coil. In the cooling mode this will result in low suction pressure and low discharge pressure. Sub cooling would be extra high.

However, heatpumps can't always be charged accurately using sub cooling alone.

Usually "a stuck check valve" can be freed-up by using a strong magnet. "If check valve is stuck in the closed or partially closed position place a strong magnet in the middle of the valve and move it toward the outlet end."

http://www.udarrell.com/ac-trouble-s...ubcooling.html

[Shophound]

Since it is a heatpump there is a possibility that the outside coil's check valve has stuck in a "partially open position." It would also could force some refrigerant through the outdoor expansion valve to the condenser coil. In the cooling mode this will result in low suction pressure and low discharge pressure. Sub cooling would be extra high.

If we're speaking about the heat pump operation while in cooling mode, I would think an outdoor check valve that is partially open is actually serving as a liquid line restriction, as in cooling mode this valve should be all the way open, bypassing the outdoor coil TXV.

If the indoor check valve was partially open in cooling mode, it would result in a flooded evaporator and reduced subcooling due to greater mass flow rate of refrigerant in condenser...doesn't have as much time to sit in subcooling loops of outdoor coil and be subcooled. Evap would show a higher suction pressure but low superheat due to flooded condition.

The reversing valve can be ruled out easily because if it was leaking discharge gas to suction, there would be a greater heat rejection load imposed on the outdoor coil, reducing subcooling as well.

I am with those who suspect an indoor airflow problem. Higher than expected head for given OAT of 78 may be due to overcharge...not uncommon to see in heat pumps.

[udarrell]

Originally posted by shophound

Since it is a heatpump there is a possibility that the outside coil's check valve has stuck in a "partially open position." It could force some refrigerant through the outdoor expansion valve to the condenser coil. In the cooling mode this will result in low suction pressure and low discharge pressure. Sub cooling would be extra high.

If we're speaking about the heat pump operation while in cooling mode, I would think an outdoor check valve that is partially open is actually serving as a liquid line restriction, as in cooling mode this valve should be all the way open, bypassing the outdoor coil TXV.

========================================
That is correct the outdoor check valve should be all the way open, but if it were partially closed or partially open, as you stated, --"it would result in a partial liquid line restriction," resulting in LOW suction pressure, somewhat HIGH head-pressure, also somewhat High super-heat, and HIGH sub-cooling, with usually low compressor amp-draw.

A partially plugged screen at the entry of the indoor evaporator coil's TXV would result in the same symptoms I listed above.
- udarrell

If the indoor check valve was partially open in cooling mode, it would result in a flooded evaporator and reduced subcooling due to greater mass flow rate of refrigerant in condenser...doesn't have as much time to sit in subcooling loops of outdoor coil and be subcooled. Evap would show a higher suction pressure but low superheat due to flooded condition. shophound

Okay, but not releevant to my statements. udarrell

The reversing valve can be ruled out easily because if it was leaking discharge gas to suction, there would be a greater heat rejection load imposed on the outdoor coil, reducing subcooling as well. shophound

I am with those who suspect an indoor airflow problem. Higher than expected head for given OAT of 78 may be due to overcharge...not uncommon to see in heat pumps.

I am in agreement with all of you, --that it is extremely important to get the proper heatload through the evaporator coil "before" trying to attempt any adjustment of the charge.

I can't view any information posted here on that while responding. Get the cooling coil load right, and then if it does not resolve the problem check out the points I raised.
udarrell

http://www.udarrell.com/air-conditio...ator-coil.html

[corny]

I had one a while back...outdoor ambient 85, return temp of 69 db with humidity at 35 percent....suction pressure was 65 with 49 suction line temp, head pressure around 225 with a temp of 86..I believe...cant remeber actual pressures and temps but do remember subcooling was 20.... its a computer room with 2 units and you cant convince these folks that it doesnt have to be 72 or below in there... and they wont let you shut off one unit to check the other under a load.... but anyhow....my delta t was only 16 degrees....and I couldnt fiqure that out... any ideas......wouldnt the delta t be higher with low load like it would be with low airflow

[airworx]

you have to get the subcooling to design, then check superheat and suction pressure. after subcooling is normal if suction is low and superheat is high then you have a txv problem or restricted drier or coil, if superheat is low then you have a airflo problem.
you also have to remember when overcharging txvs that the txv will close off as more refrigerant is added than needed causing a lower suction.

[cobitech]

I agree with Airworx, I would check the temp drop across the liquid line drier for a difference.

A couple or three degree difference would indicate a restricted liquid line filter and cause what is going on here.

[bornriding]

What would lead anyone to think that there is a restriction in this system ???

Would a retsriction lead to 46 degree SLT ?

[cobitech]

Bornriding, started with 27°subcooling,removed charge and still have 25°subcooling.

Refrigerant is stacking in the condenser, while removing the refrigerant dropped the suction pressure resulting in a lower SLT.

I think that there may be a restriction across the liquid line drier or the TXV is not metering,(check the position of the Bulb on the Suction line.)

[cobitech]

The more I thought about it....it couldn't be a restriction or the superheat would be alot higher.

[docholiday]

The reason its stacked in the condenser is because there is no load. Born is right, there is no restriction if the information we got is correct.

[HVAC Pro]

Measure CFM. Check duct static pressures. Sounds like insufficient air across evaporator coil. Make sure blower wheel is clean and dampers/registers are open.