View Full Version : Poor Subcooling
09-14-2005, 09:30 AM
Customer has a 2 ton split with gas heat.Only return is located at bottom of stairwell to half finished basement area with no doors.Air temp at return 76F. Output at upstairs registers 68F.Both coils are clean with no airflow problems. .059 fixed orifice evaporator with excessive liquid returning to compressor.Evap temp 50F.Liguid line to evap laiden with sensible heat.We had same problem with WIC a few months back.We jumped every loop & hurdle in vain till we replaced condensing unit to solve problem.I would apreciate any info on why a seemingly good running system would slowly (within months) degrade to this condition.No service history is available on residential unit except I cleaned an extremely dirty I.D.coil with little improve ment. Thanks! 5 years,still green and learning.
09-14-2005, 09:46 AM
You didnt actually give us pressures and temps, but that said, if you are experiencing low subcooling with low superheat, you are probably overfeeding the evaporator. This means the piston is probably not seating very well. Verify properly matched equipment, replace piston or install a TXV.
09-14-2005, 10:15 AM
When you changed the condenser, did you replace the piston with the one that came with the condenser? 8 degree delta t is an airflow problem.Check the ductwork. also it sounds like you only replaced condenser and not indoor coil....you just cleaned it.
09-14-2005, 12:55 PM
A flooded evaporator will also offer lower deltas. Remember, all the work is being done as liquid changes to vapor, if you dont start boiling the refrigerant off until half the evaporator, you will not get the cooling you are looking for.
I think we both agree on the wrong or something wrong with the metereing device.
09-14-2005, 01:10 PM
Thanks for responding smokin68! I'm sorry my post was so vauge.The walk-in cooler evaporator was TXV controlled and my boss handled ordering the condensing unit from the manufacturor. I had replaced the txv before we replaced the condensing unit. I'm sorry for the confusion that these are two different instances.The installaton on the cooler was handled by a fellow installer & tech while I was on another job.Everyone in our service dept had been consulted and inspected the situation.My boss Who's very sharp made the right call even though I'm still not sure what made the old condensing unit go south.I made the call on replacing the TXV and I was wrong and Embarrased.
Also Thank-you docholiday for your help. I thought the orifice might have been the problem along with the possibility the evap might be oil-logged,maybe even overcharged.Let me give you my pressure readings ambient outdoor temp and you guy's can probably nail it for me.
O.D.ambient 80F.High side gauge 255psi.Suction,79psi.Line set less than 20'. I'm just a little gun shy from the cooler experience and need assurance fromm veterans. Thank's again, You guy's are Great!
09-14-2005, 01:37 PM
I agree with you. A coil becomes flooded because it's not removing heat, i.e. overfeeding with respect to airflow, or low airflow. Either way, I figured he should check the air across the coil, or a possible false reading from a ripped duct etc. Dr. Fluke, get all the readings and we could probably nail it. SH & SC are critical,not just pressures.
09-14-2005, 02:36 PM
Smokin & Doc
Thanks Guys for good professional advice.I think the best way to learn is to ask pros rather than trial & error.I'd rather tell someone I'm ignorant and ask Questions than to guess and prove I am.Ive already made an appoinment with the customer when he gets back in town Sat to take new measurements after cleaning the unit on first visit.I'm sure I'll get better readings since system has had plenty of time to settle in on being able to breath better. I,ll let you know what I find. Thank's so much for taking time to help me better myself.People such as yourself are a blessing to us all!
09-15-2005, 01:25 PM
one last question .....did you clean that coil in place or did you pull it and take it outside to disaasemble/clean? It might not be as clean as you think.....
09-19-2005, 05:45 AM
Sorry about not getting back to you sooner.Your correct about how to get a coil as clean as possible, but as you know you have to qualify customers as to how far they want to reach in their pocket.I did brush a lint mat off the ID coil and followed up with a chemical cleaning & rinse(poor little condensate pump worked hard too)On my return appoinment I adjusted super heat to factory spec( recoverd .5# overcharge).It's an Armstong unit with no acumulator.I found out homeowener had been diddling with downstairs dampers whitch was added on later,complete with sheetrock and popcorn ceiling covering previously exposed line set.Oh My, when his line set freezes up which in my experence happens quite often as old systems deterioate and develop leaks it could be a mess.I was able to achieve a 15F TD which is about par for most 2 tons I've seen,But remember I'm still Green.He's got 1800 sq feet and I didn,t do a heatload calc or size the unit,I thought you did that BEFORE you install the Unit.But I also know time changes things.He's comfy enough now And dosn't seem want to do the things I thouht he might want to do,( his units 10 years old and seems OK to him )besides he needs the money for more important things, like ya know eating,etc. I'd like to hear your advice or critisism either way.I've really apreciated you And Doc so far! Thanks Again!
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