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High subcool (40 Posts)
High subcoolGot called to look at a newer install not cooling well. The subcool is running at about 20. Outside temperature only about 75*, high pressure at 280, temp of 67.8 and low at 120# with 63.8*. Also they have a drier on suction side only, believe the line set was not replaced only condenser and AH. will return soon and need a plan of attack. Did not think to check inlet side of drier for pressure differential at the time. Any suggestions would be appreciated. Thanks
High SCI would check for non-condensables and restrictions first. Do a static check; with the system off for at least 20 min., your head pressure converted to temperature and your ambient should be the same. If higher than ambient you have something inside that shouldn't be there, causing an abnormally high SC. If OK, check for restrictions. You started that process by mentioning checking temperature or pressure drop across the drier or driers. If three degrees or less the driers are happy, so you might try to recover some refrigerant to see if this helps. Look for kinks in the tubing or an incorrect size. Watch your superheat too!!!!!
Hi SCWhat is the distance (vertical) from cond unit to evap ? What are those temps 67.8* and 63.8 * for? The new cond unit does not have a LL F/D ?This post was edited by an admin on June 6, 2013 7:34 PM.
overcharge ?Almost all systems have a critical charge unless they have a liquid receiver. I would follow Spence's advice, and also consider reclaiming refrig. triple evacuating, use micron guage, and than weighing in the charge, the charge listed on the cond. unit usually takes in account about a 25 ft. line set. ( check with the unit manufact.) All service people need to have a digital scale with them.RJ
High SCHmmm. A drier on the low side? Did you have a burn-out? Is the drier next to the compressor where it should be, or in the line set? A burn-out would be the only reason you should have a low side drier, so it is possible the system was not cleaned up adequately, which would again lead us to a possible restriction. I would check your PD (a suction drier will have a service port for that very reason). If not, temperature is the way. If you have to start over, follow RJs lead. Best of luck.
High subcoolLine set length is maybe 10' and not visible, down through roof and above old ceiling. Will check PD across drier when I return, static check seemed fine on 1st trip will double check. Thinking I will have to pull it down, if so would you pull drier off suction side and replace with new on liquid? Line set is the only thing there left over, don't know about possible burn out prior, also is one size larger than new equip. And yes drier is right off condenser. Thanks
Also there is a sg on liquid side, are you all still using them on 410, just thinking about future installs, consensus yes,no
SC IssueIf no burn-out, get that SLD out of there fast, as well as that sight glass. If you DID have a BO (better find out before going any farther), the SLD should hug the compressor. You should probably retest the compressor (again, only if a BO) for acid and sludge. I hope somebody didn't scab in a LLD on the low side! ANYTIME you open a system to atmosphere you have to change the LLD; get the biggest one you can. While the system is open you may as well make sure the TXV inlet screen is not plugged. If your manufacturer says it is OK, an Rx11 with nitro purge wouldn't hurt either. Also, make sure your manufacturer is happy with a 10' lineset. The factory charge is for 15' so too short may be trouble unless you add some more tubing.
The first thing that comes to mind with the SG is an overcharge, especially with your tubing so short. Lets hope for no BO, so you're down to getting rid of that SLD and sight glass, removing the LLD and installing an oversized one, checking with your unit's maker regarding the short tubing, and RJs weigh-in method. Fine-tune to manufacturer's chart.
Hi SCWith the cond unit above the evap the SC could be 1-5* ,not 20*. Do an acid test to determine if the suction F/D is needed. DO NOT get rid of the SG/MoistureIndicater. W/O the MI how can you tell if moisture is in the system??? Moisture = acid=dead compressor. A 10' lineset is not bad for a regular AC system, on a ductless split it is no good. A FULL SIGHT GLASS DOES NOT INDICATE AN OVERCHARGE. Overcharging can be accomplished with or w/o a SG/MI. Charging ANY system to a full SG in cold weather is overchargeing. Every AC system that has a TXV should have a FULL SG under all normal operating conditions!!!!!!!!!!This post was edited by an admin on June 10, 2013 3:36 PM.
MoistureYour static test will show if you have moisture.
Some More InformationAs Techman asked earlier, what are the 67.8* and 63.8* temperatures?
As far as noncondensables go, they will cause high subcooling but they will also cause the high side pressure to rise. With an outside ambient temperature of 75* and a condenser saturation temperature of only 91*, this does not seem likely.
What was your return air temperature and what was your evaporator superheat? Even though you have a TXV on the system, knowing what the valve is doing is helpful.
Also, as far as a sight glass always being full on a TXV system, such is not the case. If the load on the system is very high, the TXV will open wide to attempt to bring the superheat down. When operating like this, it is possible for refrigerant to flash in the liquid line, causing bubbles to form. I think what Techman is saying is that if the system is operating close to its design temperature, then yes, the sight glass will be clear on a TXV system.
One word about suction line filter driers. They can, and often are, installed on systems unless the manufacturer specifically states otherwise. Daikin, for example, stresses in their literature that filter driers of any kind are not to be installed on their equipment. There is nothing wrong with installing both suction and liquid line driers on new systems AS LONG AS THEY ARE PROPERLY SIZED so as not to adversely affect the mass flow rate of refrigerant through the devices.
Keep us posted with your findings on this system.
Enjoy the Summer!Eugene
High SubcoolSo I'm thinking next week on return.
1. Check static pressure
2. Check PD across SD
3. Verify problem with subcool
4. If problem, recover system, remove SLD and replace with LLD
5. Pressure test to 400, evac to under 400, weigh in correct charge.
Think system should be flushed beforehand? OK Thanks
HI SCBy saying the SG should be full under all "normal operating conditions" that excluded a "pull down situation". I am not aware that having a moisture level of 200ppm or 400ppm will show up on a static test. A ppm of 50-60 will show up as a good reading on the SG/MI ,200ppm will show up as a bad reading of the SG/MI. Can anyone tell me they can tell the difference between 50ppm or 200ppm? I rely on the SG/MI to tell me when to change the FD. Take an acid test !This post was edited by an admin on June 7, 2013 3:16 PM.
Static press/temp testYes moisture will show up as a pressure during a static test, BUT ,your gauges WILL NOT show that pressure. The water vapor press is so low that your gauges are not adequate for the job.
Static PressureI humbly and respectfully disagree with your statement. Your head pressure converted to temperature and the ambient must be equal. A change of more than 3 degrees indicates non-condensables (provided you have checked for a restricted condenser). Head pressure converted to temperature most certainly involves a refrigerant gauge and a PT chart, having no correlation to water pressure. What it is, be it moisture, nitrogen, etc. matters not. It has to come out.
Hi SCI am referring to the water/moisture part of the static test. All of those other non-condensable gases will show up on the static test, but, not the non-condensable water vapor/moisture part.
High SCOK, I'm with you. But what does it matter what is in there? It has to come out anyway, so who cares if its water, nitro, or Jack Daniels? If the removal process is done correctly, you get it all, or at least enough to get the percentage of nasties to a tolerable level. With R-410A for example, we will probably never get all of the moisture out, yet a well-done evacuation and an oversized LLD at least gives us the satisfaction of doing the best we can with what we have.
For rickhYour step-by-step plan of attack, in #5, states: "pressure test to 400." I like your plan, yet personally I would use factory test pressure. It might be 400 PSIG; it might not be. After all of this, I would hate to see you create your own leak.
Hi SCHay my friend , someone mentioned that the static test would indicate the presence of moisture. So I'm saying that the SG/MI is the best and only way to know if moisture/water is in there. So what the system was perfect on startup, let a few mechanics get there grimy ,air(water/vapor) filled hoses on a system then how do you know? Lets say I take over a new customer, how do I know the condition of the Freon/oil? Doing the best I can for the customer's money and AC unit I put in a SG/MI and usually a new FD. Then I know how the system is or is not working.This post was edited by an admin on June 10, 2013 3:43 PM.
SGMIIf you are using this device for ONLY checking moisture or oil condition, then I'm with you. However, I remain a proponent of the static test for non-condensables for one small issue and one huge issue. The small issue is two extra potential leak sources. The huge issue is with a static test, there is no way anyone before or behind you can use it a a charging method.
Hi SCI agree Spence! .I do not install ,all of my installs are done by a trusted tinknocker, I do the startup and service. I do straighten outting for a living. Around 50% or so of the new (to me) systems that I get my hands on, that I install a SG/MI FD on, need a 2nd FD. On some residential I start out with a 30cu.in. FD . The static test is good for what it is! Hay ,look at it this way, with the SG you can actually "see" 1* of SC, then 2* of SC and so on .What is that? KindaSorta it's 8* of SC ,just what my system needs!
The MI checks for moisture ONLY. No moisture ,no acid ! I use a heck of a lot of the HH version of the FD. HH is for acid neutralizing. I do not replace many comps or cond units.This post was edited by an admin on June 9, 2013 8:49 AM.
SGMIToo cool! There are two things I really dislike; losing my car keys and losing compressors. And, I'm sure you will agree that after what we see in the field, someone lost their car keys in the compressor.
I've really enjoyed being on "The Wall." With the security check, my math is getting better. Sometimes, I don't even use a calculator.
NumbersAllowable amp draw 9.3, running at 7.6
Outside temp. 77*
Static pressure 195#
PD across SLD 128 inlet of dryer and 121 at condenser
Supply air 61*
Return air 70*
SL at 70* so SH at 28
LL at 74.5* at SC at 19
Again suction line drier is 3/4" and about 12" from the condenser, is the PD too much? Subcool is high by a lot and SH is out of whack. Static is pretty close, close enough? Actually it's a little low. What do you all think. More than willing to pull down and weigh in but just want t solve problem at same time.
Hi SCWell now, either your test instruments are out of whack or the system is out of whack. Or the Freon is ! I'd start with the Freon. If that SLFD press drop is anywhere near accurate then replace it , and the LLFD w/ a 303-HH after you do an ACID TEST of the oil . This system is 410a? the new evap / TXV is for 410a?
What is the BTU/H.P. of this unit?This post was edited by an admin on June 10, 2013 3:24 PM.
NumbersSomething is amiss here. Your static test should not indicate your condensing temperature is 10 degrees COLDER than your ambient, and with an abnormally high SH, it is impossible your condensing temperature would be colder than ambient. Unless you have a heat pump operating in heating mode, of course. Also, your PD across the SLD is too much.
NumbersWell it seems quite obvious that I should recover this unit. Will remove SLD and replace with new LLD and SG. DO acid test, evacuate and weigh in charge. Had both high and low side gauges hooked up at same time to monitor PD across drier so I know that is an accurate number. The unit had been running but I shut and off and waited for it to equalize, guess I wasn't really concerned about ambient being lower than should be, was concerned about a higher reading.
There are 3 other units that are all the same as far as installation time, and equipment. Haven't paid any attention to them yet, no complaints but the weather has still been quite cool. Raining hard next day or 2, will tackle on maybe Thursday.
Not a large unit maybe 2 ton.
Any Bruins fans here. Thanks
Hi SCWhen checking the SLFD press drop with both hi & lo gauges & the readings are not "normal" try reversing the hose/gauge just to check the press again. It is much easier to change out a flare LLFD than a brazed in FD. If the other units on the roof are the same then I have gone over to then and taken comparison readings. It helps the "thinking" process.This post was edited by an admin on June 11, 2013 7:26 AM.
AgreedWill go back up tomorrow and do that and test all 4 units for comparison. Thanks
Numbers from all 4 unitsRan numbers on all 4 units, all equipment is the same and piped up with driers on SL and a SG only on LL. All equipment new and 2 tons each.
Outside ambient is 72.5*
SL62* and 117#
LL83* and 260#
WB SA 46.5* and RA is 57.2 DT 10.7*
PD across SLD is 5#
SH is 23 and SC is 3
SL70.8* and 118#
LL74* and 290#
SA49.1* and RA 53.9* DT 4.8*
SH 30.8 an SC 20
SL 56.5* and 118#
LL 82.5* and 255#
SA52.5* and RA 57* DT 4.5*
SH 17.5 and SC 2.5
SL61.2* and 120#
LL80.8* and 250#
SA48.5* and RA 52.8* DT 4.3*
SH 20 and SC 3
Original call was for poor cooling on # 2 , SC is way high. Static on 1,2 and 4 way low, and PD on # 4 looks like problem. Delta T only looks good on #1, has higher velocity. #3 Static # is the only one that looks right. SC on # 2 way high.
We have not had any hot weather yet, would like to not run into trouble when it arrives. Any suggestions, Thanks
UnitsLets look at #1 first. You listed your static at 180, which means your condenser is 9 degrees colder than ambient. This should not be the case! I'm going to guess that since your RAs are in the 50's, you are stacking liquid in the condenser because you have no load. That also makes sense as your SH is not that far out of line, if at all. Three things still bother me; why is there an SLD, why is it not next to the compressor where it belongs if it is really needed, and the PD is too high. Disregarding the SLD for the moment, I would crank on the furnace to raise the indoor temperature to the 80's. Even though it would be all sensible load, at least you would see if your SC comes up. If it doesn't, checking for an overcharge may be the next logical step. All of this is assuming your air flow is correct, of course.
Spence#1 had the highest air flow of all around 800cfm out of ceiling diffuser. How is it possible to have 9* colder than ambient? That I don't get. The SLD is within 8-12 inches of condenser, why it is there I don't know. Would they have put in SLD because of the old line set, that is the only piece that was original.
Would be difficult to bring on heat with a bunch of people working there, but may be better than waiting for hot weather and getting caught
High SCFirst lets review the test. As I mentioned earlier, condensers below ambient shouldn't be. Hook up your head pressure gauge and turn the system off. Place a quality thermometer at least 6" from the coil, but not in direct sunlight. After at least 20 minutes, your head pressure converted to temperature and the ambient should be the same. If still lower I am concerned that your ambient and the inside temperature are too close, causing liquid stacking in the outdoor unit. Only a load will tell at this point, other than recovering some refrigerant, which I'm sure you would rather not do right now. It's 72 outside, yet in the 50's and low 60's inside, so your system is loafing. That being said, your SC would still be correct with the correct charge. Since you have sight glasses installed and there seems to be a tremendous temptation to charge to a clear glass for reasons I'll never understand, I'm betting on too much refrigerant. Turn the 'stat to "pizza oven" and see what happens to your SC. We'll save the SLD mystery for later.
Mystery UnitsLook at Unit 3. Your head pressure converted to temperature and the ambient are dead on target! I'm going for air flow or overcharge. Send us your ESP and the CFM at that figure from your blower performance chart in the IOM manual that came with your indoor unit. Along with the load test, we will nail this thing.
ReplyWill go back on Monday to measure to check AH. # 3 had the lowest cfm's of all it was 315 out of ceiling diffuser.
# 1 was 826
#2 was 530
# 4 was 570
This wouldn't change static pressure of condenser though, I am baffled on that. The AH's were slid into existing spaces and reconnected to existing ductwork. Will check ESP Monday and be back Thanks
HI SCWhat is the application of these 4 units The temp diff between RA & SA is way to close. RA temps of 53 & 52 & 57*f is very low, why? If those RA temp are the same as room temp then the refrigerant/static press would tend to lower because of the "hi temp refrigeration" room temp affecting the static test.?
Why would the SH be so hi? 10-12* across the TXV /evap and the rest of the SH is absorbed by 10-15' of suction line?This post was edited by an admin on June 14, 2013 6:49 PM.
Hi scExplain the importance of mounting the SLFD as close as possible to the compressor and NOT in the suction line/line set? Thanks.
SLD PlacementFor the same reasons you place an air filter as close as possible to the blower inlet. Plus, the makers of the driers recommend it for best results.
Suction F/DI looked in my Sporlan ,Alco, &Virginia catalog . One said "as close as possible to the comp, one said "in the suction line ,by removing part of the suction line". and my good ol' reliable R.S.E.S. says in the suction line. After some burnouts the entire suction line has contaminants in it ,so, in order to better protect the new comp the closer to the comp as possible is better. But for ease of installation I put all of my SLFD in the suction line, especially the shell and core FD, I'll forsake 3' or so of dirty suction line and let the FD's do its thing. Heat pumps w/ a SLFD are recommended to have the SLFD in between the reversing valve and the comp.
I an not aware of the rule about the air filter being placed as close as possible to the blower inlet. What about return air filter grills?
I find it hard to believe that all tech are not concerned about moisture in the refrigerant. Moisture=acid= dead compressor. Oh well!This post was edited by an admin on June 15, 2013 4:47 AM.
SLDsSorry I didn't respond last night. I was charging myself to a full shot glass.
With the new coils so tall you need a bucket truck to get to the compressor, as long as the manufacturer doesn't object, putting the drier at the service valve sure makes it easier to install, read temperature and pressure across the body, and change the core if needed. I'm still at a loss as to why Rick's units have them, yet they are there, and the PDs are out of line on a couple of them at least.
LLFDAre you sure there is not a LLFD down inside the cond unit.? Some do, some don't.
YouProbely have a bad TEV and someone dumped a lot of gas in it to bring up the suction pressure.