Comparing your data to a 14-SEER 3-Ton unit of another Make: the indoor temp-split of 13F appears to be not enough; even when using 63% RH & 1175-CFM this data says 16F split.
Originally Posted by Cap'n Preshoot
At an indoor 75F dry bulb & 50% RH the indoor split should be close to 20F; I'd say @58% RH the indoor split ought to be near 17-F.
Possible refrigerant system problem;an overcharge would produce a false higher pressure reading than the actual heatload transfer would be providing. Therefore, we have to look at the outdoor split with caution...
The outdoor condenser split is on target using 50% RH but a little low using 58% RH; a higher latent/humidity load on the indoor coil increases the outdoor condenser split but decreases the indoor split.
The difference between the two splits indicates possible problems.
However, to pin point any problems we need the subcooling temperature & compressor amp-draw.
Using the Lennox Approach Method; for the 14ACX it calls for 6-F (+/- 1-F) of condenser subcooling. The liquid line temp minus outdoor ambient = approach subcooling temperature.
On non-accumulator Lennox A/C condensers with TXV metering devices the above method will indicate whether it's overcharged or undercharged according to their approach chart.
Last edited by udarrell; 08-24-2012 at 07:12 PM.
Reason: Word missing...(+/- 1-F)
Originally Posted by udarrell
From all this it's obvious to me that it's time to have (another) pro take a second look at it. In the peak heat of the day I'm seeing compressor amperage right around 12 amps. Unsure how best to get a temp reading on the liquid line. It feels warm, but not what I'd call hot. The service co. I originally called has a BBB rating of A+, but is unrated on Angie's list (shows them as out of business) and as I mentioned previously, changed hands last year. We've used them for several years under the original ownership and never had a problem. Nowadays they're subbing-out their service calls to sub contractors. I just got off the phone with Air Tech of Houston; they'll be out Monday morning. At this point I really don't mind investing in another service call, I want it fixed & done right. At the moment it's cooling and likely most homeowners wouldn't pursue it any further, but I'm still convinced something is wrong & your info seems to support my opinion.
Last edited by Cap'n Preshoot; 08-24-2012 at 08:08 PM.
Reason: addt'l comment
Right now (7 PM) it's cooled off a little, ambient temp outside approx 86. With systen on (running for 5 mins) the liquid line right at the compressor, as best as I am able to determine, is also 86 deg. Temp diff at the compressor is presently 18 degrees. (104 deg at the top of the fan - 86 ambient). Right now indoor ambient is 77, RH is presently 59% and DP is 61.4 I would like to see the RH closer to 50%
Do you have a pull down staircase in the hallway?
Originally Posted by tipsrfine
No. The attic access is via a pull-down stairway in the garage. We also have 18" of blown-in insulation. It wasn't originally that deep, but after a minor structural problem the builder sent an insulation contractor out to fix what had been disturbed and wound up dumping in quite a bit more (and I didn't complain -
Originally Posted by jtrammel
Concerning subcooling if unit ran long enough & you got an accurate read on the liquid line temp: it is liquid line temp - minus ambient temp; therefore, it has about 6-F too much subcooling, indicating an overcharge.
Originally Posted by Cap'n Preshoot
The liquid line (LL) would have to read 92-F LL temp minus 86-F ambient for 6-F SC. It sub-cooled an additional 6-F above its proper 6-F, to get even with the outdoor temp.
Therefore, it showed 6-F too much subcooling (+/- 1-F). With a light load I'd use 5-F subcooling or what would be a 91-F LL temp.
Well, the condenser temp rise of 104-F & 18-F temp-split appear right on target.
"If" your LL temp is accurate; too high a subcooling is also indicative of a possible slight LL restriction which results in too much subcooling a higher superheat & slightly starved evaporator coil with less heat being absorbed by the indoor coil; thus a lower than normal indoor split & a lower moisture condensation rate with a lower than normal amp-draw.
Non-condensibles will raise head pressure so it looks like there is plenty of subcooling but it's due solely to the additive pressures (usually poor R/H control).
Could also be overcharged with a higher than normal amp-draw or; some other problem which requires further testing...
Last edited by udarrell; 08-24-2012 at 10:23 PM.
Okay, I intermixed Approach & subcooling terminologies in my last post which could create a lot of confusion; I should have stuck with Approach only terminology.
Both methods end-up with nearly the same amount of excess subcooling...
Therefore, here is the Lennox 'stated' Subcooling method for the same Model 3-Ton unit: 104-F - 86-F LLT is 18-F SC; Lennox calls for 11-F subcooling; 18-F SC - 11-F SC is 7-F too much subcooling = overcharge with higher than normal amp-draw or, slight high-side restriction with lower than normal amp-draw, with possible poor dehumidification with both conditions.
Last edited by udarrell; 08-24-2012 at 11:58 PM.
Reason: Lennox 'stated' Subcooling method for the same Model 3-Ton unit...
I would bet a pension check on overcharge as being one of the problems, but maybe not the root cause. Remember, the original BOZO agreed the indoor temp split was wrong, but after inspecting the coil ultimately added refrigerant while watching his manifold gauges and his amprobe. The system was originally pulling less than 9 amps. When he left is was pulling close to 12 and the high side PSIG was 400. In spite of this he never was able to achieve better than a 13 degree indoor split, which is what it originally was before he did anything. After adding refrigerant w/no improvement is when he started suggesting replacing the expansion valve with a piston. We were supposed to hear back from him yesterday. I'm kind of relieved that he didn't call. All the better reason to call someone else.
Originally Posted by udarrell
I'm quite anxious to see what the tech coming from another company on Monday will have to say, especially after the little expose on TV the other night about unscrupulous A/C repairmen wanting to perform unnecessary work when the only problem was a broken wire. (Even the Angie's List guy was caught in their trap) - awfully disappointing, to say the least.
Well, initially the indoor split was considered less than it should be with the AMP Draw at 9 when rated at 14-Amps, which could indicate a mild restriction in the high-side of the system.
Originally Posted by Cap'n Preshoot
The Tech assumed, perhaps falsely, that it was undercharged, rather than, a possible H.S. restriction; therefore, he added refrigerant to bring up the amp-draw, however, that did nothing to improve or increase the indoor temp-split.
Therefore, the probability then leans toward a restriction in the high-side or TXV not feeding enough refrigerant to the coil, which will cause liquid to backup into the condenser coil resulting in excess subcooling.
If he had checked the subcooling & saw that it was too high he would have known that adding refrigerant would make the problem worse, not better.
SuperHeat should also be checked to see if the TXV is holding SH at its prescribed setting; if not you know you have a starved coil.
Good luck with the next Tech...unless he gets on the right troubleshooting track you might let him know your findings...
I would also want the Tech to check the actual indoor CFM airflow; it's always an important number to know; considering your other numbers.
I'm not going to say a hellouvalot to him/her, I'd much rather accompany them to the attic and back yard, keep my hands in my pockets and observe. They're the pro, not me. (Just don't try to sell me a contactor or capacitor). I will simply tell them that the house feels a little humid and that the air coming out of the registers doesn't seem as cold as it should be, and we can't get the house any cooler than 75. That should give them a good starting point. I will not volunteer information that the system was recently serviced unless they ask (which they should ask once they see the system pressure).
OKAY - Follow-up - some progress!
The new tech (fairly young fellow) left about 10 mins ago. Upon arrival he measured the subcooling and found it to be 22 deg. He also said the system was overcharged (as y'all suspected). Upon his departure the SC is now at 14 deg. He also said we had a 4-ton coil with a 3-ton condenser, which he felt was not optimal in this climate and was possibly contributing to the humidity problem as well as the low inside temperature split. I asked about the size misatch, tech said it's the same coil Lennox uses on both 3 and 4 ton models. Like the first guy, he also suggested we try dropping the fan speed. I said OK, but contingent upon how the wifey gets along with less air volume. I have not said anything to the wife, nor will I, waiting instead for her to notice & complain, but thinking she may not notice since at reduced fan speed we're now achieving an indoor air split of 57/75 (18 deg - wheee!) Amp draw is down to 9.8. Humidity is still a tad high (57%) but the guy has only been gone 30 mins, so we'll keep an eye on that for a while.
While here he also removed both side covers of the coil & agreed it was clean, but pointed out a very small amount of oil in one corner (nearest the TXV) and said it's a very minor (slow) leak but would qualify for a warranty replacement as long as we do it sometime within the remaining warranty period. I felt the substance and indeed it is oil. He also commented that the coil is backwards (apex pointed toward the furnace) - he said technically it could be installed either direction, but with it the way it is, the tubing assy was bucking the air flow.
His overall opinion, upon departure, was that everything seems to be working and the outside unit, although low-end, is a good unit and there's no reason not to expect 12~15 years of service. He was not as flattering about the Lennox coil, remarking that they've replaced many and that it'd be rare to have one last more than about 7 yrs. Of course there's a substantial labor charge involved in coil replacement. I'll have it done, but I'm going to wait a few weeks. Unfortunately the warranty replacement coil will also be a 4 ton model.
Minimum svc. call chg. only.
I'd say you found a good Tech.
Two bad you can't go to a smaller 3-ton indoor coil - also with a TXV, as that could help improve the pressure-temp drop for improved dehumidification; helps reduce the sensible ratio & increase latent ratio; plus increases runtime a little bit more to satisfy the RM-TH.
Dropping the blower CFM volume flow should help improve humidity control.
Well, looks like we had the diagnoses pretty well pegged; reduced the overcharge, dropped the blower CFM & the amp-draw also went down. That indoor split looks real good at 18-F with 57% RH.
Okay, @57% RH & a 4-Ton indoor coil on a 3-Ton condenser with a drop in blower CFM; that 18-F indoor split is just about right on its target point. Hopefully at 75-F IDB, it will get it around 50% or lower relative humidity.
Sometimes, it pays to do your homework, be persistent; & qualify a Tech that knows how to do the job...now you know how to get things near to where they need to be - for the remaining life of the equipment.
Unfortunately 10 hrs later there's no improvement in RH. Still hanging in there, drifting between 57 and 62%