Microchannel evap

[BaldLoonie]

They're here!

Nordyne C7 coils. This should be interesting. They say they've tested extensively and they will drain fine.

[ORV]

May drain, but how will they clean? I hope filters get changed with those!

[SBKold]

Nordyne says they clean easier because you can use a brush and not damage the fins.

I also think it interesting how the refrigerant feeds the lower right of coil and suction is from lower left header. Interesting.

[La Air Man]

I'm not a fan of the micro channel coils. I have found many leaks in the condenser coils(mostly coleman trailer condensers). Time will tell.

[a\c don]

Changed one out about a month ago on a 3 ton heat pump.
Seems to be well made.

[hvacvegas]

neat stuff. Now if only they can make them smaller somehow.....

[BaldLoonie]

Not many evaps are decent sized anymore except maybe 3rd party coils then you wonder what you are really getting.

[ar_hvac_man]

Cool, first one ive seen.

Yorks condensers are done that way now but not the h pumps yet. Drainage.

[refertecbd]

I've been seeing these come out on commercial ref. condensing units. I haven't had any issues with them but I bet they'll be a pain to fix leaks on them.

[ar_hvac_man]

I was told they had a repair kit but it didnt work out so now theyre just replacing the condenser.

[ararams]

My question is. How can an expansion feed into a header and still feed may be twenty or more tubes equally.

[gravity]

just more complicated things for the not so trained people to work on.

[hvacvegas]

My question is. How can an expansion feed into a header and still feed may be twenty or more tubes equally.

Experimental Research and CFD Simulation on
Microchannel Evaporator Header to Improve Heat
Exchanger Efficiency
"Obviously, the single inlet header concentratedly distributes refrigerant into middle portion of microchannel
coil; the dual inlet header significantly improves the refrigerant distribution uniformity inside inlet coil header; and
the distributor inlet header performs the most uniformed refrigerant distribution."

http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1752&context=iracc&sei-redir=1&referer=http%3A%2F%2Fwww.google.com%2Furl%3Fsa%3Dt %26rct%3Dj%26q%3Dmicrochannel%2520evaporator%26sou rce%3Dweb%26cd%3D5%26sqi%3D2%26ved%3D0CFYQFjAE%26u rl%3Dhttp%253A%252F%252Fdocs.lib.purdue.edu%252Fcg i%252Fviewcontent.cgi%253Farticle%253D1752%2526con text%253Diracc%26ei%3DTK5eT4Ns5bLRAZW8vbcH%26usg%3 DAFQjCNGkZKXbOSuUnObR8UIK3GHQxlNQnQ#search=%22micr ochannel%20evaporator%22

Page 7.

And, it appears these microchannel coils feed on one end of the header, not the middle, which clearly wasn't even considered acceptable enough to be tested in that study.

Read the conclusions also. They highly suggest uniform distribution, for maximizing efficiency.

Keeping in mind, thats from 06.


I wonder if there is something INSIDE the header that acts as a distributer.

[71CHOPS]

very cool.....haven't done much research on them yet....guess I've got to add to the list of reading material!!

[Cooked]

I wonder if the static pressure drop is comparable to a typical tube and fin?

[mark beiser]

I'm not a fan of the micro channel coils. I have found many leaks in the condenser coils(mostly coleman trailer condensers). Time will tell.

I'm not sure a Coleman product is a good weather vane for the state of any technology....

[behappy]

Experimental Research and CFD Simulation on
Microchannel Evaporator Header to Improve Heat
Exchanger Efficiency
"Obviously, the single inlet header concentratedly distributes refrigerant into middle portion of microchannel
coil; the dual inlet header significantly improves the refrigerant distribution uniformity inside inlet coil header; and
the distributor inlet header performs the most uniformed refrigerant distribution."

http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1752&context=iracc&sei-redir=1&referer=http%3A%2F%2Fwww.google.com%2Furl%3Fsa%3Dt %26rct%3Dj%26q%3Dmicrochannel%2520evaporator%26sou rce%3Dweb%26cd%3D5%26sqi%3D2%26ved%3D0CFYQFjAE%26u rl%3Dhttp%253A%252F%252Fdocs.lib.purdue.edu%252Fcg i%252Fviewcontent.cgi%253Farticle%253D1752%2526con text%253Diracc%26ei%3DTK5eT4Ns5bLRAZW8vbcH%26usg%3 DAFQjCNGkZKXbOSuUnObR8UIK3GHQxlNQnQ#search=%22micr ochannel%20evaporator%22

Page 7.

And, it appears these microchannel coils feed on one end of the header, not the middle, which clearly wasn't even considered acceptable enough to be tested in that study.

Read the conclusions also. They highly suggest uniform distribution, for maximizing efficiency.

Keeping in mind, thats from 06.


I wonder if there is something INSIDE the header that acts as a distributer.

Like Don, I have replaced one already.
Plus we have installed one complete Microchannel inside and out system.

I give each client the "talk", keep it clean or else!

The static is the same (new).

The refrigerant feeds in the bottom header on one side, up and across the coil and out the header on the bottom on the other side.

We will be seeing them in HP soon, now that may be a trick?
We'll see....

[BaldLoonie]

3 ton C6 coil is .21 at 1200 CFM, the microchannel is .27 but compare that to .41 for a 3 ton Rheem N coil.

[71CHOPS]

I just wonder if the "A" design is gonna be the best way to go with these. I'm sure manufacturers have tried other configurations, with having some interesting distribution challenges, there may be other configurations that are more effective.

I bet they are more effective in package units...

Anybody have any engineering data for these???

[La Air Man]

I'm not sure a Coleman product is a good weather vane for the state of any technology....

HaHa. Completely agree on that one, but the condenser coil on the coleman and on the trane xb300 look exactly the same. Are they made by each manufacturer or do they all purchase them from another company. Plus Trane requires them to be charged by a max temp or pressure instead of subcooling.

[BaldLoonie]

The XB300's coil is Chinese. Wonder if York & Nordyne make their own?

The Tappan system with the coil pictured is up & running. Had to go online for a piston charging chart since the manual from the outdoor unit was written when Nordyne called for a TXV inside.

[garyed]

With the experience I've had so far with microchannel coils I wouldn't install one on a bet. I'll change brands until there's no company left that doesn't use them before I ever install a microchannel coil for one of my customers.

[Cooked]

Care to elaborate Gary, I'd love to hear your thoughts.

[amd]

What's the purpose of microchannel coils?

The one in the picture is as large as the standard coil.

[garyed]

I've replaced between 25 to 30 leaking all aluminum microchannel outdoor coils under warranty this summer. Its left bad taste in my mouth for microchannel coils. The only reason for using them is to get the same heat transfer with less materials & less freon so its basically cheaper to make the units. I'll stick with the old stuff as long as I can.

[gravity]

I stand beside gary on this one.

[BaldLoonie]

I would gather these were on York products, Gary?

[ar_hvac_man]

Worries me a bit ive been setting a ton of York for the past 6 months. No problems so far except for one dry charge that had a leak right out of the box. Tiny nick in the channel which they quickly shipped any paid 4 hrs labor for me to replace

[garyed]

I would gather these were on York products, Gary?
yes - about 3 years old

[hvacvegas]

I stand beside gary on this one.

I think I'm leaning that way too.

Goodman came out with their "smart coil" "technology" making it more "efficient". Aka, cheaper, and harder to charge.

*sigh*

The thing that worries me, is the size of the internal channels. Anyone here have blocked cap tubes?

I see microchannel coils as nothing other than a long series of cap tubes, due to the size of the internal channels.

[garyed]

Another negative is that they don't recommend pumping units down with microchannel condenser coils either.

[ga-hvac-tech]

That evap looks like an atomotive radiator or condensor coil... :grin2:

[ChrisTechMech]

Another negative is that they don't recommend pumping units down with microchannel condenser coils either.

So we have to recover the refrigerant to repair a leaking lineset? Do they still come pre-charged?

[gravity]

I'm pretty sure if you tried to pump the unit down, the condenser coil wouldn't have enough volume to hold all the refrigerant. you might be able to pump some of it down.

I have also heard these units are critical charge. if there overcharged by 6oz they can trip on high head.

This just isn't a good thing with all the non-skilled hvac guys that dont know how to properly charge a unit and gauges/temp clamps that havent been calibrated for years.

[SBKold]

They got in there somehow.

I think if u were to add any then it wouldn't fit in.

I also think they say don't do it because it would be easy to completely fill the condensor and then be in danger of rupture with expansion just like overfilling a cylinder.

[gravity]

They got in there somehow.

I think if u were to add any then it wouldn't fit in.

I also think they say don't do it because it would be easy to completely fill the condensor and then be in danger of rupture with expansion just like overfilling a cylinder.

good point.

looks like i gotta carry around extra britches and some wipes when these things really hit the market :grin2:

[craig1]

Do they still come pre-charged?

I have been installing Nordyne units with microchannel condensers for 3 years now and have not had a single problem with them except that in the beginning they were coming shipped with very little refrigerant in them. Some only had vapor. I was concerned that they had leaks. Tech support assured me the problem was they were having trouble getting the full charge into the microchannel coil at the factory.

I filled them up and there were no leaks. They apparently figured out how to jam all the refer in now because they seem to be shipping with something at least close the nameplate charge.


I have not had to pump one down yet, but I suspect it will be a challenge for all but the shortest linesets.

[gravity]

how do these units run craig? what is the common metering device? do the evap coils still get as cold? how about condensers? whats the temp?

[JBM1000]

I've replaced between 25 to 30 leaking all aluminum microchannel outdoor coils under warranty this summer. Its left bad taste in my mouth for microchannel coils. The only reason for using them is to get the same heat transfer with less materials & less freon so its basically cheaper to make the units. I'll stick with the old stuff as long as I can.

Add the fact that Carrier plasters about ten stickers warning to use "Only Water" to clean the cond coil. Guess any Chemicals will murder the coil.

[steveoo]

say it not so!

[ga-hvac-tech]

Add the fact that Carrier plasters about ten stickers warning to use "Only Water" to clean the cond coil. Guess any Chemicals will murder the coil.

THX for posting this! Will make a mental note to be careful using 'Nu-
Brite' on those condensers.

I hope we are not heading towards a C-F with this new technology. The old aluminum indoor coils GE And Trane made were good coils... I guess they were too good for the bean counters.

[craig1]

how do these units run craig? what is the common metering device? do the evap coils still get as cold? how about condensers? whats the temp?

They work just as good as condensers with copper coils. Nordyne's copper indoor coils use txvs which I really like. I hear the new evaps will use pistons but txv will be optional.

The one thing you have to be careful of with the microchannel condensers is they are REALLY easy to overcharge.

[cuchulain]

we sold a few of the condensers when thy came out. With our corrosive atmosphere they last about 2 yrs before leaks start showing up. Once we started having a run of these leaks we quit selling them. Every once in awhile we'll get a new customer who has one and the first place we go is to pull the leak detector out. If it's in warranty we'll replace the coil and be done with it, until it's out of warranty then we change the condenser.

Charging is critical. Do it by weight. They give the chart for lineset length/size/rise and how much extra you should add.

[JWB]

why would they be easy to overcharge? also, we can't clean with foaming coal cleaner, but is that because it breaks down the aluminum too quickly?

[cuchulain]

they are easy to over charge because they don't require near the same amount of refrigerant as a standard unit does. I've seen them listed as needing as little as 4 lbs for the particular lineset it was on (don't remember the length). Now you simply add 1 lbs of gas and suddenly your over charged by a full 25%.

[BaldLoonie]

I see Dave is joining Carrier, Rheem & McQuay with MC in light commercial.

http://www.lennoxcommercial.com/landing/environ.asp

[tkirkpatrick09]

why would they be easy to overcharge? also, we can't clean with foaming coal cleaner, but is that because it breaks down the aluminum too quickly?

A trane rep told all of our guys that chemical cleaners of any kind were not to be used on multi-channel coils. Water is sufficient and should be the only means of cleaning these coils to preserve coil life. Not sure how well this will work on some of the extreme cases but time will tell.

Let me know if anyone knows of a cleaner that is safe for this type coil.

[ADillon]

Just as stated above, the main problem with these types of coils is over charging. They are very charge critical and can be very unforgiving with high pressure faults. Technicians must wait the required 15 minutes inbetween adjustments or they will overcharge these systems all day long.

[dakotakid]

The XB300's coil is Chinese. Wonder if York & Nordyne make their own?

Was just at the nordyne factory about a month ago. They do not make their own microchannel coils. They come from china.

Said they would possibly start making their own as more of their product line gets switched to microchannel coils, but as of now they do not.

[mh heating]

I just checked out the microchannel coils for Nordyne. What a nightmare. Im doing a new construction job right now and am planning on installing a nordyne system. Looking at they're "quick data reference sheet" (match AH to OD unit) I have to add an additional 29 oz right off the bat. Then add for the additional lineset.

Now if Im installing a a 3.5 ton 14seer hp with the new c7 MC coil, I have to change the orfice to an field supplied txv and add 29oz. But heres the kicker, you have to change the outdoor units orfice from .55 to a .57... How stupid is that

Might have to reconsider using nordyne equipment on future jobs.

MH

[ChrisTechMech]

...I have to change the orfice to an field supplied txv...

External on internally equalized TXV? If external, is it mechanically installed or sweat?

[garyed]

Microchannel is just another way to make the system cheaper & still get the SEER. The bottom line is it all comes down to profit. The only slight advantage for our side is they can cut the size of the equipment down a little for some of those tight spaces. Other than that the disadvantages so far have far outweighed any advantage for us.

[ar]

Mfg could install a liquid receiver and help us out a lot in terms of pumping a system down and over charging possibility. TXVs are the best defense against a low skilled tech when charging.

[behappy]

we sold a few of the condensers when thy came out. With our corrosive atmosphere they last about 2 yrs before leaks start showing up. Once we started having a run of these leaks we quit selling them. Every once in awhile we'll get a new customer who has one and the first place we go is to pull the leak detector out. If it's in warranty we'll replace the coil and be done with it, until it's out of warranty then we change the condenser.

Charging is critical. Do it by weight. They give the chart for lineset length/size/rise and how much extra you should add.

Just got a flyer from Nordyne. They now have "coated" MC coils for corrosive areas like ours. It seems to be like the black coating that Goodman used on their inside coils.....

[mh heating]

This is from the new b6 series from nordyne. Its not a mc but what the call there ant eater coil. Has special coating to block the corrosive.


http://www.use.com/media/2012/0501/2419086/p_003.jpg

[La Air Man]

I just picked up a replacement coil for a b3bm-060k and it was a standard tube/fin design. This will be the second coil replacement in this unit since 96. Are their higher seer air handlers going to stick with tube/fin coils?

[behappy]

I just picked up a replacement coil for a b3bm-060k and it was a standard tube/fin design. This will be the second coil replacement in this unit since 96. Are their higher seer air handlers going to stick with tube/fin coils?

Tech Service told me last week no MC for 2 speeds. All AHU's A/C and H.P 13 seer have been MC lately. FWIW...

[mh heating]

The picture I posted is a variable speed AH and rated for a 16 seer system and its a standard tube/fin design with their special coating. It might be different from supplier to supplier.

MH

[billygoat22]

I had a 13 seer Gibson this week that I replaced the coil on. the new coil was one of the MC coils, so read the install instructions carefully. Then I called tech help about the piston size- it seems all the 13 seer units (B5BM---) need txv on the evap coil.

I had to add one because whoever put the system in 5 yrs ago didn't (the copper coil required a txv also). Found the paperwork behind the unit- across tonnages were all txvs. txv is non-bleed, so you also need to add PTC or some other start assist, which doesn't come on a condensor that requires a txv on evap?

Worked on another Fri, next neighborhood over, same model, no txv either. wonder what that says about all the homes in both these new 'hoods?

Took care to double check for proper airflow and watch charge carefully. at least this was a r22 system

I would have thought these coils would have a reverse return setup- headers tapped off opposite ends to get the same distance of travel for all the circuits. Or a tube internal to one header to get the same effect.

Another concern- if one side of coil gets hotter/colder than the other due to all supplys starting on one side- wouldn't you have noticeably different temps off the right or left side ducts runs off plenum? I've seen offset electric heaters in a/hs do this.

As far as pump down on MC condensors- if they had a tap on discharge like Trane, you could connect recovery tank there to store extra, and pull it back out by dumping into evacuated lineset after work is done. no recovery machine required at least.

[Texas-Tech]

I see Dave is joining Carrier, Rheem & McQuay with MC in light commercial.

http://www.lennoxcommercial.com/landing/environ.asp

We just did a change out of a store in Austin last month, all Lennox. The 25ton package units had microchannel condenser coils. They looked to be about an inch and a half thick. The also moved the high pressure switches out of the compressor compartment and put them at the liq line drier in the condenser compartment.

[craig1]

Found the paperwork behind the unit- across tonnages were all txvs. txv is non-bleed, so you also need to add PTC or some other start assist, which doesn't come on a condensor that requires a txv on evap?


I believe these were all scroll compressors. No start assist required.

[mh heating]

I had a 13 seer Gibson this week that I replaced the coil on. the new coil was one of the MC coils, so read the install instructions carefully. Then I called tech help about the piston size- it seems all the 13 seer units (B5BM---) need txv on the evap coil.

I had to add one because whoever put the system in 5 yrs ago didn't (the copper coil required a txv also). Found the paperwork behind the unit- across tonnages were all txvs. txv is non-bleed, so you also need to add PTC or some other start assist, which doesn't come on a condensor that requires a txv on evap?

Worked on another Fri, next neighborhood over, same model, no txv either. wonder what that says about all the homes in both these new 'hoods?

Took care to double check for proper airflow and watch charge carefully. at least this was a r22 system

I would have thought these coils would have a reverse return setup- headers tapped off opposite ends to get the same distance of travel for all the circuits. Or a tube internal to one header to get the same effect.

Another concern- if one side of coil gets hotter/colder than the other due to all supplys starting on one side- wouldn't you have noticeably different temps off the right or left side ducts runs off plenum? I've seen offset electric heaters in a/hs do this.

As far as pump down on MC condensors- if they had a tap on discharge like Trane, you could connect recovery tank there to store extra, and pull it back out by dumping into evacuated lineset after work is done. no recovery machine required at least.
Thats weird. All my nordyne Ah has the txv already installed on the indoor coils. Even the old R-22 ones. THese are all Kelvinator units

mh

[behappy]

Thats weird. All my nordyne Ah has the txv already installed on the indoor coils. Even the old R-22 ones. THese are all Kelvinator units

mh

The MC coils that we have been getting are piston and you have to add the Xvlv.

[mark beiser]

TXVs are the best defense against a low skilled tech when charging.

Hey Andy!

True, but they sure are nice to have for more skilled techs too. ;)
Also better for the system at our normal summer operating conditions since we spend a lot of time in the "--" range on superheat charging charts.:gah:

[mh heating]

The MC coils that we have been getting are piston and you have to add the Xvlv.

Then it has to be with the suppliers we are using. Different models I guess. More than likely we'll be doing the mc coils soon enough

mh

[ga-hvac-tech]

Hey Andy!

True, but they sure are nice to have for more skilled techs too. ;)
Also better for the system at our normal summer operating conditions since we spend a lot of time in the "--" range on superheat charging charts.:gah:

I think TXV's are a good thing... they solve a lot of issues a piston will not accurately address.

Remember the old adage 'one size fits all'... ever try that with shoes or a shirt? Yeah, one large enough will do... but do you really want to walk around in size 15 EEEEE wooden boxes; or stuff your 12D feet into 10B canvas? Or perhaps we would look more professional wearing a gunny-sack in place of our proper fitting workshirts?

With metering of refrigerant; a TXV will meter more accurately over a wide range of demands... where a piston is just a middle of the road compromise.

In my climate where temps can range over 30+ degrees; and we can be sticky humid, then rain, then sticky humid again--all in one day.... Expecting a piston to handle all those indoor climate changes... well IMO the customer gets the short end of that deal.

This is not aimed at anyone, just my opinion: If the tech staff does not understand TXV's (proper installation and proper charging)... then perhaps they need some professional training???

[71CHOPS]

had to work on a 1 year old 4 ton york, MC inside and out, 410A, fixed orifice. System is leaking, we were told to add charge. It took a VERY long time for the superheat to start to come down to target. Subcooling was normal in short order though.

I'm thinking the next fixed orifice MC I have to charge is gonna be done by subcooling. What do you guys think about that?? I guess you run the risk of incorrect charging, but if it's a matched system, you should still end up pretty close. Also, we were only getting about a 16 degree difference in supply/return air....I thought it should have still been closer to 20. that was at 76 room temp.

I may have been over complicating things, as i hadn't had any experience with the MC's.

[DOGBOY]

only problem with txv's are they are now not as reliable as they were once. We are having to change out entirely too many. For this reason i would be glac to go back to piston or fixed metering devices.

dogboy



I think TXV's are a good thing... they solve a lot of issues a piston will not accurately address.

Remember the old adage 'one size fits all'... ever try that with shoes or a shirt? Yeah, one large enough will do... but do you really want to walk around in size 15 EEEEE wooden boxes; or stuff your 12D feet into 10B canvas? Or perhaps we would look more professional wearing a gunny-sack in place of our proper fitting workshirts?

With metering of refrigerant; a TXV will meter more accurately over a wide range of demands... where a piston is just a middle of the road compromise.

In my climate where temps can range over 30+ degrees; and we can be sticky humid, then rain, then sticky humid again--all in one day.... Expecting a piston to handle all those indoor climate changes... well IMO the customer gets the short end of that deal.

This is not aimed at anyone, just my opinion: If the tech staff does not understand TXV's (proper installation and proper charging)... then perhaps they need some professional training???

[ga-hvac-tech]

only problem with txv's are they are now not as reliable as they were once. We are having to change out entirely too many. For this reason i would be glac to go back to piston or fixed metering devices.

dogboy

Literally EVERY item in every product has issues once in a while. This does not mean they are bad items... just issues... like anything else.

I remember when Firestone built bad tires... does that mean we should stop having rubber tires on cars? Of course not.

TXV's have been used in refrigeration and AC for longer than any of us have been alive... they are a tried and proven technology.

Just because we get some poor quality ones from China does not mean the technology is bad... just we need to not buy cheap stuff from China.

[dna]

TXV's are tried and proven but the quality is rather poor today. Given the option on an evap, I will not use one.

[ga-hvac-tech]

Lessee... we sacrifice the quality of a better design... simply because a few run-offs are sub-quality.

I guess we need to go back to solid rubber tires because inflatable tires have flats.

Just about EVERY system I install has a TXV on it. And when I mount a dry charge AC unit (always insist on a 13 SEER coil) I always add a refrigeration quality Danfoss R-22 TXV (adjustable). After setting the SC, I go back to the evap and set the SH. Systems work like a champ, and I have NEVER had a call-back on a TXV I custom installed.

Yes, I had a couple of bad TXV's on Ruud coils in 2010... however the distributor gave me a new coil and paid me some labor to change it. I hear trane had bad blower wheels, Goodmana has all kinds of bad electric and electronic parts, and lets not forget the MC coils... :whistle:

Again: The technology is not bad... just a few items. Better to not throw out the baby with the bathwater... unless we are too lazy to see if there is a baby in the pan. :whistle:

[timkav]

I'm pretty sure if you tried to pump the unit down, the condenser coil wouldn't have enough volume to hold all the refrigerant. you might be able to pump some of it down.

I have also heard these units are critical charge. if there overcharged by 6oz they can trip on high head.

This just isn't a good thing with all the non-skilled hvac guys that dont know how to properly charge a unit and gauges/temp clamps that havent been calibrated for years.
So is this a problem with the systems or with the personnel installing and servicing them?

[timkav]

I just checked out the microchannel coils for Nordyne. What a nightmare. Im doing a new construction job right now and am planning on installing a nordyne system. Looking at they're "quick data reference sheet" (match AH to OD unit) I have to add an additional 29 oz right off the bat. Then add for the additional lineset.

Now if Im installing a a 3.5 ton 14seer hp with the new c7 MC coil, I have to change the orfice to an field supplied txv and add 29oz. But heres the kicker, you have to change the outdoor units orfice from .55 to a .57... How stupid is that

Might have to reconsider using nordyne equipment on future jobs.

MH
By way of responding to this situation, Nordyne is making a running change, with a new service valve. This service valve will incorporate the restrictor inside the valve as opposed to inside the distributor. The new valve will provide the flexibility necessary to accommodate multiple indoor products. Your distributor can refer to the tech bulletin dated May 1, 2012.

[bb]

Oh gawd. Those will be a pain to clean.

Back when my wife and I ran a large paper route, our van's evap coil would get a build up of paper dust on it every year or so, and it was a royal pain to clean. Brushing only makes it worse IMHO.

[garyed]

So is this a problem with the systems or with the personnel installing and servicing them?
Since this thread started back up I have to say this is a problem with the systems. No system should be designed so sensitive as to not account for minor variations. There is no way that a system can be perfectly charged in the field no matter how skilled the tech is. We do not work in a controlled environment. SH & SC take about 15 minutes to stablize if not longer. In that time the indoor wet bulb can change a few degrees & then your numbers won't be totally accurate again. If you get two excellent techs charging the same unit under different conditions there is no way they would both put in the exact same amount of refrigerant in the system. Sure they'll get close but with a system with 5 lbs. of gas where a few ounces make the difference between working & not that is absolutely ridiculous.