Can I PLEASE get the truth about Refrigerant Charge???

[Tommy Torero]

Is a refrigerant charge test affected by the temperature of the airflow across the evaporator coil? Assume that airflow is confirmed at 400 cfm per ton.

I am trying to do a mild weather refrigerant charge and am being told by an "expert" that I ABSOLUTELY, POSITIVELY MUST maintain a Return Air dry bulb above 70F in order to get an accurate verification of charge.

I received an email from my boss with the following assertions:
-Normal operating conditions for Refrigerant Charge, (RC), test of HVAC systems are defined as having an indoor return db temperature range of 70 degrees F to 84 degrees F.

-low entering air temperatures will reduce the evaporator load.

-With less heat load on the evaporator coil, the flow of refrigerant is reduced, thus, more refrigerant can back up in the condenser causing a higher subcooling reading.

-Remember: with a low heat load on the evaporator coil the TXV will throttle back on the refrigerant flow, which effects the subcooling and superheat.


How is this possible when a refrigerator and freezer both operate with approximately the same subcooling (10) and superheat (12-ish) and their return air db is NOT 70F?

I have been placing a load on the condenser and increasing it's head pressure to simulate a 85-90 degree day (placing the access panel on top of the condenser to somewhat restrict the air leaving the condenser) It has been 55 degrees outside and I am getting the following numbers

Supply dry-bulb 44.5
Return dry-bulb 64.2
Return wet-bulb53.2
Evaporator saturation temperature 37 (112psi)
Condensor saturation temperature 90 (274 psi)
Suction line temperature (Tsuction) 50.9
Liquid Line Temperature (Tliquid) 79.3



My subcooling is +10.7F
My superheat is +13.9F

My cold weather numbers are consistent with my numbers from warmer days.

Am I missing something here.... or is this guy trying to drive me prematurely insane?

I am being told outright that these are not valid numbers because there isn't enough HEAT flowing over the evaporator.

Is this a valid refrigerant charge?

thank you.

[m-cooling]

Go back and re examine the refrigeration circuit.What state liquid or gas it should be in different parts of the loop.

[SBKold]

In short, no its not valid.

That is valid for if you are running a/c with indoor temp of 64. And outdoor of 55.

If you resample the data on a 80IDB/90ODB day you will have much different pressures.

If you dont-then you have duct problem IMO.

[cehs]

His super heat and sub cooling are in the normal range.
I do not think he is asking if the pressures are correct, but is the charge correct.
I would say that if he is not starting with an empty system, that his thinking is correct.
If the system were empty to begin with, I would recommend that he weigh it in.....

[timebuilder]

Is a refrigerant charge test affected by the temperature of the airflow across the evaporator coil? Assume that airflow is confirmed at 400 cfm per ton.

I am trying to do a mild weather refrigerant charge and am being told by an "expert" that I ABSOLUTELY, POSITIVELY MUST maintain a Return Air dry bulb above 70F in order to get an accurate verification of charge.

I received an email from my boss with the following assertions:
-Normal operating conditions for Refrigerant Charge, (RC), test of HVAC systems are defined as having an indoor return db temperature range of 70 degrees F to 84 degrees F.

-low entering air temperatures will reduce the evaporator load.

-With less heat load on the evaporator coil, the flow of refrigerant is reduced, thus, more refrigerant can back up in the condenser causing a higher subcooling reading.

-Remember: with a low heat load on the evaporator coil the TXV will throttle back on the refrigerant flow, which effects the subcooling and superheat.


How is this possible when a refrigerator and freezer both operate with approximately the same subcooling (10) and superheat (12-ish) and their return air db is NOT 70F?

I have been placing a load on the condenser and increasing it's head pressure to simulate a 85-90 degree day (placing the access panel on top of the condenser to somewhat restrict the air leaving the condenser) It has been 55 degrees outside and I am getting the following numbers

Supply dry-bulb 44.5
Return dry-bulb 64.2
Return wet-bulb53.2
Evaporator saturation temperature 37 (112psi)
Condensor saturation temperature 90 (274 psi)
Suction line temperature (Tsuction) 50.9
Liquid Line Temperature (Tliquid) 79.3



My subcooling is +10.7F
My superheat is +13.9F

My cold weather numbers are consistent with my numbers from warmer days.

Am I missing something here.... or is this guy trying to drive me prematurely insane?

I am being told outright that these are not valid numbers because there isn't enough HEAT flowing over the evaporator.

Is this a valid refrigerant charge?

thank you.

This is a discussion better suited for our Pro Technical area. Apply for Pro membership after you get 15 posts, and you will get more detailed responses there.

[Special Ed]

are you checking the charge right now when it's so cool out? Is it a heat pump? If not, I say leave it alone for now & wait for warmer days to check the charge. If it is a heat pump, I say you're probably fine for now but the system may need a little balancing when it gets warmer. It's always easier to charge a heat pump in warmer weather than it is when it's colder out, & you really have to be almost a super tech to get it right on in the winter.

Besides all that, why are we even discussing this topic outside the PRO'S forum?

[beenthere]

First. Refrigerators and freezers have a higher volume of refrigerant flow per BTU then an A/C does. So while the SH and SC may be the same on many as an A/C, the flow rate is vastly different. Since they are designed to remove heat from low temp air.

A/C is not designed to remove heat from low temp air.

To charge an A/C in low ambient conditions as you described. Its best if you raise your indoor temp to 70 degrees or more. So I would say your boss is correct that at the low indoor temps your using. The charge is not correct.

How far off is it? Can't say from this chair.

The indoor air at the conditions you posted. Would have 87 BTUs less per pound of air(5,228 BTUs less heat an hour per 400 CFM), then at 70°F DB and 50%RH. So I wouldn't expect to have the same sub cooling and SH at that low indoor load, as I would at 70°F and 50%RH.

As a test. The next time your doing a start up or spring check when its 80 outside. Set the thermostat to 60, and let it run. See what SH and SC you get then when the house gets down to 64°F.

[Tommy Torero]

Quick back story as to why it's SO IMPORTANT for me to crack the nut of mild weather refrigerant charge testing. Out here in California our new Energy Code MANDATES refrigerant charge test {RCT} on retrofits ( it even mandates RCT on Furnace Only changeouts when an existing, busted up, old broken down R-whoknowswhat.... is present! The tech is responsible for charging it even if it all leaks out the next day) So we cannot final a permit if we cannot do an RCT. What this means is No Permit=No Close of Escrow No Permit=No Rebates etc etc. I am a HERS Rater who had 1 contractor with 600 jobs that needed to be RCT'd when it got "warmer" outside. Sounds great for me? It wasn't.... and it cost the contractor an extra$150 per job in tech/ inspection fees = $90,000!!! Not to mention the time-suck.

I will take this to the PRO forum but don't know how to get the required 15 points.

My premise is simple; the AC is A MACHINE. It does not inherently "know" or "care" what the outdoor/indoor temp. An AC is required to operate under a variety of situations that preclude Return Air temps BELOW 70F. And while AC is different than Refrigeration... the operating parts and process are "basically" the same.

Per the California Energy code my refrigerant charge test is to ensure 3 things;

1) Airflow above 300cfm over the evaporator coil
2) Subcooling +/- 3F manufacturers spec (7-13 degrees subcooling)
3)Superheat between 3 & 26 degrees (to verify TXV operation)
(PLEASE do not get into the accuracy, thought process, infinite or finite wisdom of this. It's California and it is what it is)

So- my Refrigerant Charge Test needs to verify that enough Liquid hits the TXV
- A subcooling of 10 degrees means that the refrigerant is 10 degrees away from being a vapor (forget about pressures... 10 degrees subcooling is what it is) which makes the TXV very happy. Pure liquid.

-The evaporator load (or how much liquid refrigerant is needed) is determined by the sensor on the suction line as the R410 leaves the Evaporator coil headed back to the compressor as a vapor. This suction line temp "tells" the TXV how much refrigerant is needed and determines the "LOAD" placed on the Evap coil. A superheat of 12 degrees means that the refrigerant is 12 degrees away from being a liquid. This makes the compressor very happy. Pure vapor to compress.

If there was an insufficient "LOAD" would we see a higher or lower amount of superheat? (I'm guessing higher)

We read this temperature of the Saturated Evaporator (theoretically/hopefully the same temp that the TXV sensor is "seeing") on our low-side gauge. And if I see that the pressure is 112psi (37F degrees) and that my Suction Line dry-bulb is 50.9F we have a superheat of 13.9F. PERFECT. The AC does not KNOW or care that it is 55F outside because I have, in effect placed a load on the Condenser (I partially block the condenser but to prove my point you can buy a low ambient kit that will slow down your fan and increase the head pressure to operate under mild-cold conditions) It is how Commercial AC units work .... a series of fans to control head pressure, because sometimes you have to operate an AC year-round regardless of outdoor temp.

I have done a few hundred RCT's this year and my numbers from last week
are more or less:
Evaporator saturation temperature 37 (112psi)
Condensor saturation temperature 90 (274 psi)
Suction line temperature (Tsuction) 50.9
Liquid Line Temperature (Tliquid) 79.3

Subcooling of 10F Superheat of 12F

This was with an average outdoor temp below 60F and Return db less than 65F

These numbers are EXACTLY in line with 200+ tests done with an out door temp of 70-78F and a Return db of 70-75F

So I am trying to see how the evap coil "sees" or "feels" the difference in Return db if it still is sending 12F superheat (pure vapor) to the compressor?

THANKS! and if anyone can tell me how to go "PRO*" I'd appreciate it.

Tommy

* I know practice, practice!

[beenthere]

Per the California Energy code my refrigerant charge test is to ensure 3 things;

1) Airflow above 300cfm over the evaporator coil
2) Subcooling +/- 3F manufacturers spec (7-13 degrees subcooling)
3)Superheat between 3 & 26 degrees (to verify TXV operation)
(PLEASE do not get into the accuracy, thought process, infinite or finite wisdom of this. It's California and it is what it is)

You can't verify that the SC is within 3° of manufacture specs. if you are increasing the head pressure artificially. And if the indoor temps are below the manufacturer's specs for charging.

You can falsify your report to say that it is. But you can not actually verify it.

Call the manufacturer/manufacturers of the brand/brands you are doing this too. And ask them to send you a letter saying that your way is accurate enough to verify the +/- 3°.

They won't do it. And will probably tell you that the indoor must be at the printed min temp, and the outdoor must be at the printed min temp to actually verify it.

[SBKold]

Quick back story as to why it's SO IMPORTANT for me to crack the nut of mild weather refrigerant charge testing. Out here in California our new Energy Code MANDATES refrigerant charge test {RCT} on retrofits ( it even mandates RCT on Furnace Only changeouts when an existing, busted up, old broken down R-whoknowswhat.... is present! The tech is responsible for charging it even if it all leaks out the next day) So we cannot final a permit if we cannot do an RCT. What this means is No Permit=No Close of Escrow No Permit=No Rebates etc etc. I am a HERS Rater who had 1 contractor with 600 jobs that needed to be RCT'd when it got "warmer" outside. Sounds great for me? It wasn't.... and it cost the contractor an extra$150 per job in tech/ inspection fees = $90,000!!! Not to mention the time-suck.

I will take this to the PRO forum but don't know how to get the required 15 points.

My premise is simple; the AC is A MACHINE. It does not inherently "know" or "care" what the outdoor/indoor temp. An AC is required to operate under a variety of situations that preclude Return Air temps BELOW 70F. And while AC is different than Refrigeration... the operating parts and process are "basically" the same.

Per the California Energy code my refrigerant charge test is to ensure 3 things;

1) Airflow above 300cfm over the evaporator coil
2) Subcooling +/- 3F manufacturers spec (7-13 degrees subcooling)
3)Superheat between 3 & 26 degrees (to verify TXV operation)
(PLEASE do not get into the accuracy, thought process, infinite or finite wisdom of this. It's California and it is what it is)

Not a bad program. You can verify that charge is not too far off. Of course we are discussing fine tuning it.

So- my Refrigerant Charge Test needs to verify that enough Liquid hits the TXV
- A subcooling of 10 degrees means that the refrigerant is 10 degrees away from being a vapor (forget about pressures... 10 degrees subcooling is what it is) which makes the TXV very happy. Pure liquid.

-The evaporator load (or how much liquid refrigerant is needed) is determined by the sensor on the suction line as the R410 leaves the Evaporator coil headed back to the compressor as a vapor. This suction line temp "tells" the TXV how much refrigerant is needed and determines the "LOAD" placed on the Evap coil. A superheat of 12 degrees means that the refrigerant is 12 degrees away from being a liquid. This makes the compressor very happy. Pure vapor to compress.

If there was an insufficient "LOAD" would we see a higher or lower amount of superheat? (I'm guessing higher)
Wrong you will see lower superheat. But TXV will be throttling down mind you.

We read this temperature of the Saturated Evaporator (theoretically/hopefully the same temp that the TXV sensor is "seeing") on our low-side gauge. And if I see that the pressure is 112psi (37F degrees) and that my Suction Line dry-bulb is 50.9F we have a superheat of 13.9F. PERFECT. The AC does not KNOW or care that it is 55F outside because I have, in effect placed a load on the Condenser (I partially block the condenser but to prove my point you can buy a low ambient kit that will slow down your fan and increase the head pressure to operate under mild-cold conditions) It is how Commercial AC units work .... a series of fans to control head pressure, because sometimes you have to operate an AC year-round regardless of outdoor temp.

I have done a few hundred RCT's this year and my numbers from last week
are more or less:
Evaporator saturation temperature 37 (112psi)
Condensor saturation temperature 90 (274 psi)
Suction line temperature (Tsuction) 50.9
Liquid Line Temperature (Tliquid) 79.3

Subcooling of 10F Superheat of 12F

This was with an average outdoor temp below 60F and Return db less than 65F

These numbers are EXACTLY in line with 200+ tests done with an out door temp of 70-78F and a Return db of 70-75F


So I am trying to see how the evap coil "sees" or "feels" the difference in Return db if it still is sending 12F superheat (pure vapor) to the compressor?

THANKS! and if anyone can tell me how to go "PRO*" I'd appreciate it.
[COLOR="Red"]You are doing it right now. After 15 posts, submit your EPA card and other info

Tommy

* I know practice, practice!


Thats a lot to take in Tommy!

But to answer a few questions.....

Basically it comes down to the fact that operating the system below design temps...well you just can get a accurate reading that way. By artificially blocking condensor fan, heating Indoor, there are too many variables.

Some systems you check may have great numbers at your cool temps, and then suffer in the warmer weather.

Modern TXVs do an excellent job at controlling the numbers though and you are experiencing that.

[beenthere]

Call the manufacturer of the brand/brands your doing this to. And ask them to send you a letter saying that your method is accurate enough to "verify" that the unit is operating with a SC within 3° of their spec.

To apply for pro membership. You must make 15 first before applying.
Then use this link. http://hvac-talk.com/vbb/showthread.php?threadid=116113

And follow its instructions.

[Tommy Torero]

Call the manufacturer of the brand/brands your doing this to. And ask them to send you a letter saying that your method is accurate enough to "verify" that the unit is operating with a SC within 3° of their spec.

To apply for pro membership. You must make 15 first before applying.
Then use this link. http://hvac-talk.com/vbb/showthread.php?threadid=116113

And follow its instructions.

Hi BeenThere

I am taking my initial specs from the Energy Consortium paper
"Specification of Energy-Efficient Installation and
Maintenance Practices for Residential HVAC Systems"

http://www.cee1.org/resid/rs-ac/reshvacspec.pdf

This was written by the same people who wrote the ACCA standard for High Quality Installations which s referred to by almost every above-standard utility rebate program. I see that this forum constantly refers to ACCA MAnual J as the STANDARD for load calcs.... this is their Gold STandard for installs;
http://www.cee1.org/resid/rs-ac/HVACQIspec.pdf

on page 41 of the "Specification of Energy-Efficient Installation and
Maintenance Practices for Residential HVAC Systems" they have a section called
3.14.3 Simulating Design Temperatures for Superheat and Subcooling Tests.

It would appear that the 40+ HVAC techs, engineers and industry professionals who are acknowledged in the forward of this paper would want to retract something so patently wrong.

I still don't see how my numbers are false or manipulated... subcooling is subcooling and superheat is superheat.... it's either vapor/liquid or it's not. Right? TXV wants liquid. Compressor wants vapor. SC and SH confirm this. Or not??


Thanks Tommy

[beenthere]

That document. Says outdoor ambient at typically 60° or higher. Says nothing about simulating an outdoor temp.

Also says to manufacturers specs. Which manufacturer says to do what you are doing.

It also says you may have to come back to check charge on another day due to ambient conditions.

No where did I see anything that said to do what you are doing.

Perhaps you can point out those paragraphs, sentences. Cause I don't see them.

[DavyB]

Quick back story as to why it's SO IMPORTANT for me to crack the nut of mild weather refrigerant charge testing. Out here in California our new Energy Code MANDATES refrigerant charge test {RCT} on retrofits ( it even mandates RCT on Furnace Only changeouts when an existing, busted up, old broken down R-whoknowswhat.... is present! The tech is responsible for charging it even if it all leaks out the next day) So we cannot final a permit if we cannot do an RCT. What this means is No Permit=No Close of Escrow No Permit=No Rebates etc etc. I am a HERS Rater who had 1 contractor with 600 jobs that needed to be RCT'd when it got "warmer" outside. Sounds great for me? It wasn't.... and it cost the contractor an extra$150 per job in tech/ inspection fees = $90,000!!! Not to mention the time-suck.

I will take this to the PRO forum but don't know how to get the required 15 points.

My premise is simple; the AC is A MACHINE. It does not inherently "know" or "care" what the outdoor/indoor temp. An AC is required to operate under a variety of situations that preclude Return Air temps BELOW 70F. And while AC is different than Refrigeration... the operating parts and process are "basically" the same.

Per the California Energy code my refrigerant charge test is to ensure 3 things;

1) Airflow above 300cfm over the evaporator coil
2) Subcooling +/- 3F manufacturers spec (7-13 degrees subcooling)
3)Superheat between 3 & 26 degrees (to verify TXV operation)
(PLEASE do not get into the accuracy, thought process, infinite or finite wisdom of this. It's California and it is what it is)

So- my Refrigerant Charge Test needs to verify that enough Liquid hits the TXV
- A subcooling of 10 degrees means that the refrigerant is 10 degrees away from being a vapor (forget about pressures... 10 degrees subcooling is what it is) which makes the TXV very happy. Pure liquid.

-The evaporator load (or how much liquid refrigerant is needed) is determined by the sensor on the suction line as the R410 leaves the Evaporator coil headed back to the compressor as a vapor. This suction line temp "tells" the TXV how much refrigerant is needed and determines the "LOAD" placed on the Evap coil. A superheat of 12 degrees means that the refrigerant is 12 degrees away from being a liquid. This makes the compressor very happy. Pure vapor to compress.

If there was an insufficient "LOAD" would we see a higher or lower amount of superheat? (I'm guessing higher)

We read this temperature of the Saturated Evaporator (theoretically/hopefully the same temp that the TXV sensor is "seeing") on our low-side gauge. And if I see that the pressure is 112psi (37F degrees) and that my Suction Line dry-bulb is 50.9F we have a superheat of 13.9F. PERFECT. The AC does not KNOW or care that it is 55F outside because I have, in effect placed a load on the Condenser (I partially block the condenser but to prove my point you can buy a low ambient kit that will slow down your fan and increase the head pressure to operate under mild-cold conditions) It is how Commercial AC units work .... a series of fans to control head pressure, because sometimes you have to operate an AC year-round regardless of outdoor temp.

I have done a few hundred RCT's this year and my numbers from last week
are more or less:
Evaporator saturation temperature 37 (112psi)
Condensor saturation temperature 90 (274 psi)
Suction line temperature (Tsuction) 50.9
Liquid Line Temperature (Tliquid) 79.3

Subcooling of 10F Superheat of 12F

This was with an average outdoor temp below 60F and Return db less than 65F

These numbers are EXACTLY in line with 200+ tests done with an out door temp of 70-78F and a Return db of 70-75F

So I am trying to see how the evap coil "sees" or "feels" the difference in Return db if it still is sending 12F superheat (pure vapor) to the compressor?

THANKS! and if anyone can tell me how to go "PRO*" I'd appreciate it.

Tommy

* I know practice, practice!
I hope you are not failing inspection on unit charges during cold months. You need to give a conditional pass until the weather warms up.

[airtight_tech]

I process paperwork for California title 24 and you can still complete the CFR's for permit purposes even when RCA (refrigerant verification) hasn't been performed. The RCA portion for that particular project stays incomplete on the CalCerts website so you can complete it at a later date. If you really want to have it done now, the correct way to do it in low ambient conditions is to weigh the charge in. Option #3 if iam not mistaken on CalCerts


Or if your real set on using Sh and Sc I suppose you could do it east coast style and use a charging tent and warm the home up to create false load condition for accurate charging

[bmb]

On various brands, that I have installed, most of the charging charts don't apply below 65-70 degrees.

[Tommy Torero]

His super heat and sub cooling are in the normal range.
I do not think he is asking if the pressures are correct, but is the charge correct.
I would say that if he is not starting with an empty system, that his thinking is correct.
If the system were empty to begin with, I would recommend that he weigh it in.....

Thank you CEHS.

Has anyone here ever had to install a HVAC system that was required to maintain a constant temp of BELOW 70F? This would indicate a Return Air db of LESS than 70F on a constant basis.

2 scenarios:
#1) I have a flower shop. It is a brisk 60F day...my Rhododendrons and Daisies prefer a 55F indoor ambient. My Return db sometimes pushes 58F (gasp!) before the thermostat kicks the AC on. I have installed a Low Ambient Kit which does ONE THING; it slows down my condenser fan to increase head pressure so that I can run my AC on cool days. My subcooling is 10F and my superheat is 12 with 400cfm across my evaporator coil ( my compression ratio is stellar and my fan watt draw is spot-on)

#2)I have a flower shop. It is a warm 80F day...my Rhododendrons and Daisies prefer a 55F indoor ambient. My Return db sometimes pushes 58F (gasp!) before the thermostat kicks the AC on. My subcooling is 10F and my superheat is 12 with 400cfm across my evaporator coil ( my compression ratio is stellar and my fan watt draw is spot-on)

My gauges and thermocouples show 2 things;
I am sending Liquid to the TXV.
I am sending Vapor to the compressor.

Isn't this HVAC&R 101? The ambient air outside is ultimately determined by the airflow across the condenser coil.... the condenser fan has one job; dissipate the heat. I can limit the heat transfer by slowing down the fan or using a homemade heat-recovery device (toolbox, tent, garbage bag etc...) It's my contention that head pressure (with confirmed proper airflow) is what matters in a proper refrigerant charge test... NOT ambient air temp. When I limit the fan my MEASURED air temp across the condenser coil is 75-95F.


I feel as though some are stating that it's impossible to verify the charge of a system (unless weigh-in) that will NEVER,EVER see a 70F Return Air db. If that is true then subcooling and superheat would never,ever be done on a whole lot of systems.

Thanks, Tommy

BTW- as I mentioned I am a HERS Rater, I do 300-400 duc/blower door tests a year and EVERY SINGLE DAY I "ARTIFICIALLY" manipulate pressures with Flow Rings so that I can conduct a certifiable test. .

[stvc]

Welcome Tommy, you came to the right place to find the answers and lively discussion you are looking for. But, the site rules are NO do it yourself info. This being the open forum, this discussion is kind of crossing the line. You need 15 posts to apply for pro status, then you can go to the technical forums. You have 5 now, search "please cancel my membership" I'm sure you can get your post count up there!

[Some Dude]

Dealing with and city , county or state is a game.
Do they give you a sheet they want filled out?

Regardless of what the machine should or should not be doing, follow their stupid guidelines and fill out the paperwork, send it in and get paid.

If you want to check your system out as close as possible.
Heat up the space in heating mode.
Turn on the A/C, if your head is low block off part of the condenser with a plastic garbage bag until its right.

Take their stupid readings, send your paperwork in and get paid,game over.

You engineers make things way to hard, remember, theres some hillbilly driving down a dirt road with a pair of gages hanging in the back window of his pickup whoes forgotten more than we will ever know.

[beenthere]

I feel as though some are stating that it's impossible to verify the charge of a system (unless weigh-in) that will NEVER,EVER see a 70F Return Air db. If that is true then subcooling and superheat would never,ever be done on a whole lot of systems.

Sounds like you asked a question that you only wanted to hear an answer of your right to.