Checking the Charge

[MKC]

Hi,

I am new to this site and think it would be a great place to learn and share knowledge with each other so here it goes:

I was taught that I could check the charge in a residential AC system as follows:

For a TXV system - 1. check the outdoor ambient air temp going across the condenser coil. 2. check the wet bulb temp of the air flowing into the "A" coil. 3. Cross the two figures on a chart that tells you what the sub cooling should be and adjust the charge to obtain the subcooling as prescibed on the chart.

An example would be 85 degree dry bulb at the condenser, 67 degrees wet bulb at evap inlet crosses over to 18 degrees of subcooling.

Is this accurate? If so, where do I check the subcooling - at the inlet to the TXV?


For a restricted orifice system: 1. Same procedure but use superheat vice subcooling.

I welcome your comments on this procedure and other ways that you guys check the charge. I was told this was a very accurate method and that being correctly charged is critical to the longevity of the compressor.

MKC

[jeff520]

The first thing to check is that the airflow thru the A coil is correct for the size of the system.

Think about it - superheat will be low if the airflow is low because there would not be enough heat in the limited amount of air, and if there is too much air (something you may not see too often!) then there may be enough heat available to enable excessive superheat. This will effect the amount of liquid freon in the A coil.

If your decision the add or remove charge is based only on the subcooling on the condenser side (which indicates the amount of liquid in the condenser coil) then you are going to be adjusting the total charge with no idea of how much charge is in the A coil.

Adding/removing charge sure impresses homeowners with the technicality of your expertise, but you should never need to do it on an installation that has been working for a season and does not have a leak.

It is not quite so simple as just a couple of temperature measurements and a chart, and I do not see adjusting the charge as being any part of a preventive maintenance call. Check the efficiency, yes, and compare it to last years number or the published figures for the equipment, but you better have a really good reason to convince this homeowner before you pull that can of juice off the truck.....

[beenthere]

Your method is incorrect, and would end up over charging many systems.
Within reason, the SC on a TXV system doesn't vary with the ID wetbulb.
Older systems generally required more SC then newer 13 SEER units.
Some new systems can have SC down around 8*, and its not varied by the ID WB.

SH is determined by the ID WB, and OD DB.

After you get 15 post, apply for pro membership, then you can ask these thype of questions in the pro forum, and get the whole answer.

[macdaddy]

When the suction line is six-pack cold, grab another beer:cool:

[hvacrmedic]

*

[hvacrmedic]

Hi,

I am new to this site and think it would be a great place to learn and share knowledge with each other so here it goes:

I was taught that I could check the charge in a residential AC system as follows:

For a TXV system - 1. check the outdoor ambient air temp going across the condenser coil. 2. check the wet bulb temp of the air flowing into the "A" coil. 3. Cross the two figures on a chart that tells you what the sub cooling should be and adjust the charge to obtain the subcooling as prescibed on the chart.

An example would be 85 degree dry bulb at the condenser, 67 degrees wet bulb at evap inlet crosses over to 18 degrees of subcooling.



This is an accurate "superheat" calculation, but not for a TXV system, nor for subcooling. It's for superheat and a fixed restrictor (piston or cap tube) system.

There is no formula for subcooling. Set it per the manufacturer's specs. It's regarded as remaining constant once it's set.

[beenthere]

No, it doesn't work for higher OD temps, with low ID wet bulbs.
Take ARI design conditions. That method would give you a target SH of 18*.
SH slide chart gives 13*.

[hvacrmedic]

No, it doesn't work for higher OD temps, with low ID wet bulbs.
Take ARI design conditions. That method would give you a target SH of 18*.
SH slide chart gives 13*.

85º OAT
67º IWB

Target superheat 17º (Carrier/Bryant) Tolerance +/- 5º

http://blueguitar.org/new/misc/hvac/bdp_sh.pdf


Target Superheat (3*iwb - 80 - OAT)/2 = 18º (hvacrmedic)

What method are you referring to that doesn't work? Also, when are you going to have low indoor wet bulbs and high ambients with a unit that you're adjusting the charge on? And when is the formula going to give a value that falls outside of the Carrier chart's tolerance?

I don't know where your 13º came from.

[beenthere]

Subtracting ID WB from the OD DB is not acturate.

85* OD temp, is not ARI design.
Recheck SH for ID WB 67*, and OD DB 95*.
See what you get.

[hvacrmedic]

Subtracting ID WB from the OD DB is not acturate.

85* OD temp, is not ARI design.
Recheck SH for ID WB 67*, and OD DB 95*.
See what you get.

Target Superheat = (3*iwb - 80 - OAT)/2 = 13º (hvacrmedic)
_________

Carrier/Bryant

http://blueguitar.org/new/misc/hvac/bdp_sh.pdf

12º

Now I don't know where you got the 18º from!! :)

[beenthere]

The 18, was a finger slip, it should have been 28.

All the OP was doing was subtracting WB from DB, so 95-67=28

[hvacrmedic]

The 18, was a finger slip, it should have been 28.

All the OP was doing was subtracting WB from DB, so 95-67=28

He was using a superheat chart and trying to apply it to subcooling.

"Cross the two figures on a chart that tells you what the sub cooling should be. An example would be 85 degree dry bulb at the condenser, 67 degrees wet bulb at evap inlet crosses over to 18 degrees of subcooling."

If you replace the word subcooling with the word superheat, then his statement would be correct. The difference does happen to be 18 as well, but that's sheer coincidence.

[beenthere]

Well, I'll wait for the OP to say thats what he meant.

[MKC]

Wow! Thanks for all the replies but now I am totally confused. I was told this was a rock solid method for charging or checking the charge in a system. Question 1. Please clarify for me that I do, or do not, use superheat for charging a fixed orifice metering device system. Question 2. Should I, or should I not, use subcooling for charging a TXV system.

It was a coincidence that the numbers I used as my example was mathematically correct (85-67=18). The rest of the chart does not match up like that.

If you give me examples or formulas on how you charge a system, please explain the formula to me and others that may not understand it.

Question 3. When I need to check superheat, do I check it right at the tail coil or back at the compressor? When I check sub-cooling, do I check it at the outlet of the condenser or just before the metering device?

Again, thanks!

[BigJon3475]

You have it right on which to use for what type of system....piston type metering use superheat......TEV use subcooling...You'll learn to use both on all systems though once you realize why taking these measurements matters.

Superheat at the evap is useful superheat....superheat at the condenser is total superheat.....useful is good for knowing what the evap does....total is good for knowing how the lineset effects your reading and if there is something wrong with it.....same idea for subcooling.....taking it at the condenser tells you what the condenser did and taking it at the evap tells what's going on before the metering device.....though not as useful for subcooling because the temp will obviously drop more once it leaves the condenser.....unless it's running through an area that is hotter than the line.

Get 15 post and join the pro forum where we can discuss in detail.....I probably already said more than I should....If so I may get edited....in which case join the pro forum to learn more....




Edit: See Shophounds post.....

[Shophound]

Wow! Thanks for all the replies but now I am totally confused. I was told this was a rock solid method for charging or checking the charge in a system. Question 1. Please clarify for me that I do, or do not, use superheat for charging a fixed orifice metering device system. Question 2. Should I, or should I not, use subcooling for charging a TXV system.

It was a coincidence that the numbers I used as my example was mathematically correct (85-67=18). The rest of the chart does not match up like that.

If you give me examples or formulas on how you charge a system, please explain the formula to me and others that may not understand it.

Question 3. When I need to check superheat, do I check it right at the tail coil or back at the compressor? When I check sub-cooling, do I check it at the outlet of the condenser or just before the metering device?

Again, thanks!

Could you state for us, in your words, what you understand subcooling and superheat to be, and their affect on the refrigeration cycle?

Reason I ask: If you know what something is, you'll then know why you are measuring it.

[model m-man]

You're on the right track. Go to General & up your post count as has been mentioned. What can be posted in this forum is limited.

[MKC]

Could you state for us, in your words, what you understand subcooling and superheat to be, and their affect on the refrigeration cycle?

Reason I ask: If you know what something is, you'll then know why you are measuring it.

Sure, superheat is tailcoil temperature minus suction pressure converted to temperature, ie: suction pressure of an R22 system is 70 degrees and the refrigerant is changing state at 41 degrees. If the tail coil temp is 61 degrees, I have 20 degrees of superheat. Too much superheat means I am starving the evap.

Subcooling is the difference in temp between the condensing temp and the temp of the liquid refrigerant entering the metering device. Subcooling ensures nothing but liquid entering the metternig device. Subcooling is high side pressure converted to temp minus liquid entering M/D. Not enough subcooling and I have too much flash gas which makes the system less efficient.

MKC

[beenthere]

If you have a SH chatging chart.
On the back of it, it should have a chart that gives you rough SC.
Rough because length on line set can effect how much SC you need to have a solid head of liquid at the TXV.

[Shophound]

Sure, superheat is tailcoil temperature minus suction pressure converted to temperature, ie: suction pressure of an R22 system is 70 degrees and the refrigerant is changing state at 41 degrees. If the tail coil temp is 61 degrees, I have 20 degrees of superheat. Too much superheat means I am starving the evap.

Subcooling is the difference in temp between the condensing temp and the temp of the liquid refrigerant entering the metering device. Subcooling ensures nothing but liquid entering the metternig device. Subcooling is high side pressure converted to temp minus liquid entering M/D. Not enough subcooling and I have too much flash gas which makes the system less efficient.

MKC

Those are good answers.

A note regarding superheat: your superheat can read high with a system that is optimally charged and metering properly. The reason is due to high heat load imposed on the evaporator.

The amount of subcooling not only affects whether the metering device has a solid column of liquid, but also the efficiency of the evaporator. Dirty condensers hurt subcooling, which in turn hurts evaporator refrigerating efficiency.

Superheat and subcooling temperatures, on residential split systems, are typically measured at the suction inlet and liquid line discharge, respectively, at the condensing unit.

One more note: TXV systems are charged via monitoring subcooling. TXV performance and troubleshooting is had by checking superheat at the evap outlet, and verifying adequate subcooling.