Bestway to determine refrigerant levels accurately in residential A/C systems

[54regcab]

The most accurate way is weighing it in.

The second most accurate is manufacture charging chart. But keep in mind that manufacture charging chart assumes the airflow is correct, coils are clean, filters are clean, lineset and ductwork are sized properly. Here's one that trane uses for charging a heat pump in heat mode.

Attachment 367401

+1, manufacturers charts are excellent.

[SouthTex]

Don't forget checking the temp drop accross the indoor coil, I have seen units that have the correct subcooling / superheat but still have a low temp drop.

[quiet mike]

If your new in the field, I would like to suggest to you that you not get to hung up on trying to get all the numbers exactly like you think they should be or that your teacher said they should be. When you are working on the job, especially when the clock is ticking, it's easy to get rushed and add too much refrigerant or not let a system run enough to get an accurate read on it.

I personally look at a few things assuming that the other basic criteria has been met, filters and coils clean ect. I use Testo digital guages so this is fairly quick to do. First obviously is charge taking into considerations the minimums and outdoor ambient. I always check the delta T and make sure I have the drop I'm looking for. If I can't ever get the superheat and subcool where I want it despite everything, at minimum I try to get the good delta T if possible.

[penderway]

Thanks for the reply. But, doesnt weighing it in ensure a proper charge also ?
Im just assuming that if you go to a service call and you suspect an incorrect charge, cant you recover all the freon and then weigh it- and then add / subtract freon to get to the design amount ?
This may not be the most efficient way to do it but it seems like the most accurate way to charge a system with the correct amount of refrigerant.
I see guys on youtube adding freon into a system and just feeling the suction line ( with their hand )to determine if they added enough refrigerant. And thats something the school instructors warned us about..... that is the absolute wrong way to do it.

Weighing it in works perfectly if system delivers rated airflow with matched equipment. Other than that its only a guesstimate

[motoguy128]

The contractor I use at my work only weighs in new system and replacement comrpessors. They check pressures when needed on PM's if there seems to be a performance issue.

THe residential contractor i use on my home also weighs in, but will also verify superheat (will return later on a hot day if needed).

Just a note, the 2nd contractor on a new install also has found units all too often, not charged correctly from the factory (maybe units made on Mondays and Fridays ) , so they always completely recover the unit and weight it back in and and adjust the required amount for the lineset length.


Also as mentioned, airflow is crictical. On resdintal splits with communicating equipment you'll probably have to use a test mode to get nominal airflow, since some system may run at reduced airflow much of hte time. You also have to force it to high stage as well if 2 stage equipment.

[SBKold]

Some manufactures spec out charging in low stage

[Dowdhvac]

[QUOTE=jpsmith1cm;15440951]There are two questions being asked here with three possible answers.

#1. Are we trying to determine exactly how much refrigerant is contained in the system? This could really only be done accurately by recovering and weighing in the charge.

#2. Or are we trying to determine if the system is properly charged? The answer here is far more complex...

Before we can determine if a system is properly charged, we've got to establish a few things.
Coils clean?
Filter clean?
Airflow across evaporator correct?
Airflow across condenser correct?
What type of metering device does the system have?
Is it a matched system?

Now, we'll measure superheat and subcooling, compare them to either the manufacturer's required numbers or to a field computed value absent manufacturer's data and adjust the charge as required.[/QUOTE

X2 !!!!

[Barrettservices]

The most accurate way is weighing it in.

The second most accurate is manufacture charging chart. But keep in mind that manufacture charging chart assumes the airflow is correct, coils are clean, filters are clean, lineset and ductwork are sized properly. Here's one that trane uses for charging a heat pump in heat mode.

Attachment 367401

I like the manuf. charts as well but they are not for all scenarios. I do a lot of rheem and their chart is just like tranes above but if you look closer it says your db inside temp needs to be 80 degrees for the chart to line up and db for h/p inside needs to be 70 degrees. I have yet to do a job that the house is right at those temps so i get my charge close to their chart and if my subcool is in line with what they recommend i leave it.

[atomdude]

Hello Guys,

Im new on here and this is my first post. Im looking forward to learning much from you guys.
Im new in the HVAC field, just about finished with school.

My question :

What is the most reliable way to find out how much refrigerant is in a residential system ?
I would think recovering the refrigerant and weighing it will obviously be the most accurate. Then you know how short or overcharged the system is, right ?
My instructor said the only way it should be done is by taking superheat readings
Can you please advise ?
Thanks,
Bob

you do not need to know exactly how much ref is in a system. you must make sure the system is working properly. if it is not then you may need to add ref. in that case you weigh it in so can charge the customer accurately.
to add carge you use super heat for fixed metering devices and subcool for txv systems.
nowdeterming if it is low on charge is a whole other story so if you want to address that issue let me know.
p.s. the panel plate will tell you approximately how much ref is in the system in ounces for a system with a 15 foot line set. if the line set is longer the installer most likely added ref.