a/c precharge question

[stiz]

I am wondering about something. From what I am understanding, when you buy a new central air system to add onto the furnace, the condenser unit comes packed with a precharge of refrigerant. Now correct me if I am wrong-there is enough refrigerant in this precharge for the condenser, the evaporator , and a set amount of line set lenght, which the manufacturer will specify ,but is usually around 25'. One thing that is a little confusing to me is how can you be sure there is enough in the precharge for the evaporator too? don't they come in different sizes? I am wondering mostly about the Goodman brand as to how accurate the precharge is? And typically how much total refrigerant would a 3 1/2 ton system hold? and then if the installer has to add more ( and the line length is the same as what the manufacturer has allowed for) what are typical amounts needed to top off the system?  stiz   

[TECH X]

The outdoor unit is what dictates the tonnage.  In many cases I have to mismatch my coils to exceed the energy star rating and qualify for rebates.  Often times I am installing a 2 ton condenser with a 2.5 ton evaporator to achieve a higher SEER.  I refer to the engineering data from my manufacturer for proper coil matchups.  Our provincial government is very specific before handing out rebates.  Each coil matchup + furnace matchup come with an ARI reference number.

Most systems these days require a TXV valve installed.  In this case you must verify your charge by calculating your subcooling, and use the manufacturer's data to ensure the proper charge.

I usually add approximately 0.6 ounces per additional foot of line set.  For example if my unit allowed for 15' of line set, and my line set was actually 50', then I would add about 1 pound 5 ounces of refrigerant.

It depends on the manufacturer and the type of refrigerant, R410A or R22, but you would find anywhere from 3 to 8 pounds of refrigerant in a residential A/C.  The unit precharge is found on the condenser rating plate.

[stiz]

The unit I am looking is a cased coil and is equipped with a piston (check style flowrator). Now from what you have said in your post is it advisable to remove this piston and install a txv instead? A txv kit is offered as an accessory. The specs on the condenser say it has 180 refrigerant charge, then that would be 180oz.'s of charge or 11.25 lbs. correct?

[TECH X]

If you use the piston type then calculate superheat to verify the proper charge.  I have seen many Goodman units that come with more refrigerant than what's stated on the rating plate.  Yup 180 ounces = 11.25 pounds. 

I think you need to use a TXV for 14 SEER and above systems.

[HVACJOE]

 I agree, listen to Teck, it is always the most benficial for you to use a tx valve for your best energy savings. An orifice is always supplied by the manufacturer due to cost but it is a very ineffective way to meter your refrigerant through the evaporator. Using the proper charge and the suggested added rate per foot by teck-x and a tx valve you will have the best efficiency.

[ProTemp]

When charging a txv system i found the best and most accurate way is to use the subcooling methode with an amprobe reading on the compressor windings as your adding freon. Using ounces per foot to me is  the most inaccurate way but more of a ball park figure. If your charging a system and your RLA from your rating plate is close to the reading on your ampbrobe and if your subcooling is 0 to 1....watch out, your overcharged! Use the outdoor amb + 30 deg F rule to estimate your pressures and then verify with your gages. If the pressures are way higher on your gages then your outdoor amb rule ...your overcharged again....lol. The superheat methode is useless when a TXV is being used. So try using this methode for R-22 and R-410A. After awile your not even going to use the factory charts anymore once you get the hang of it.  Here is the formula ex:  amb 75 deg F + 30 deg F = 105 deg F wich makes 200 psig on your gages for R-22. At this point your charge is pretty good. Now check to see if your subcooling is in th 10 to 15 deg F range. If so your charge is good. Then check the amp draw and if your 30% to 40% less than the RLA of the compressor, your charge is good. Don't attempt a super heat reading cause its a TXV system. Once again this info was not posted from some HVAC site but true 100% field experience.