Charging Techniques on a rooftop packaged unit

[lancaster619]

Hi There,
I am relatively knew to the HVAC field and have worked mainly on splits, I am now a mechanic in a factory and in charge of about 3 rooftop units, and have not delt to much with them, I am just seeing if any one has good accurate techniques on finding my charge, and charging packaged units with multiple refrigeration circuits. Currently I am troube shooting packaged unit with three refrigeration circuits. Two compressors are on one od fan, and the third is its on OD fan. I noticed that my OD fan motor for the third was not coming on, after verifying the motor was good and everythin back to the controls was in check I noticed what looks to be like " a fan control switch for head pressure" I put my gauges on and determined the unit was way low on charge thus never reaching the appropiate head pressure to bring on the fan. The site Glass is only half full and a steady stream of bubbles are flowing through it. A. Have a diagnosed my charge correcty, I used subcooling and super heat, and the fact that there are bubbles in the sight glass i am pretty sure is a dead give away, but I am young in the trade. and B. if so what is the proper procedure for adding R22. THank you so much i am sorry for my greeness, I just figured i would let experience guide me

[jpsmith1cm]

Maybe if you have a model and serial number, we can help you find the manufacturer's data.

[basshound71]

weigh it in

[maxster]

main thing on checking a charg eon a multi faned condensing section is get alll the fans running and guages on and see where the charge is.if you don't you willl be chasing the charge all over the unit.on a dead hot day in the summer all the fans should be running mornings maybe 1 to 2 so keep that in mind.check the simple stuff first belts filter coils and stay away from that TXV if it has one.keep and eye on that compressor amperage and mostly charge just GAS not liquid:gah:...

[stevrock]

I thought all rooftop units had charging charts right on the side of them?

Sight glasses can be misleading. For example, a clear sg on a 410a system will give you about 20* sh, aka way overcharged.

[mizzouhvac]

As mentioned before if you get the model and serial you can look up the information from the manufacture online and see their recommended procedures for troubleshooting a system low on charge. Since you are fairly new to the field I would recommend you study up on the unit first to familiarize yourself with how it works. And I agree, a sight glass with bubbles doesn't necessarily mean a undercharged/or full charge. The best and easiest way for you maybe to recover the ref and weigh in the correct charge. Remember alot of variables go into place charging a system, airflow,etc be sure to verify all is correct. Best of luck.

[david chamberla]

best place to start model # by that number it will tell the what typ of refrigerant some times its better to weigh in ,but also if you have manufactors specks you will know if they wont you to use subcool for txv or supperheat for charging ,,ihave been on jobs were its better to weigh it out found out it was grossly over charged ,than weigh it in and find the real problem . R.S.ES SAY BETTER KNOWLEDGE THROUGH EDUCATION keep asking questions all the time .

[lancaster619]

Thank you fellows, for your help, I think I am going to do what the one gent recomended, Fully recover the charge in that circuit and just add the 9.6 factory charge, This unit is fairly old and I have no idea what was done before me, or after me, One gentlemen stressed to charge only as a gas, if i may ask, why, I have charged units slowly jogging in liquid before but that was only on splits, not packaged. I am sorry to ask so many questions, but I am really anxious to learn and do my job right, Thanks guys

[HVAC-matt]

Thank you fellows, for your help, I think I am going to do what the one gent recomended, Fully recover the charge in that circuit and just add the 9.6 factory charge, This unit is fairly old and I have no idea what was done before me, or after me, One gentlemen stressed to charge only as a gas, if i may ask, why, I have charged units slowly jogging in liquid before but that was only on splits, not packaged. I am sorry to ask so many questions, but I am really anxious to learn and do my job right, Thanks guys

I would have recommended fully recovering the gas and then weighting it in as long as it wasn't a huge circuit. You can charge gas but it will take a long time. We usually liquid charge on the low side and then wait to start the unit so that all the liquid that may have gotten into the compressor can boil off. Just my :.02: good luck.

[ruuufuhbock]

I thought all rooftop units had charging charts right on the side of them?

Sight glasses can be misleading. For example, a clear sg on a 410a system will give you about 20* sh, aka way overcharged.

Could be misleading but 20* superheat does not sound too high. Your proper superheat changes with a lot of factors id wb, od db, etc.. . Also only charge by superheat if its fixed metering device( cap tube, piston, etc) If its txv make sure to get the charge right by taking subcooling. The unit should have design subcooling chart on it if not they make charts. As far as charging by gas only, I remember from the 410A class that if you charge by gas and not liquid you can fractionate and ruin the bottle because its a blend. So they recommend metering liquid through the suction side. There are ways to charge liquid into the liquid line, but not sure if we can get into that in this part of forum.
:.02:

[frideman1]

find your leak and then recover repair leak test . if leak free. then charge by weight factory charge, done

[stevrock]

Could be misleading but 20* superheat does not sound too high. Your proper superheat changes with a lot of factors id wb, od db, etc.. . Also only charge by superheat if its fixed metering device( cap tube, piston, etc) If its txv make sure to get the charge right by taking subcooling. The unit should have design subcooling chart on it if not they make charts. As far as charging by gas only, I remember from the 410A class that if you charge by gas and not liquid you can fractionate and ruin the bottle because its a blend. So they recommend metering liquid through the suction side. There are ways to charge liquid into the liquid line, but not sure if we can get into that in this part of forum.
:.02:

I meant sc, not sh. Sorry for the confusion.

[jingoism]

I had to do quite a few very old package units at my last job, and pulling the charge and weighing back in really is the best way. Just don't be like my dumb self... I found a massive leak on the suction line and said "AH HA!". Cut the offending section out and replaced with some copper... brazed her in and... gah still not holding!

So I cut it out again thinking I screwed up the braze somehow, and put in a new piece. DANGIT STILL LEAKING! :gah:

I am at wits end thinking I suddenly forgot how to braze, and tell my partner ( who has 0 hvac experience ) to look at it. Long story short... I missed the two holes the case had rubbed through the U bends on the opposite side. So make sure you find all the leaks. :lol:

[hvacpope]

I charge by subcooling, jump the fan cycle/outdoor temp swt. to get all condenser fan motors going. disable unloaders, hgbp etc to get the unit loaded 100% then add refrigerant till 5 degrees of subcooling are reached. depending on the fan cycle settings the subcool would go up to 20+ degrees when the fans are not running but drop fast when all fan are going, my goal is to ensure a solid liquid column to the txv all the time.

[hvac526]

explain to me how unloaders work

[Moparmyway]

They usually energize unloaded, and de-energized is loaded

[hvacpope]

explain to me how unloaders work

the most common types of unloaders are mechanical and electrical, both types are configure to sense and maintain suction pressure and as it gets bellow setpoint the unloader unloads one or more cylinders by keeping the compressor valve open recirculating the same refrigerant back and forth when pressure builds up the system goes back to normal (loaded):hijacked:

[Timmy414B]

def best idea. remove, pull vac, weigh in factory.

[LT1 FUN]

I thought all rooftop units had charging charts right on the side of them?

Sight glasses can be misleading. For example, a clear sg on a 410a system will give you about 20* sh, aka way overcharged.

yeah 410a likes 30-40* superheat, about double what r-22 has

[hvacpope]

yeah 410a likes 30-40* superheat, about double what r-22 has

Very interesting! I never hear of such thing, superheat is a necessary evil why would it be different with 410a???