Basic troubleshooting question - your opinions, please!

[747ken]

Hi, I'm learning HVAC technology and need help with a basic troubleshooting question.

I'm working on a TXV, 7 ton unit that uses 17.5 lbs of R-22.
It was reported that during the day, with temps in the high 90s, the unit produced hot air. I wasn't around when it failed, but the diagonstic software said the HPS opened at the time of failure. Now that the outside temp has cooled, the unit works ok. I suspect the unit may be overcharged with refrigerant. My question: how can I tell if it is overcharged? I believe checking the superheat would be the first step, right? Also, the unit has a receiver with a sightglass and float-ball. What will the position of the float-ball tell me about the amount of charge?

BTW, I suspect overcharge due to the way some techs recharge the units. If there is flashing in the liquid line sightglass, they will add "a few squirts of refrigerant" until the bubbles dissapear. From what I've read, this method usually results in an overcharged condition.

Thanks for your advice!

[coolwhip]

Did somebody reset the high pressure switch? Could be a bad cond. fan motor. It might run for a while and then slow down or stop. Diry cond coil could also be a culprit.

[747ken]

This HPS doesn't have a manual reset - its contacts close when pressure drops to a normal level. I've done a visual inspection on the unit - all coils clean, filters fairly clean. The condensor fans seem ok - no noise or burning odor. But the condensor fans could be the problem, as they are controlled by the computer.

Thanks for your help!

[maintenanceguy]

With a system that has a txv, superheat is constantly adjusted by the txv and should stay the same under all conditions. So checking superheat won't work. With this type of system you need to check subcooling which is exactly the same procedure except you switch high pressure side for suction side. For a really good explination and subcooling charts check out the reference section here.

There are lots of reasons for high pressure. Overcharge one but not the most likely. Dirty outside coil, bad condenser fan, or anything that prevents proper heat exchange through the outside coil.

[air1]

If you are tripping on high pressure there are several possiable causes. Could be overcharge, lack of air flow across condenser coil or faulty pressure switch.
Lack of air flow is the most likely cause. Even coils that look clean can be pluged up. If you live in an area with cotton wood the fibers will plug the coil. The next thing to check is the fan motor. Is the motor the correct one, correct rotation, fan blade installed correct relative to the shroud?
Often the motor will run fine untill it runs long enough for the bearings to heat up and seize.
The best way to check for charge is sub-cooling, super-heat and pressures. If you suspect that others have over charged the system you may have to remove the charge and weigh the charge in. Many times inexperianced people will mis diagnose low air flow or bad txv with under charge and add refrigerant.
If you suspect a faulty pressure switch you can disconnect the cond. fan and watch the pressures to see if switch cuts out too soon. This is a extream test and should be done with caution.
Some units use the receiver sight glass with float ball to check the charge. You will need the manufacture instructions to see under what conditions you would see liquid in the sight glass.

[Edited by air1 on 07-14-2004 at 04:07 PM]

[Green Mountain]

Originally posted by 747ken
This HPS doesn't have a manual reset - its contacts close when pressure drops to a normal level. I've done a visual inspection on the unit - all coils clean, filters fairly clean. The condensor fans seem ok - no noise or burning odor. But the condensor fans could be the problem, as they are controlled by the computer.

Thanks for your help!

There are a couple of things that give me a red flag in this quote.
"I visually inspected coil" I nailed a few of my techs on this same statement. Blow that coil out with nitrogen. Or better yet wash it with high pressure water. You are at design conditions.(90 degs) The manufacturers aren't going to give you a bigger condenser just to be nice guys.
When it is 90 deg you need 100% of that condenser. Visual inspections don't make it. Clean it.

Second, " The condenser fans are controlled by 'the computer'" What computer??? Condenser fans should be controlled by outdoor temperature. But the best way is by discharge refrigerant pressure. At 90 degrees the condenser fans should be running 100%


Next study the history of the machine. Tell old uncle benncool what it is and I can tell you more stuff to check.

[len]

uncle beenie hit it right on the head,CLEAN IT FIRST,clean it juice it adjust it,FORGET ABOUT IT,but seriously,you must clean them babys first,ive got guys that will go into a store cooler not cooling,i get there all it need is good doucheing,

[Diceman]

Buy a set of gages too, they come in handy once in awhile.

[glgto]

ok, yes when in doubt clean it. i think a few other things need to be known here if its a r-22 system it should hold around 24 lb of refrigerant . superheat? what is the subcooling! did you check for noncondensables? are the fan blades in the proper position are the motors slipping?are the moters the ones that belong in this unit? etc.

[rob10]

Benn hit every nail on the head. A good washing never hurts them.

[R12rules]

All of the above. More or less....


If you got a bad fan motor, IF ... then you need to catch it in the act of failure.

Without the history available to us, we are handicapped in advising you.

[twosticks]

The only clean coil is the one you just cleaned.

[Shophound]

Did you check the shafts on the condenser motors for end play?

Agree with others. Visual coil check...BZZZTTT!!...wrong answer, Charlie!