carrier won't cool Tech wants to replace TXV

[Shophound]

For those of us techs who have had a TXV fail on us, but our initial reaction was to think the system was low on charge, I will maintain that your initial tech was correct on the TXV failure (as evidenced by the new tech replacing it and you have cooling again). The thing I have to remind myself as a tech is that my first response when I find a system with what appears to be a low charge, don't add any refrigerant until any other factor that can contribute to the same symptoms has been found and eliminated. Just the other day I had a rooftop unit that indeed was low on charge. I started to add gas but stopped myself. Partly because my eyes zoomed right in on where the leak was. I could see it even without breaking out leak detector. Partly because even if the leak had not been that obvious, it was foolish for me to keep adding gas to a system with no other factors causing it to indicate a low charge but a low charge itself, which can only be due to a leak.

Old habits are hard to break. Knee jerk reactions in service aren't always the best.

[hvacvegas]

Glad to hear they've got it working.

I agree with shophound. Sounds like he figured on bad txv, which it was. I've had txv's bad out of the box, and I've had txv's go bad after a week.

Kevin, I'm not arguing, but:
IF you thought there was the possibility of a loss of charge, why would you pump a unit back into itself? Your not worried about sucking moisture in, especially with a possible malfunctioning txv?

[catmanacman]

if a system is properly charged on start up and then the txv fails you have high sub cool and high superheat if he added refrigerant to get the proper sub cool then either the unit was not charged correctly on start up or the refrigerant leaked out.if the tech that replaced the txv followed proper procedures then the system should leak free

[Chuck]

Glad to hear they've got it working.

I agree with shophound. Sounds like he figured on bad txv, which it was. I've had txv's bad out of the box, and I've had txv's go bad after a week.

Kevin, I'm not arguing, but:
IF you thought there was the possibility of a loss of charge, why would you pump a unit back into itself? Your not worried about sucking moisture in, especially with a possible malfunctioning txv?

If you don't let it pump down into a vacuum you can't suck any moisture into it. I usually just get it down to 5 psi and shut it off.

[cuchulain]

In all reality your not suppose to pump a scroll compressor into a vacum state anyway

[dandyme]

A new tech came out. Said first tech had added refrigerant to do diagnostics. He replaced TXV, emptied system and refilled. Said should be good to go. Did not give me new pressure readings though.that doesn't say much for the "tech" Seems to be cooling now. I'll be back if any more problems. Thanks again.

you should ask his boss for those numbers

saying it is charged correctly and not documenting it is the same as a lie in my book

[LKJoel]

In all reality your not suppose to pump a scroll compressor into a vacum state anyway

they say not below 25psi, not enough oil pressure to maintain that thin film between the scrolls, and they start scraping against each other

[hvacvegas]

In all reality your not suppose to pump a scroll compressor into a vacum state anyway

We aren't talking about pumping a scroll into a vacuum. Not in that way.

Chuck, I know what your saying, but my only concern is with the customer. You can't tell me that needle hasn't dropped like a rock on you before. It's done it to me. On something as much of a crapfest of a happy customer as a warranty replacement coil (which btw, we are starting are rounds again....*sigh*), why take the chance? We usually just close it off, recover, repair, pump, recharge (virgin).

[JakRabbit]

TXV, he added refrigerant thinking it was low most likely but the pressures never changed. Since they didn't TXV. just let them change it a TXV is a simple device that can go bad easily.

[hvacrmedic]

if a system is properly charged on start up and then the txv fails you have high sub cool and high superheat if he added refrigerant to get the proper sub cool then either the unit was not charged correctly on start up or the refrigerant leaked out.if the tech that replaced the txv followed proper procedures then the system should leak free

A restriction at or within the metering device will cause a drop in SC.

[Shophound]

A restriction at or within the metering device will cause a drop in SC.

Thank you! I was gonna say the same but you beat me to it.

Hey folks, if you ever hear anyone dissing "all that refrigeration theory crap", tell them to take a long walk off a short pier.

[hvacrmedic]

Thank you! I was gonna say the same but you beat me to it.

Hey folks, if you ever hear anyone dissing "all that refrigeration theory crap", tell them to take a long walk off a short pier.

The problem is that some of the theory presented is wrong. We hear things like "SH and SC tell you where the refrigerant is in the system.". Well that's a nice thought. It is however a misguided thought. With a complete restriction at the MD all of the refrigerant gets trapped in the condenser and LL. Nevertheless the SC will stabilize at exactly 0.

[Shophound]

The problem is that some of the theory presented is wrong. We hear things like "SH and SC tell you where the refrigerant is in the system.". Well that's a nice thought. It is however a misguided thought. With a complete restriction at the MD all of the refrigerant gets trapped in the condenser and LL. Nevertheless the SC will stabilize at exactly 0.

Yes, my statement was based on the assumption that correct refrigeration theory was being taught. Regardless the point you raise is a good one. I would rather say "Superheat and subcooling tell you what is going on in the evaporator and condenser" vs. "SH and SC tell you where the refrigerant is in the system". The refrigerant is everywhere in the system! It's undergoing changes throughout the entire system! It's up to the tech to know what these changes signify, and whether such changes are normal or not.

With a restricted metering device, there is little heat load picked up in the evaporator entering the condenser to be rejected. Head pressure falls due to lack of heat load. Pressure and temperature are relative; if the temperature via heat load is reduced, so will the pressure be reduced. And with that the saturated condensing temp approach to outdoor ambient temp (the basis for subcooling). Liquid stacks in the condenser, reducing the amount of coil that can actually condense superheated vapor from the compressor back to a liquid. If the vapor is able to condense at temperatures near outdoor ambient temp, there won't be much subcooling.

[aircooled53]

I really appreciate all the replies. A new tech came out. Said first tech had added refrigerant to do diagnostics. He replaced HVX, emptied system and refilled. Said should be good to go. Did not give me new pressure readings though. Seems to be cooling now. I'll be back if any more problems. Thanks again.

I would ask what the pressure where /temps and T.D. after charge. You are owner of system..Required info..

[jpsmith1cm]

Skilowyo,


Please start a new thread if you need help.

[openclasspro]

What was the ambient temperature? He didn't calculate superheat, which would have been really nice to know. Shooting from the hip, yes you have a leak and a 7* split (with missing information) could be a TXV, but I'd need more info to be exact.

He should have noted superheat,on a fixed txv,mfgs wants 10-11 degrees subcooling,the superheat is what it is at that designed subcooling number.