[TechmanTerry]

What do you have for head press control?

 

[pmck94]

What do you have for head press control?

Sorry for not knowing but what is that? Example?

 

[Snapperhead]

I once almost got fooled by a YORK , same pressures as you have , and I found the unit had a 2 stage compressor ( never seen one before) and the board had it stuck in 1st stage. I bridged the board 1st and 2nd staqe , fired it up and pressures were fine. I have no idea how the compressor can have 2 stages , but it did

 

[CircusEnvy]

Okay, so with what you guys are saying is making sense. The more I think about the clearer it becomes. But I am still lost. I feel like I need a refresher course with refrigeration..

It comes with practice, you'll get it eventually. Let's start with a normally operating system with a TXV. What do you expect to see? 10 degrees SH and 10 degrees SC (assuming SC target is 10 degrees). What does that mean? It means that the TXV is throttling the right amount of refrigerant into the evaporator coil to absorb the right amount of heat from air passing over it. If we increase the amount of heat for that evaporator to absorb, our SH will climb, so the TXV will open more to compensate. If we decrease the amount of heat for the evaporator to absorb, our SH will drop, so the TXV will close more to compensate.

What if we were undercharged? Let's say we have 10 degrees SH and 5 degrees SC. Even though we lost refrigerant, the TXV can maintain normal SH because there's still a column of liquid behind it, but if we add more heat to the evaporator, the TXV is going to open more and there might be a point where it's fully open and can't maintain SH because there's simply not enough refrigerant to meet demand. Our head pressure/saturation temperature will driven be closer to the LL temperature.

What if we were overcharged? Let's say we have 10 degrees SH and 18 degrees SC. Again, the TXV maintains normal SH despite the heavy amount of refrigerant behind it. But if we continue to add refrigerant, it will keep stacking up behind the TXV, so our SC increases more and more. Our head pressure/saturation temperature will be driven further away from the LL temperature.

What if we have a restriction? It depends on where the restriction is, but it's most often at the metering device. So let's say we're seeing 35 degrees SH and 15 degrees SC. This tells us the TXV is closing down when it shouldn't because there's a good column of liquid behind the TXV, more than normal, but it's not maintaining 10 degrees SH.

What if we have a restriction and we're undercharged? We know in an undercharged scenario, our SH will be high, but our SC will be low and most often 0. So if we see our SH is high and SC is slightly low, that gives a good indication that not only is the TXV not doing it's job, but there's not enough refrigerant in the system either.

 

[TechmanTerry]

On a PERFECT system, what is the PERFECT head press on a 72*day? 72* ambient is typically the lowest ambient in which AC is used, as in OPEN THE WINDOW. What is the minimum head press that a Carrier MotorMaster(head press control device) is set for? It's 95* SCT! What is the minimum head press that the ORIGINAL Alco HeadMaster maintained for r12,r22,r502? Its right around 95* SCT. What is a cond fan cycling control set for ? Its right around 95* SCT +- a little. On those "charging charts" that many units have, what is the head on a 72* day?

Something like that!

 

[kamersoutdoor]

68 ambient with a 64 degree liquid line is what he posted. The liquid line can not be cooler than ambient with a dry coil. He's flashing liquid before the service valve.

I see!!
Original post would indicate restriction before he took temp. OP changed ODT to 60, but you were correct!

 

[pmck94]

On a PERFECT system, what is the PERFECT head press on a 72*day? 72* ambient is typically the lowest ambient in which AC is used, as in OPEN THE WINDOW. What is the minimum head press that a Carrier MotorMaster(head press control device) is set for? It's 95* SCT! What is the minimum head press that the ORIGINAL Alco HeadMaster maintained for r12,r22,r502? Its right around 95* SCT. What is a cond fan cycling control set for ? Its right around 95* SCT +- a little. On those "charging charts" that many units have, what is the head on a 72* day?

Something like that!

I have a chart on this phone from a unit and it tells me that with a 65 degree liquid line temp with 10 degrees design subcooling the pressure should be 218 psig. So I'm off by 8 psig.

 

[pmck94]

I see!!
Original post would indicate restriction before he took temp. OP changed ODT to 60, but you were correct!

Please I insist call me Michael

 

[TechmanTerry]

This is a r410 system right ? For years and years on a r22 system on a 72* day you added 25*to the ambient and you got the matching CT that you used your w/ the p/t chart to see what the head was supposed to be. So 72+25=97CT, and the p/t chart shows a press of 186psi or so on that 72* day. Then on a 95*day (the typical highest temp we get on LI,NY,,Ct,,R.I, Jersey) you add 30*. so ,95*+30*=125* SCT. So,on a 95* day you should have a r22 press of 276psi or so. The higher SEER stuff has dropped that adding of 30 to the 95*day down to 25* or even 20* on an ULTRA hi efficiency r410 system. So , on any decent 410 unit the same rules still apply and work.I think!

 

[1SikLBZ]

68 ambient with a 64 degree liquid line is what he posted. The liquid line can not be cooler than ambient with a dry coil. He's flashing liquid before the service valve.

Bingo!

Id start here, The unit wont have a discharge line port other then the liquid line service port so your going to have to work with temps. Open up the sides of the unit and you will find a copper spun liquid line drier. Check for temp drops across the drier. Like stated above, You cannot have a lower temp reading on your liquid line then your Ambient temp.

I agree with a earlier post of the cope land compressors having the oil additive added to them to supposedly help with rust? This oil additive has clogged up a few new Install small server room systems I've worked on. Inside of the TXV looks like molasses, Add In AC Renew and change the Drier. Monitor for a few hours and you can see the super heat start to drop, Return after a few days and everything Is back to normal.

Not saying this will be your problem, Just stating to check your drier thats hidden behind the panel and Its possible something Is clogging It up.

 

[CircusEnvy]

Like stated above, You cannot have a lower temp reading on your liquid line then your Ambient temp.

I've seen it sometimes on severely undercharged systems, to the point where the liquid line is sweating before the metering device. At that point though, there's about 25% left in the system. I had one just last week that was down by 5 pounds (6 pounds total charge). Recharged it and it was running perfectly, no restrictions.

Don't ask me for an explanation though, just an observation! :grin2:

Nevermind - I should clarify that I measured the liquid line just before the evaporator, not at the service valve.

 

[1SikLBZ]

Haha ok I'll except that one.

I should change my post to never having a lower then ambient liquid line, Unless your severely under charged or Its raining :gah:

 

[Core_d]

He never said it was below ambient just sweating. Maybe the basement had a higher wb than outdoor conditions unhooked drier vent ect

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[CircusEnvy]

He never said it was below ambient just sweating. Maybe the basement had a higher wb than outdoor conditions unhooked drier vent ect

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That's what I was probably seeing.

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[Keifer13]

Trane 2 stage compressors have a secondary wire harness on the opposite side of primary harness of compressor(2 white wires). If it has second stage, you can verify it by tracing white wires to the DC bridge board in control panel on a little black square. There is a yellow and blue wire on this bridge and if you get 24 volts AC at blue and yellow, AND 24+- volts DC on the white wires, it's calling for second stage. You should check in 2nd stage if possible. Also, you can pull yellow or blue off DC bridge and see if pressures change. If it's single stage, bad TXV and I agree with possible low charge.

 

[Core_d]

Trane 2 stage compressors have a secondary wire harness on the opposite side of primary harness of compressor(2 white wires). If it has second stage, you can verify it by tracing white wires to the DC bridge board in control panel on a little black square. There is a yellow and blue wire on this bridge and if you get 24 volts AC at blue and yellow, AND 24+- volts DC on the white wires, it's calling for second stage. You should check in 2nd stage if possible. Also, you can pull yellow or blue off DC bridge and see if pressures change. If it's single stage, bad TXV and I agree with possible low charge.

Thanks for that tidbit I see those a lot. Ever replaced a black box?

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[Keifer13]

I've replaced one bridge. It operates the solenoid inside the compressor to engage the second stage plate. I've found that the solenoid in the compressor goes out more often than the bridge and there's only 1 fix, new compressor .

 

[mr_guy]

Check temp drop across drier..., likely it is a blocked drier, seen it several times!

 

[pmck94]

Guys what about airflow? not having enough airflow could create a high superheat right?

 

[kamersoutdoor]

Guys what about airflow? not having enough airflow could create a high superheat right?

Low superheat