Low suction, normal head pressures. Confused.

[pmck94]

Hi all (yall)

I was doing a PM for a customer and when taking the refrigerant readings, I have come to a complete brain fart as to why the system was doing this.

Readings:

Indoor temp: 70 F
Outdoor temp: 68 F
Suction Pressure: 65 PSIG (roughly 10 degrees)
Discharge Pressure: 205 PSIG (roughly 72 degrees)
Suction temp: 61 F
Discharge temp: 64
Superheat: 51 F
Subcool: 8 F

Trane R410A system has a design subcool of 10 +/- 3 degrees so my subcooling is within the threshold.

I did not see a filter drier on either the suction or discharge lines to take a temperature drop across to see if there is more than a 2 degree difference aka change that out. I was going to say something along the lines of a restriction after the TXV within the cap tubes that is starving the evap coil giving me a low suction/very high superheat.

Since my job at this moment is to keep a unit running and make sure that happens through the most basic of ways (contactor changes, new dual run cap or start, etc) I have to call into dispatch and turn the PM into a service call.

So, with that said, anyone want to give their 2 cents as to what the problem might be? I want to know just in case I come across this again.

Thanks,

Michael

[catmanacman]

I would say a combination of low charge and restricted metering device.
on tranes the drier is inside the unit

[CircusEnvy]

With such high superheat and normal subcooling, I would be willing to bet that TXV is malfunctioning and it's undercharged by 1-2 pounds. Sensing bulb could be low on charge or something fouled up the needle. Fortunately, that's probably a bolt on TXV so it's real easy to change out.

[TechmanTerry]

The SC might be normal , but the normal head for a r410 system w/ a 68* ambient should be closer to 300psi and the normal suction press should be closer to 110psi w/70* IDB. Those p/t's sound more like r22.

[BNME8EZ]

As others have said it sounds like a TXV and maybe low on juice. Check the serial number, it may need the snake oil to clean the goo out of the TXV.

[pmck94]

But if the system was low on refrigerant wouldn't that show a low head pressure and suction as well? Right off the bat I would have a high superheat and low subcooling, no?

By seeing that it had 8 degrees subcooling would mean that the condenser has the refrigerant it needs to operate at design subcool specs with the threshold of +/- 3 degrees. Unless I'm not reading the data plate right isn't that what it means?

I don't want to sound like I know the game and that I'm better because I'm not but looking back on what I learned in school as a reference seems to conflict. Then again theory and reality are opposites most of the time.

The serial started with a 14 and I remember something trane made that did in fact clog up the TXV and yes some oil had to be put in to dissolve it. Our company has been going around and applying the oil to those with such serial numbers that were infected.

[BNME8EZ]

But if the system was low on refrigerant wouldn't that show a low head pressure and suction as well? Right off the bat I would have a high superheat and low subcooling, no?

By seeing that it had 8 degrees subcooling would mean that the condenser has the refrigerant it needs to operate at design subcool specs with the threshold of +/- 3 degrees. Unless I'm not reading the data plate right isn't that what it means?

I don't want to sound like I know the game and that I'm better because I'm not but looking back on what I learned in school as a reference seems to conflict. Then again theory and reality are opposites most of the time.

The serial started with a 14 and I remember something trane made that did in fact clog up the TXV and yes some oil had to be put in to dissolve it. Our company has been going around and applying the oil to those with such serial numbers that were infected.

You are showing low head.

Look at it this way. If everything is working properly you would have 8°-12° SC and 10°-20° SH.

So lets look at just the SH side first. What causes high SH, low on charge or a restriction. 8° SC would discount the low on refrigerant right. That leaves a restriction.

If you have a correct charge and a restriction you would have higher SC right. Since you have normal SC and a restriction you must be short on charge.

Does this explain better or is there other info I should consider?

[CircusEnvy]

With a normally operating TXV on an undercharged system, you'll find that SH may be normal with some subcooling. Not until you reach 0-3 degrees SC will your SH start to increase.

See, as the system loses refrigerant, the TXV will open more and more to keep the SH normal (10 degrees), so there's always a column of liquid before the TXV. When it's fully open and still not enough refrigerant is in the evaporator, then your SH starts to rise.

In your case, you have an almost normal column of liquid behind the TXV, but it's closing down way more than necessary. There's no reason to have 50 degrees SH if you have 8 degrees SC other than a restriction (TXV being the restriction). It's also undercharged because if it were charged fully, your SC would be well above normal (15-30 degrees).

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[martyinlincoln]

Only ways your liquid line can be colder than the ambient is if there's a restricting before the pressure port or the condensing coil is wet.

[kamersoutdoor]

Only ways your liquid line can be colder than the ambient is if there's a restricting before the pressure port or the condensing coil is wet.

But it's not!
68 deg ambient/liquid line saturation 71.4 deg........ close, but not....

With a normally operating TXV on an undercharged system, you'll find that SH may be normal with some subcooling. Not until you reach 0-3 degrees SC will your SH start to increase.

See, as the system loses refrigerant, the TXV will open more and more to keep the SH normal (10 degrees), so there's always a column of liquid before the TXV. When it's fully open and still not enough refrigerant is in the evaporator, then your SH starts to rise.

In your case, you have an almost normal column of liquid behind the TXV, but it's closing down way more than necessary. There's no reason to have 50 degrees SH if you have 8 degrees SC other than a restriction (TXV being the restriction). It's also undercharged because if it were charged fully, your SC would be well above normal (15-30 degrees).

Sent from my XT1080 using Tapatalk

To the op, correct answer was given above with txv malfunction on top of low charge!

[jtrammel]

Bad TXV. Not necessarily low on charge. Many times I've had normal subcooling and a ll restriction. The subcooling skyrockets when some knucklehead jams a bunch of refrigerant in it to get the suction pressure above freezing.

If it's a copeland compressor from 2014 I can almost guarantee the TXV needs some of the magic miracle additive ac renew

[pmck94]

Bad TXV I get but someone please tell more about a low charge on top of that?

[jtrammel]

I don't think it's low on charge

[TechmanTerry]

How can the SC be perfect and be used as a "cornerstone" for a good running system when the lo side is lo and the hi side is also lo?

A good running system has a good suction press, a good hi side press, a good SH and a good SC.

[Core_d]

At start up you'll likely have high head for a while until you get a large column of liquid built up. With a real good restriction you are cooling more than you are pumping and the pressures get close to ambient saturation points when the start to happen subcooling goes down..... Much like checking subcooling in say fifty or below outdoor temps. You would think it would subcool real good yet remains around 4 or 5 degrees

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[kamersoutdoor]

Correct me if I'm wrong, but if I close my txv(restrict), subcool increases (as in refrigeration, close your valve and sight glass clears)

[kamersoutdoor]

Or high subcool with restricted captube.
Restriction causes liquid to stack in condenser, so if txv is restricted, he would have higher subcooling than under normal operation....... thus low charge

[martyinlincoln]

But it's not!
68 deg ambient/liquid line saturation 71.4 deg........ close, but not....

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.

[pmck94]

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..

[pmck94]

I just looked back on my notes,

Low side pressure: 65 PSI (10*F)
High side pressure: 210 PSI (72*F)
Liquid line temp: 64*F
Suction line temp: 61*F
Superheat: 51*
Subcooling: 8*
Outdoor ambient: 60*F
Indoor ambient: 70*F

Trane R410a system

Model: 4TTR3036E1000AA
Serial: 1410386J3F

Have at it