Head Scratcher

[garyed]

I'd suspect a Lower subcooling with no metering device to slow the liquid down.


The problem described is missing something. It doesn't add up.

I agree, that's why I called this a head scratcher.
I don't know what I'm missing. Having no metering device should be similar to a stuck open TXV but even worse, so low SH & low SC. I had low SH but normal to slightly high SC. The only thing I can think of is that it has something to do with the long lineset & small microchannel coil combination.

[hvacrmedic]

You said that initially the suction pressure was 0 psig. I suppose that could happen by accident, but it's most often encountered when the system has run in a vacuum and equalized to atmospheric pressure, as in a leak in the low side. One thing is certain, with 0 psig suction you would also have close to 0° SC. Your condenser TD would also be near zero, or IOW, SCT would be nearly equal to ambient temp. Instead your TD and SC were 16°. How do you get a TD that high when the evap coil isn't picking up any heat? Only noncondensables can account for those numbers.

You then bypassed the TXV. Again you had 16° SC. That makes perfect sense, iF the same nonconsables are still in the system. The "actual" SC in both cases was very nearly zero, which is what would normally be expected under both of those sets of conditions respectively.

You then installed a TXV. Sub cooling is still 16, and you still have nonconsables in the system. But now you have a working TXV, so SP and SH are now normal. The 90' fall is gaining you some actual subcooling at the TXV inlet, so the system works, despite the noncondensables.

What was the condenser TD when you finished, and what should it have been under those conditions according to the unit product data? Also keep in mind that you removed some of the noncondensables each time you valved off the condenser to open the low side.

[garyed]

You said that initially the suction pressure was 0 psig. I suppose that could happen by accident, but it's most often encountered when the system has run in a vacuum and equalized to atmospheric pressure, as in a leak in the low side. One thing is certain, with 0 psig suction you would also have close to 0° SC. Your condenser TD would also be near zero, or IOW, SCT would be nearly equal to ambient temp. Instead your TD and SC were 16°. How do you get a TD that high when the evap coil isn't picking up any heat? Only noncondensables can account for those numbers.
I never said I had any TD across the indoor coil until after I pulled out the TXV & ran it with no metering device & had 0 SH.
You then bypassed the TXV. Again you had 16° SC. That makes perfect sense, iF the same nonconsables are still in the system. The "actual" SC in both cases was very nearly zero, which is what would normally be expected under both of those sets of conditions respectively.

You then installed a TXV. Sub cooling is still 16, and you still have nonconsables in the system. But now you have a working TXV, so SP and SH are now normal. The 90' fall is gaining you some actual subcooling at the TXV inlet, so the system works, despite the noncondensables.

What was the condenser TD when you finished, and what should it have been under those conditions according to the unit product data? Also keep in mind that you removed some of the noncondensables each time you valved off the condenser to open the low side.

It all sounds logical but I have a hard time believing that I have enough non condensables to cause me to have the same SC with a working TXV as with no metering device without showing some other obvious symptoms.

[hvacrmedic]

It all sounds logical but I have a hard time believing that I have enough non condensables to cause me to have the same SC with a working TXV as with no metering device without showing some other obvious symptoms.

I was referring to your condenser TD, not indoor deltaT. Two entirely different things.
IMO, it's a classic case of NC, not even any question about it. You may or may not have had a bad TXV too. That's going to be difficult to determine after the fact.

Maybe I should add that the primary symptoms of noncondensables are artificial elevation of SC and condenser TD. It's really that simple. The noncondensables add to the refrigerant vapor pressure, causing you to get a falsely high SC measurement.

[SandShark]

Interesting thread. Just curious, but what are your liquid line sizes on these long line set systems?

[garyed]

Interesting thread. Just curious, but what are your liquid line sizes on these long line set systems?

3/8 liquid & 5/8 suction - 2 ton system

[ar_hvac_man]

Happen to remember your model number? First 4 digits?

[bluecool]

could your by-pass turned out to be correct metering device

[hvacrmedic]

could your by-pass turned out to be correct metering device

It was overfeeding, as expected with no TXV. High suction low superheat.

Even with no metering device there is still some restriction provided by the distributor assembly. I once found a system that had been used for two summers with the piston missing. It was cooling the house, but wouldn't keep up when the outdoor temp went above 85, which is why they called me.

[ar_hvac_man]

Yep. Some will cool to some degree with no metering device. Ive had countless calls where I found the wrong piston, and even no piston.

[AStudent]

I'd suspect a Lower subcooling with no metering device to slow the liquid down.


The problem described is missing something. It doesn't add up.

Would the micro channel condenser slow the liquid down enough on it's own to stack the refrigerant?

[garyed]

I was referring to your condenser TD, not indoor deltaT. Two entirely different things.
IMO, it's a classic case of NC, not even any question about it. You may or may not have had a bad TXV too. That's going to be difficult to determine after the fact.

Maybe I should add that the primary symptoms of noncondensables are artificial elevation of SC and condenser TD. It's really that simple. The noncondensables add to the refrigerant vapor pressure, causing you to get a falsely high SC measurement.

I usually agree with your diagnostics but I can't on this one.
I'm not arguing about NC's elevating subcooling but to suggest they would bring your suction pressure down to zero is not very likely. I've seen too much moisture in a system freeze & cause a restriction in a heat pump on a cold day or in a walk in cooler but it is not very likely to see it on a straight air in warm weather. First, the system has to run a while for enough moisture to freeze up at the metering device which didn't happen. The system started pumping down to 0 psi immediately after start up. For anyone who has seen a restricted TXV that is a classic sign. Second is if you have a totally restricted TXV you can't even blow through it in the direction of the refrigerant flow. This TXV had an internal check valve because I could blow through it in the reverse flow direction but in the cooling direction I couldn't blow through it at all.

[hvacrmedic]

Would the micro channel condenser slow the liquid down enough on it's own to stack the refrigerant?

Pressure drop through the condenser coil would act to lower the subcooling at the outlet.