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Old 06-03-2020, 03:34 PM   #1
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Subcooling Target


Hi all!
I'm helping a neighbor (80yo) with his HVAC. The unit is a Trane, 4 ton heat pump, 4TWX6048 with 410a and a txv.
Facts determined with Testo 550
T1 - 64.2 f @ 136.5 psi
T2 - 96.6 f @ 379.1 psi
Ev 47.7 f
Co 112.4 f
Delta T 32.2 f
Superheat 16.4 f
Subcool 16.0 f

Outside temp 90 f
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Old 06-03-2020, 03:46 PM   #2
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Re: Subcooling Target


Isn't the target normally printed on the nameplate/label if not the sticker facing into ?

delta-t 32 , is that the drop across air handler?

The other readings at first glance look pretty normal.
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Old 06-03-2020, 05:44 PM   #3
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Thank you for the quick reply. If the numbers look good, then I think he's in good shape for the summer. I thought the subcooling was going to be lower for some reason.

I couldn't find the target anywhere on the unit. I believe the Delta T that the Testo refers to is the temp difference between the high and low lines.

Thanks again
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Old 06-03-2020, 08:30 PM   #4
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Re: Subcooling Target


I'll throw my hat in the ring for the numbers NOT looking good. You've got a pretty fancy set of gauges there. Do you also have a fancy recovery machine and a recovery tank and an EPA license to go with them?
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Old 06-03-2020, 08:44 PM   #5
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Re: Subcooling Target


^so tell him what you think is wrong.

did say "at first glance".

don't know what the required subcooling is, maybe it's a bit over but nothing terrible.

wise to make sure the condenser coil is clean b4 making assumptions.

Maybe the return air temp is high hence slightly high sst. Edit: just looked it up, 2 stage machine and may have been running on low when checked.

Incomplete data was given.
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Old 06-03-2020, 09:20 PM   #6
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Quote:
Originally Posted by user_12345a View Post
^so tell him what you think is wrong.
The last sentence in my above post was a hint.

Alright, I think it's overcharged a little bit. You made a good point about making sure the coils (and filter) are clean and you made a good point about making sure it's running in both stages before "adjusting" anything.

Must AC units with TXVs that I've worked on like to be closer to 10 of subcooling. I will admit that 16 subcooling probably isn't that far off in the grand scheme of things. Might just take filling up the hoses and "de-minimisly" purging them a couple of times to get it dialed in. 😉

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Old 06-03-2020, 10:11 PM   #7
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Re: Subcooling Target


I don't see anything in those figures to be ashamed of... the subcooling is fine... the more the better. Most manufacturers will want a minimum of 10 F. but it'll work down as low as 5 F. so you're far and away to the good side. I'm retired from commercial refrigeration so I always had sight glasses on all systems and never bothered much with subcooling except when picking the right condenser coil on the build up... after that, you get what you ask for unless you fail to keep it unclogged.
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Old 06-03-2020, 10:38 PM   #8
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Re: Subcooling Target


Quote:
the subcooling is fine... the more the better.
but the higher it is, the less area of the condenser actually used to condense refrigerant.

just need a solid column of liquid, right?
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Old 06-03-2020, 10:53 PM   #9
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Re: Subcooling Target


Well yes, but if the head isn't pushed out of normal, it's not an issue. My build ups always had a receiver... those little a/c's get by without them so charging them is more of a science, especially on the cheap units that try to get by with as little heat rejection surface as possible and run coils with fin spacing so tight that they get plugged twice per season. The buyers take the brunt of that kind of progress (progress indeed???) since it's hard to sell quality except to the very rich.

With the price of refrigerant, you won't find any overcharged systems unless some dummy has been in them.
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Old 06-04-2020, 08:32 AM   #10
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Thanks everyone.

After reading your replies, I think everyone is correct. I think the previous technician misdiagnosed the issue as low liquid, but I believe the issue is a liquid-restricted system, hence the hint of recovery system. I'll kick this one up to a professional.

Thanks again
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Old 06-04-2020, 09:28 AM   #11
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Re: Subcooling Target


There isn't much evidence of a restriction, the txv is probably just set to maintain around 16F sh and the unit may be very slightly over-charged.

The manufacturer should have specific charging instructions - which stage to check it on, subcooling value.

I looked up the guide for it out of curiosity and the pressures you got line up with the pressure curve.

What issue prompted this check? it doesn't sound like anything is wrong.
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Old 06-30-2020, 03:00 AM   #12
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Hi All,

I just wanted to follow up. The unit is running great and their house is staying cool. Thanks for your feedback.

Prompted concern: my neighbor told me that he had someone coming out every year to add freon. He wanted to give it attention before it stopped cooling this year. So far, so good. Thanks again.
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Old 06-30-2020, 05:12 AM   #13
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Re: Subcooling Target


I have read plenty of threads here that say SC should be ~10 degrees or whatever but my old Bryant R22 condensing unit is labeled for 20 degrees SC when paired with an indoor TXV coil and that is how I have it set up. They do seem to vary a lot.

Quote:
Originally Posted by bfrabel View Post
The last sentence in my above post was a hint.

Must AC units with TXVs that I've worked on like to be closer to 10 of subcooling. I will admit that 16 subcooling probably isn't that far off in the grand scheme of things. Might just take filling up the hoses and "de-minimisly" purging them a couple of times to get it dialed in. 😉
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Old 06-30-2020, 08:08 AM   #14
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Re: Subcooling Target


If I was the OP's neighbor, I'd have a sight glass installed on the liquid line so I could know in advance when I needed to call for a shot of gas. I never built any txv systems without a sight glass.

I'd also check into getting that leak fixed, but that's another subject altogether.
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