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Old 08-31-2014, 10:41 PM   #16
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Conflicting super heat and sub cooling


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That super heat chart is for 400 CFM per ton. And you only have 300 CFM per ton. The low super heat and sub cooling is showing you a low air flow condition.
I understand the low SH but the low SC has me baffled. I just did some searching and found a troubleshooting chart but my scenario is not listed. Do the pressures that I listed sound normal?

http://www.refrigeration-engineer.co...ling-scenarios

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Old 08-31-2014, 11:32 PM   #17
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Conflicting super heat and sub cooling


They are with in range of normal for a 13 SEER. A tad low on the high side. But that is part of the low indoor air flow.
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Old 09-01-2014, 05:52 AM   #18
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Conflicting super heat and sub cooling


OK thanks....the outdoor is 10 SEER and the indoor is so old that it doesn't have a rating on the brochure. I wish I could measure the airflow right at the unit instead of through the registers. The wall cavities and joist spaces could have some leakage.
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Old 09-01-2014, 07:12 AM   #19
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Conflicting super heat and sub cooling


For a 10 SEER, the high side a pretty low. Again, this is an indication of low ir flow/low load on the indoor coil.

Mismatched outdoor and indoor units are often problematic to get the charge correct.

You can do a temp rise check at the air handler, and calculate the air flow through the it that way. Just need to know the actual running/operating voltage while the strip heat is on, and their amp draw. Convert to BTU.

Formula: BTU output/(temp riseX1.08)=CFM
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Old 09-01-2014, 08:01 AM   #20
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Conflicting super heat and sub cooling


Thanks BT, that is something I can easily do but I will wait until winter. I already have small holes and rubber grommets for the little thermometers right at the unit on the return and supply. There is 20kw of resistance heat which I have found to be too much so I pulled fuses and run it at 10kw.
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Old 09-04-2014, 12:04 AM   #21
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Conflicting super heat and sub cooling


no, what I'd do is check static pressure versus heat gain/loss versus lineset pressure drop paralleled to length and lift to loss to insulation to cleanliness (air flow) to ambient temp to dew point to amps to cfm.

or I'd check if you had piston/fixed orifice or txv and not compare sh to sc as believe it or not one metering factor directly relates to sh or sc in the first place and then come to the conclusion as to why or why not the target may or may not be off.

All heck, I don't know enough.
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Old 09-04-2014, 12:13 AM   #22
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Conflicting super heat and sub cooling


fixed orifice metering device can and do change superheat directly due to temp. It can be scientifically correct one day (minute) due to invested recordings and change the next due to change in recordable data.

thus subcool changes as flood/famine does as sh changes rapidly. can happen within minutes.

txvs attempt to maintain a constant sh, thus sc is the recommended way to diagnose charge.

Now what are all the other 100's of variables?
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Old 09-04-2014, 12:20 AM   #23
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Conflicting super heat and sub cooling


btw, if txv where is it mounted, is copper clean, is bulb mounted tight, calibrated? mounted properly directly relates to it's ability to sense temp properly. it needs to read gaseous and only gaseous state,

Most cases, it's not right, thus is bipolar as it tries.

Anywho...
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Old 09-04-2014, 12:36 AM   #24
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Conflicting super heat and sub cooling


I'm not Bt or Yuri, not by a long shot nor did I intend, albeit in hindsight I can see how presumably I did, come off as a know it all, but there is simply too much to consider and present into any variable to simply believe an indifference in sh and sc means anything at all let alone catastrophic failure.

While one can and does directly relate to the other, it's the refrigerant metering device, txv or fixed orifice, in which determines the diagnostic tool to assist in diagnosing a system. That's the main factor, now what of the variables?

It's never ending. Good luck.
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Old 09-04-2014, 05:43 AM   #25
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Conflicting super heat and sub cooling


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no, what I'd do is check static pressure versus heat gain/loss versus lineset pressure drop paralleled to length and lift to loss to insulation to cleanliness (air flow) to ambient temp to dew point to amps to cfm.

or I'd check if you had piston/fixed orifice or txv and not compare sh to sc as believe it or not one metering factor directly relates to sh or sc in the first place and then come to the conclusion as to why or why not the target may or may not be off.

All heck, I don't know enough.

I don't have the equipment to do that but I'm a new tool slut...so just maybe.

I mentioned in my first post about changing orifices and thought most readers would know that I don't have a TXV. I said piston so maybe that was the wrong word to use.
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Old 09-04-2014, 05:56 AM   #26
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Conflicting super heat and sub cooling


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Originally Posted by Doc Holliday View Post
I'm not Bt or Yuri, not by a long shot nor did I intend, albeit in hindsight I can see how presumably I did, come off as a know it all, but there is simply too much to consider and present into any variable to simply believe an indifference in sh and sc means anything at all let alone catastrophic failure.

While one can and does directly relate to the other, it's the refrigerant metering device, txv or fixed orifice, in which determines the diagnostic tool to assist in diagnosing a system. That's the main factor, now what of the variables?

It's never ending. Good luck.
I find HVAC very interesting and use my home system to learn and as a side benefit keep it well maintained. If I can't figure out what the numbers are telling me I try to dig deeper and understand.

The SH is telling me I'm overcharged and the SC is saying undercharged. I know the air flow is a little restricted, maybe? I purposely mentioned that I have mismatched units and talked about the orifice maybe not being changed to see if anyone thought that might be part of the problem.

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Old 09-05-2014, 08:02 AM   #27
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Conflicting super heat and sub cooling


I can't add a lot to what has been said but I will offer an explanation to your question. If your air flow is low it will cause your SH to be low(low heat absorption),which will give less heat to be rejected at condenser(low sc). Hopes this helps a little.
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Old 09-05-2014, 06:49 PM   #28
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Conflicting super heat and sub cooling


Any idea if your system had a vacuum pulled on it when it was originally installed/charged? If not it may throw your SC calculation out of whack by a fair bit. I have found that before where the non-condensibles drive hop the head pressure and then make out look like you have less SC than you actually do. The airflow issue may be addressable with a fan speed change(if possible), then of course there is always the thought of how clean is the evaporator if it's been there for 20 yrs, perhaps it could be holding a bit of flow back itself by this point.
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Old 09-05-2014, 08:26 PM   #29
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Conflicting super heat and sub cooling


The outdoor unit was replaced in about 1995 and he pulled a vacuum. I can't remember for how long or if he used a micron gauge. The fan speed should be set on high for A/C but I can double check it. The evaporator is 31 years old and very clean. I just foamed it 2 weeks ago to make sure but no dirt came out and the readings didn't change after cleaning. It's an all aluminum angled slab and I can get to both sides easily. The filter is a normal 1" pleated and very restrictive. I really have to tug on it when pulling it out or in while the blower is running. Again though, no readings change whether the filter is in or out.

Here is something interesting that may or may not mean anything. I can listen to the liquid line at the evaporator and like clockwork I can hear a change corresponding to the time it takes for the refrigerant to make a complete trip. For the most part it's nice and quiet and I can tell that it's 100% liquid. Then for about a 15 second time span I can hear what sounds like a combination of liquid and vapor going through the orifice. Then it gets quiet again until it has made its way through the system and back again, over and over. The suction line temperature goes up by 2* when it's making the noise (after a short delay while making it's way through the evaporator).
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Old 09-06-2014, 01:23 AM   #30
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Conflicting super heat and sub cooling


oh, sounds like non condensibles and or air in the refrigerant. too bad no sight glass.


txv or piston metering device?

Return air is too small judging by your description of how hard it is to pull..

What size is the system and what size is the return grill and return chase/duct?

Interesting subject, if were in my hands I'd recover all refrigerant, triple evac lineset, weigh in charge and clean coils and go from there.

Let us know.

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