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Old 06-30-2015, 04:29 PM   #1
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Pressure / temperature readings


Hi, I am looking to figure out the next step of my process and thought this may be the place to look. I have a 15 TON Trane split unit that just isn't keeping up. It is outputting air @ 61*F with a current indoor temperature of 83*F

Pressure readings outside were nearly identical on either side with 250 head pressure, 60 suction pressure on R22.
Subcool of 15*F superheat of 39*F. ambient outdoor temperature of 90*F

Coil and evap have been cleaned and new filters have been installed..

What should I check next?

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Old 06-30-2015, 09:04 PM   #2
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Pressure / temperature readings


What is the square footage of your home?

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Old 06-30-2015, 10:37 PM   #3
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Pressure / temperature readings


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What is the square footage of your home?
15 ton I do not think this is a house??? But said pressures were the same???
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Old 06-30-2015, 11:26 PM   #4
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Pressure / temperature readings


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Originally Posted by rhop824 View Post
Hi, I am looking to figure out the next step of my process and thought this may be the place to look. I have a 15 TON Trane split unit that just isn't keeping up. It is outputting air @ 61*F with a current indoor temperature of 83*F

Pressure readings outside were nearly identical on either side with 250 head pressure, 60 suction pressure on R22.
Subcool of 15*F superheat of 39*F. ambient outdoor temperature of 90*F

Coil and evap have been cleaned and new filters have been installed..

What should I check next?
Sorry but don't quite under stand the pressure reading statement, also need WB indoor temp along with the other temperatures to get the needed target superheat.
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Old 07-01-2015, 12:56 AM   #5
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Pressure / temperature readings


Do you remember a time when it did keep up. Is airflow the same as always?


Not coming from a pro here - anyhow:

You have to see if it has a txv or orifice and check wetbulb return air temperature.

The split across the coil is very good unless there's a low airflow condition or very low humidity.

The superheat is very high, but a high return air temp can increase it. 60psi seems low to me with such a high return air temperature, so there's a chance the coil is being underfed with r22. (granted, 60 may be normal if this is an older lower seer system with a small evap coil) If there's a txv, it's suspect for sure; its supposed to maintain pretty constant superheat. Suspect slight restriction as well, if there are any filter driers check for a temperature drop across them, check liquid line for kinks.



With low airflow, you'll have low superheat and low suction.

With 15f subcooling and 250 psi head, you've got plenty of refrigerant in the system.

Maybe have it checked at a more normal indoor temp (75F?) and see if there's a manufacturer's charging chart.

Another test you can do is check the split across the outdoor unit; if you can look up the model number and condenser cfm, you can calculate heat rejection with q = delta-t x 1.08 x cfm

Heat rejection will exceed heat collected by the indoor unit - the lower the efficiency, the higher the heat rejection will be relative to capacity.

You if can get compressor amp draw measured and convert watts to BTUs, you can roughly determine how much heat this system is pulling out and compare it with the tonnage.

Last edited by user_12345a; 07-01-2015 at 01:03 AM.
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Old 07-01-2015, 01:18 AM   #6
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Pressure / temperature readings


Unless the thing is ancient, it probably has a txvs at that size. You said, "either side" so I'm thinking it's a dual circuit? I would suspect the txv is starting to fail. Check to make sure the bulbs haven't fallen off, and if they haven't, pull one off and hold it in your hand. If your sh drops and evap pressure comes up, you can try relocating the bulb or leaving it un-insulated, but it'll have to be changed sooner or later. (One unit i had started working fine after getting the valve to open. Something must have gotten stuck, and freed itself when the valve opened. Hopefully my filter drier caught it after. You might have the same luck) If nothing happens, you'll need to change your power head. (or whole txv) I'd be very confident if you pulled out the factory charge when you start on it.

Cheers!
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Old 07-01-2015, 04:42 PM   #7
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I mistyped, it is a 20 ton unit, not a 15.

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Originally Posted by DOUG1111 View Post
15 ton I do not think this is a house??? But said pressures were the same???
Sorry, this is not a home but a business. It is a split unit and the pressure are about the same for both halves of the unit.

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Originally Posted by evapman View Post
Sorry but don't quite under stand the pressure reading statement, also need WB indoor temp along with the other temperatures to get the needed target superheat.
I don't have a way to measure indoor wet bulb, but I can buy something if it is important.

Quote:
Originally Posted by user_12345a View Post
Do you remember a time when it did keep up. Is airflow the same as always?


Not coming from a pro here - anyhow:

You have to see if it has a txv or orifice and check wetbulb return air temperature.

The split across the coil is very good unless there's a low airflow condition or very low humidity.

The superheat is very high, but a high return air temp can increase it. 60psi seems low to me with such a high return air temperature, so there's a chance the coil is being underfed with r22. (granted, 60 may be normal if this is an older lower seer system with a small evap coil) If there's a txv, it's suspect for sure; its supposed to maintain pretty constant superheat. Suspect slight restriction as well, if there are any filter driers check for a temperature drop across them, check liquid line for kinks.



With low airflow, you'll have low superheat and low suction.

With 15f subcooling and 250 psi head, you've got plenty of refrigerant in the system.

Maybe have it checked at a more normal indoor temp (75F?) and see if there's a manufacturer's charging chart.

Another test you can do is check the split across the outdoor unit; if you can look up the model number and condenser cfm, you can calculate heat rejection with q = delta-t x 1.08 x cfm

Heat rejection will exceed heat collected by the indoor unit - the lower the efficiency, the higher the heat rejection will be relative to capacity.

You if can get compressor amp draw measured and convert watts to BTUs, you can roughly determine how much heat this system is pulling out and compare it with the tonnage.
It is a Txv unit, I don't have a device to measure wet bulb. There does not appear to be a low airflow condition or low humidity.

No kinks, no temperature change across the filter drier. its a 7 year old Trane unit. Manufacture charging chart calls for 255 - 270 PSI head pressure @ 90* (temperature these were measured at) and 60-75 psi SP (depending on wet bulb which I cannot measure)

Heat rejection:
Outdoor unit - (151-96) * 1.08 * 16120 = 957528
Indoor unit (measured after txv and directly after evap) - (66-55) * 1.08 * 8000 = 95040

15 Amp per phase, 480V three phase = 42549 BTU = ~3.5 Tons.. did I do something wrong here, or is it 3.5 * 3 = 10.5 which is what half a unit would be?

Quote:
Originally Posted by supers05 View Post
Unless the thing is ancient, it probably has a txvs at that size. You said, "either side" so I'm thinking it's a dual circuit? I would suspect the txv is starting to fail. Check to make sure the bulbs haven't fallen off, and if they haven't, pull one off and hold it in your hand. If your sh drops and evap pressure comes up, you can try relocating the bulb or leaving it un-insulated, but it'll have to be changed sooner or later. (One unit i had started working fine after getting the valve to open. Something must have gotten stuck, and freed itself when the valve opened. Hopefully my filter drier caught it after. You might have the same luck) If nothing happens, you'll need to change your power head. (or whole txv) I'd be very confident if you pulled out the factory charge when you start on it.

Cheers!
They were both secured but one side wasn't insulated well. I pulled each off and held it in my hand for a few seconds and there was no change on the SH or PH as measured from the compressor side ports.
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Old 07-01-2015, 05:02 PM   #8
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Pressure / temperature readings


How old is the unit. Strange that both circuits are giving you identical readings.

Head pressure is low if its a low efficiency unit. SC is okay, but with that SH. It points to either both circuits are low on charge(unusual for both circuits to be as low as each other), and the condense coil are dirty(its probably a double coil, looks clean on outside, but between the coils its a blanket).

Does it have dehumidification.
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Old 07-01-2015, 05:52 PM   #9
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How old is the unit. Strange that both circuits are giving you identical readings.

Head pressure is low if its a low efficiency unit. SC is okay, but with that SH. It points to either both circuits are low on charge(unusual for both circuits to be as low as each other), and the condense coil are dirty(its probably a double coil, looks clean on outside, but between the coils its a blanket).

Does it have dehumidification.
The unit is 7 years old

It is a double coil. I used super brite coil cleaner on both inside and outside of the unit twice and sprayed it with a pressure washer (40* angle tip, 2 ft minimum from coils) I also used the same cleaner to clean the evaporator and used a water hose trying to keep most the water in the condensate pan/drain.

I was under the impression it dehumidified as part of the process? it does not have any separate dehumidifier attached though.
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Old 07-01-2015, 06:32 PM   #10
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Certain Trane RTUs have a special set up in them for better dehumidification.

You posted a return air temp at the evap coil of 66. But said the area's room temp is 83.
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Old 07-01-2015, 06:47 PM   #11
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Certain Trane RTUs have a special set up in them for better dehumidification.

You posted a return air temp at the evap coil of 66. But said the area's room temp is 83.
Oh, I was using a probe to measure the line temperature, not air temperature. Is that what I should be doing for that measurement?
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Old 07-01-2015, 07:42 PM   #12
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Pressure / temperature readings


The dehumidify function works like in resi units, right? Slows down the blower.

With plenty of refrigerant in the condenser (lots of subcooling going on), shouldn't the txv be opening up to maintain a proper superheat, even if a little low on charge?

Not familiar with 3 phase, don't think its as simple as adding up the individual phases, rms isn't the sum with 3 phase for voltage, not sure about current - here's a wattage calculator: http://www.rapidtables.com/calc/elec...Calculator.htm

If this is a 20 ton single stage and it's only rejecting 95000 btus, you've got a major problem. (unless there are 2 10 ton circuits with separate fans and u only measured one)
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Old 07-01-2015, 07:58 PM   #13
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The dehumidify function works like in resi units, right? Slows down the blower.
No, doesn't do anything with the blower.
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Old 07-01-2015, 10:33 PM   #14
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TOO all. Take a look @ MSN.com Great HVAC info from canada people!!
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Old 07-01-2015, 10:46 PM   #15
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TOO all. Take a look @ MSN.com Great HVAC info from canada people!!
HAPPY CANADA DAY!!!!!

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