Is it possible to have sufficient coolant charge, but low split temps?

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  #1  
Old 12-16-12, 11:49 AM
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Is it possible to have sufficient coolant charge, but low split temps?

I'm about to schedule for a service call, but I don't really anticipate having an insufficient charge. Just wondering if there is any situation or condition where the coolant charge is sufficient, but the split temps inside and outside are only in the 12 to 15 degree range?

edit: The return air ducts have been sealed.

Thank you
 
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Old 12-16-12, 03:15 PM
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Yes, there are situations where that can happen. Causes need to diagnosed by a professional.
 
  #3  
Old 12-19-12, 08:30 AM
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Thanks for the reply

I tested this again, though the outside temp was only @ 76. I continue to get an upper average of around 15 degree split between return and supply air with maybe a top reading of 17 degrees. However on the outside unit the max split with the outside temp was around 12 degrees, more consistently 10 degrees. It took nearly an hour to bring the inside temp down 4 degrees and the RH from 63 to 58. I'm going to have it looked at but I'm leery of having another tech tell me it's fully charged and working fine. One thing I have noticed is one of the canisters in the outside unit (Goodman cplt30 heatpump) builds up condensation. I don't thinks it's the main compressor, but I guess it could be part of it. The compressor is a Copeland scroll which I believe is directly under the center of the fan and has a blanket on it. This other "canister shaped" part that sweats has a couple of 1/2 in copper tubes going to it.

Is this sweating normal for that part?

edit: I guess it's the accumulator according to a diagram I found. Is it normal for the accumulator to sweat?
 
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Old 12-19-12, 11:33 AM
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Yes you will see condensation on it. ALSo 4 degrees in an hour is great. Your RH is high.
 
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Old 12-20-12, 09:42 AM
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Thank you.

Hmm. There's my dilemma...I may be expecting too much. It should be an oversized system, but it doesn't seem to keep up during the summer even after I did a lot of envelope tightening. And it doesn't work as well as at least one other same size system I know of that does achieve 20 degree ra/sa split temps with @ 50 rh and brings down the temp to whatever you want. (It's a trane)

Am I expecting too much than the split temps I'm getting? I've had more than one person imply that to me, even a/c techs. Seems to me I should be able to achieve 76 degrees even when it's 95 out, if not lower.

Thank you
 
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Old 12-20-12, 11:49 AM
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If the unit is sized for design temp then yes it should be able to cool twenty degrees below outside temp. But at 96 degrees outside it will never stop running to do this. I do think your delta T is low but it's not that far off. Can you post M# of your equipment
 
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Old 12-20-12, 10:29 PM
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Goodman cplt30 and aruf32.

I appreciate your input, thanks.
 
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Old 12-21-12, 05:03 PM
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Is it possible to have sufficient coolant charge, but low split temps? (Yes, for numerous reasons.)
I'm about to schedule for a service call, but I don't really anticipate having an insufficient charge. Just wondering if there is any situation or condition where the coolant charge is sufficient, but the split temps inside and outside are only in the 12 to 15 degree range?

edit: The return air ducts have been sealed.
--------
There's my dilemma...I may be expecting too much. (You're not expectying too much!) It should be an oversized system, but it doesn't seem to keep up during the summer even after I did a lot of envelope tightening. And it doesn't work as well as at least one other same size system I know of that does achieve 20 degree ra/sa split temps with @ 50 rh and brings down the temp to whatever you want. (It's a trane)

Am I expecting too much than the split temps I'm getting? I've had more than one person imply that to me, even a/c techs. Seems to me I should be able to achieve 76 degrees even when it's 95 out, if not lower.

I tested this again, though the outside temp was only @ 76. I continue to get an upper average of around 15 degree split between return and supply air with maybe a top reading of 17 degrees. However on the outside unit the max split with the outside temp was around 12 degrees, more consistently 10 degrees. It took nearly an hour to bring the inside temp down 4 degrees and the RH from 63 to 58. I'm going to have it looked at but I'm leery of having another tech tell me it's fully charged and working fine.
When there is a higher indoor humidity the temp-split indoors will be less, however, then the outdoor condenser temp-split should be higher, say around +20-F for a 10 or 12-SEER condenser.

The higher the humidity load the lower the indoor split; however, the higher the outdoor condenser split will be; both of your splits are too low; the outdoor split being too low tells me the indoor coil is not absorbing enough heat!

The evaporator coil may not be getting an adequate heat load through it; could be due to low airflow or dirty coil. Also could be problems within the refrigerant system.

Shut power off & inspect the blower wheel blades for lint in the cup of the blades. If it's there its also in the evaporator coil.

Do to numerous reasons, a large percentage of evaporator coils are under heat-loaded.


What large city do you live in or near?
What's the sq footage of your home?
What SEER is the condenser?
Do the tests when it is hot outdoors.
Do you remember some of those readings?
 
  #9  
Old 12-22-12, 04:37 AM
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If you oversized your system then the duct work is likely undersized or filter is undersized. This will cause problems.
 
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Old 12-22-12, 06:01 AM
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Nearly all 10 to 20 SEER systems at 50% indoor Relative Humidity & the nominal 400-CFM per ton of cooling should get an 18 to 22-F indoor temp-drop between Return air & Supply Air.

Example: a 3- ton unit, 400-CFM * 3-Ton is 1200-CFM of airflow through the indoor coil.

The humidity is taken away from the Supply Air & closer to the Return Air grille; SA is very high in humidity, it needs mixing with the room air...

Your system is short on both ends of the temp-split formula.

hvactechfw
If you oversized your system then the duct work is likely undersized or filter is undersized. This will cause problems.
RIGHT-ON!
 
  #11  
Old 12-22-12, 04:24 PM
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Thank you both very much for your input. I understand and agree that appears to be the major symptom.


Tampa, FL is about 20 miles away(hot and humid). The conditioned space is 1100 sq. ft. The system is a 13 seer Goodman cplt 30 heat pump with a Goodman aruf 32 air handler; 2.5 ton system. (Now I would choose a smaller system & just air conditioner) When I tested it in the summer I did not have an infrared thermometer, just a wall thermometer. However, I got about the same split temps for the return and supply. I'm not sure about the outside unit but I can't imagine it was much better since the lack of performance is more obvious during the hotter weather.


The system is most likely oversized but this is what I have to work with and I have done what I can to improve the airflow. I repositioned the air handler(in the attic) to obtain a straight line run from return air plenum to far end of trunk. I replaced the entire return air side as well as added return air capacity. The main return filter grill is 18 x 24 with 14 inch flex to return plenum. I added two 12 x 12 filter grills with 10 inch flex junctioned to 10 inch flex to return plenum, and one 10 x 10 filter grill with 4 inch flex to return plenum. I replaced the take off supply plenum for a better transition to existing supply duct, and I sealed all the ductwork.


Replacing both plenums gave me access to coil and blower wheel and they are in good to like-new condition. There is nothing obviously visible on either of the coils or the blower wheel.


I have experimented with air scoops and they will significantly increase the velocity at the supply grills but it seems this may hinder more than help, especially in the area where the thermostat is located. I'm trying to eliminate all the possibilities that I can take care of myself first and maybe have an idea of what the tech should be looking for. However, I may be at the end of the line as to what I can do as mere diy'er. It sounds as if it may be beneficial to wait until spring before I have a service call done so there is sufficient heat load.


So what else may cause “under heat loading”?


Thank you for your consideration and expertise.
 
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Old 12-23-12, 04:29 AM
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Insufficient airflow causes a high split.

Causes of low split other than charge:
- Blower speed set too high
- High humidity
- Metering device problem (bad txv - not feeding enough refrigerant. A cold suction line rules this out)

High humidity in the return can be caused by return air leaks if the system is in the attic or garage.

A 12F split at the condenser may or may not be okay depending on the amount of air it's designed to move; it's not uncommon for a manufacturer to use the same condenser cabinet/fan configuration for two sizes.

Post the model number of the condenser.
 
  #13  
Old 12-24-12, 09:58 AM
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Post the model number of the condenser.
Thanks for the reply.

Post 7 & 11; Goodman cplt30 and Goodman aruf32 air handler.
 
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Old 12-24-12, 09:04 PM
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Spec sheet: http://johnstonesupply9.com/TechDocs...%20SS-342D.pdf

It's a 13 seer heat-pump; unfortunately, the condenser cfm isn't published in the spec sheet.

Until recently, 10 seer was the standard.

The amount of heat rejected by the outdoor unit is equal to the energy consumed by the compressor and heat removed from the house.

Hence, more efficient units (such as yours) reject less heat than older 8-11 seer models, so the split might be okay.

The air handler is rated for 2.5 tons (http://www.dnmech.com/lit/PDB_ARUF.pdf), so chances are that it's not moving more air than it should.

---------------------------------
There's a somewhat time consuming test you can do to determine how much heat your heatpump is removing:

1. If your heatpump was installed with electric backup heat, determine what size the heat kit is.

Turn the heat on (emergency heat without heatpump running) and measure the split at the air handler after 10-15 minutes.

Plug the split into this formula to calculate the amount of air the air handler is moving:

CFM = (heat kit kw x 3400) / (1.08 x delta-t)

2. Turn on the a/c and wait 15 minutes.

Wrap the end of a digital stem-type thermometer with a thin, wet (not dripping) piece of fabric.

Measure the wetbulb (takes heat and humidity into consideration) temperature of the return and supply air at the air handler.

Determine the enthalpy of the return and supply air using this chart: https://www.consumersenergytradeally...%20Chart_0.pdf

Plug enthalpy and cfm into the formula...

Capacity in BTUs = 4.5 x CFM x (return air - supply air enthalpy)

If all is well, it should be pulling out 30 000 BTUs/hr give or take 3000 BTUs. (27000 - 33000)
 
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Old 12-24-12, 09:12 PM
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Measuring the return air wetbulb temp as close to the air handler to account for leakage is very important.
 
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Old 12-26-12, 09:14 AM
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Whoa! Thanks for the reply. I'll do that test soon and report what I find here. The units are over 8 years old, I didn't think they would be that efficient as to make the split less. Sounds backward; as if it were more efficient, the outside unit split temps would be higher, removing more heat. Go figure.
 
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Old 12-26-12, 11:08 PM
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Efficiency actually has no bearing on capacity.

The electricity consumed by the compressor gets converted into heat which gets absorbed by the refrigerant (prior to going through the outdoor coil + cooling down); the higher the SEER rating is, the less electricity is needed to do the job.

SEER is BTUs per watt over the entire season (at a low outdoor temp), while EER is BTus per watt at 95F outdoor/80F indoor. (cooling capacity divided by wattage)


Typically a 10 SEER unit has an EER of 9, and a 13 seer model has an EER of 11.

So to pull out 30 000 BTUs per hour in hot weather, a 10 seer machine would have to reject roughly...

30k BTUs per hour + (3333 watts x 3.4) = 41 333 BTUs / hour outdoors...

...while a equivalent 13 seer / 11 eer model would have to reject only...

30k btu/hr + (2772 watts x 3.4) = 39 273 btu/hour outdoors.

The more efficient outdoor units also may move more air relative to capacity, contributing to a lower split across the condenser.
 
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Old 01-13-13, 09:04 AM
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Hello and Happy New Year.

I finally ran the test, which produced good results I guess, but I'm not sure I like the answer since it doesn't leave me with any way to improve performance and makes it seem I may have wasted some time and money on the improvements I did make.

With only the 5kw strip running, the ra was 79 and supply was 98.1
With a/c: the wet bulb ra was 62.9 and supply was 51.1

My calculations:
cfm: 824.122
btu: 28,110.801

So it appears to fall within the range you gave but of course the cfm is about 1/2 ton less than it should be. The split at the grills was 17. It appears this may be the best I'll get out of these units, which while is in btu range, in my opinion is sub par at best. Especially when considering the humidity removal. The only thing I can do is experiment with the air scoops at the supply boxes, but after doing this test I don't see it helping with the humidity either.

I could have it checked by a tech but I don't see what they're going to be able to do aside from checking the charge which would appear to be "in range".

Thanks again for all the help and info. Any other suggestions are appreciated.

Edit: Something I've wondered about, but disregarded; Would the size of the electrical wire(s) running to either unit have anything to do with these conditions/results?
 

Last edited by brimac; 01-13-13 at 09:31 AM.
  #19  
Old 01-13-13, 02:38 PM
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This Goodman pdf shows the actual performance of the heat pump; right click data & turn the data sheets clockwise to read the data; click the + sign to fill the window area. Eliminate the left side for enlarged area for the data.

http://www.goodmanmfg.com/Portals/0/pdf/SS/SS-GSZ13.pdf

Look-up the 2.5-Ton 13-SEER performance data. Goodman has this good performance data on all their equipment.

You can use a R-22 or 410A pres/temp chart to convert pressures to temperatures...

At 75F indoors & 63-WB or about 50% RH; the SA/RA temp-split drop is listed at 19-F.

A 1.5-Ton might have handled the cooling or, 2-Ton at the most for 1100-sf.
 
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Old 01-15-13, 03:32 PM
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Thank you for that information. If I'm reading that chart right, if the cfm was where it's suppose to be and assuming my older unit would perform similarly, the split should be somewhere near 22 as it was around 80 outside and the return wet bulb recorded was 62.9. I would sure like to get that type of result. I agree a smaller unit should be all that is needed for the space. I think the original was likely a 1.5 ton all in one unit back in the 1970's. I just hope I can find a tech that will know what to do to attain at least near those results.

Edit: However, I don't think I am reading it right. The ra temp at the filter grill was 69.5 and outdoor temp was @80
 

Last edited by brimac; 01-15-13 at 04:11 PM.
  #21  
Old 01-15-13, 07:36 PM
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At 80-F dry bulb outdoors; if the indoor dry bulb was 80-F, then at that 62.9 nearly 63-wet bulb temp, then around 22-F would be correct.

That number will vary some with different make units & it is rare to get a perfect install & setup; perfect duct system, no Return or supply air leaks, properly insulated, etc.

However when things are setup right, with those exact numbers data it ought to be close; not far from that figure.
 
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Old 01-15-13, 10:35 PM
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The numbers don't look right - to remove 28k btu below 900 cfm, the split should be higher.

How close is the filter grill to the air handler?

Measurements need to be taken at the air handler itself. Even an insulated return air duct can pick up a lot of heat.
 
  #23  
Old 01-16-13, 08:56 AM
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Hello. The largest filter grill (18 x 24) is about 20 ft away, and the two other (12 X 12 each) filter grills are about 40 - 45 ft away from the air handler. I took the temps about 18 inches into both the supply and return plenum for the heat and a/c run-times. ie. @ 18 inches from each end of the air handler

I thought of it after the test, but I didn't take dry bulb temps at the air handler during the a/c run-time. I understand what you're saying about the duct, but it isn't even close to what it gets like in the summer.

I'm not sure if this applies, but for clarification, the inside temp of the house was not 80 like outside. The house cools down overnight and during the a/c run-time it came down to around 72 inside.

I'll go over my notes and recalculate the numbers.

Edit: during 5kw heat run time: at the air handler: return 78.8, supply 98.1
wet bulb during a/c run time: at the air handler: return 62.9, supply 51.1
temp gun at the return and supply grills: return 69.5, supply 52.5
 

Last edited by brimac; 01-16-13 at 09:14 AM.
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Old 01-16-13, 09:30 AM
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"Edit: during 5kw heat run time: at the air handler: return 78.8, supply 98.1
wet bulb during a/c run time: at the air handler: return 62.9, supply 51.1
temp gun at the return and supply grills: return 69.5, supply 52.5"

The infrared gun will read way below the actual air temps!
 
  #25  
Old 01-16-13, 12:51 PM
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ok I just ran the a/c to get a dry bulb at the air handler. Now I feel like when I go to the auto mechanic and think something is wrong with the car but the car isn't making the noise I heard. Today, I'm actually getting 19 degree split at the grills. Today is almost the same kind of day I ran the test the other day.

outside temp: 82 F, 66 rh
indoor temp: 74.5 F, 66 rh

ran air for 1/2 hour

AT THE AIR HANDLER dry bulb
ra: 69.8 F
sa: 53.9 F

At the return and supply grills(infrared gun)
ra: 71.5 F
sa: 52.5 F

end temp/rh: 73 F, 61 rh
 
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Old 01-16-13, 07:04 PM
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The infrared gun will read way below the actual air temps when checking heat temp-rise at registers; it is not accurate by pointing to metal areas, etc.
Use a digital air thermometer.

end temp/rh: 73 F, 61 rh
That is about a 9F drop or 64F wet bulb.

Therefore, look that up on the Performance Data pdf; you could use 75F DB Temp at 63-F wet bulb, the difference between that & 70F between 59 & 63-F wet bulbs.

Looks like around 16-F temp-drop.

AT THE AIR HANDLER dry bulb
ra: 69.8 F
sa: 53.9 F
That is 15.9 or 16F close to on the money!

I'd forget the other infrared gun readings; it won't be getting 19F drop, even with somewhat low airflow; humidity level is the biggest temp-drop variable.
 
  #27  
Old 01-16-13, 07:13 PM
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What was the return air wetbulb at the air handler?

------------------------------
If the humidity reading was accurate...

66% rh at 75F = 63F dewpoint

r/a 69.8F at 63F dewpoint = 65F wetbulb.

You should have a s/a temp of 55F, a 15F split - sounds like your system is operating properly.

The airflow might be significantly higher than what was calculated; if not, there is a problem -> split should be higher.

------------------------------

High humidity seems to be the problem; the more moisture there is in the air, the less actual (sensible) cooling gets done. Condensing moisture on the coil is energy intensive.

What kind of house is the system cooling? (size, number of stories)

In hot weather, the indoor humidity shouldn't exceed 55% (50% is ideal), so either too much moisture is getting into the house, the a/c isn't dehumidifying as it should, or it's oversized.
 
  #28  
Old 01-16-13, 10:30 PM
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Muggle & HVAC RETIRED, I can't thank you enough. This has become a real diy learning experience thanks to you.

I didn't do the wet bulb today, but when I did the suggested test the other day, the wet bulb was 62.9 F and supply was 51.1 F. The weather and indoor conditions were very similar today.

I agree it must be a moisture problem and it is more than likely oversized. So I guess the question would be: How do I determine whether it's removing the optimal amount of moisture it should be capable of?

If I understand the logic of it, being oversized, if I crank the temp on the thermostat down, eventually the rh will come down to 55% to 50% and the temp will drop to whatever it is at that point. The heat of summer ended before I finished my improvements so I haven't tested it in those conditions yet. But when I did run it to test, my experience is the temp begins to stall say around 76 F (when it was upper 80's outside). Am I flawed in my thinking here?

The house is a single story ranch, block construction with lower than normal ceilings; @ 1100 sq ft conditioned space. This has always lead me to believe it should cool down pretty well. I know of one house in the neighborhood that's the same model, similar size, and previous owners told me their a/c worked great. It did appear to be a smaller outside unit, maybe 1.5 ton Trane.
 
  #29  
Old 01-17-13, 04:05 PM
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Overall, if the air handler is indeed moving 824 cfm, something is wrong.

If it's moving the 1000 +/- cfm it should, the problem isn't with the a/c itself -> leaky or poorly insulated ducts can reduce capacity dramatically.

Smaller single story houses without much if any insulation in the walls require more cooling per sq ft than more modern two story homes; however, a 2.5 ton unit shouldn't have any trouble maintaining the setpoint when it's below 95F outside.

------------------
Note: Low airflow itself reduces capacity and throws off refrigerant pressures. Your system may have been overcharged to compensate for insufficient airflow, which further reduces capacity. A tech who only measure pressures and not refrigerant temperatures is likely to misdiagnose the problem.
 
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