AC ? - DF

Dan Foley

Eugene,

We have not been back to this job yet. I'll keep you posted. -DF

<A HREF="http://www.heatinghelp.com/getListed.cfm?id=305&Step=30">To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"</A>

Dan Foley

Insufficient Cooling

One of my techs was on a call yesterday for insufficient cooling. The system is a five year old Bryant 12 SEER system (597CNX030) with matching Bryant TXV coil. Here are his readings:

96* ODA

83* Return air temp

62* supply air temp

79 PSIG @ 76* SLT

285 PSIG @ 89* LLT

30* SH

37* SC

Compressor drawing 11.4A (Rated 15A FLA)

When the weather is hot, like yesterday, the system cannot keep up and the temperature starts to creep up. During milder weather it is able to maintain temperature. Any suggestions? -DF

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Eugene Silberstein 3

Suctin Line Temp?

Dan,

Where is your technician taking the suction line temperature reading?

For example, if he is taking the reading right at the outlet of the evaporator coil, the superheat is way too high and the evaporator is starving.

If the sucitonline temperature is being taken at the condensing unit and the line set length is relatively short, you need to check the sucitnline insulation, as the 29 degrees of superheat could definitely stand to be lower.

Your evaporator saturation temperature is right on, given the return air temperature of 83 degrees.

High superheat and high subcooling is a textbook sign of a liquid line restriction, which could be caused by a damaged liquid line, blocked liquid line drier, or an underfeeding metering device.

Once again, find out where the SL temp reading was taken and the condition on the SL insulation and we should have an answer here.

Thanks for the post.

Techman

Not enough cooling

Hi Dan.If that liquid line temp is accurate I'd go for a plugged filter /drier;crimped liquid line ,service valve related,or a sneaky one,a factory installed filter/drier down in the bottom of the cond unit.I think the head is a little hi for a 12 SEER. Has the cond unit been washed lately ?

don_156

!

Jim Bergmann_2

A/C Problem Non Condensibles

Given the information, it appears that the evaporator is working properly for the load, which also means the evaporator airflow is correct. It also appears the TXV is operating properly because the suction pressure is correct. (superheat is high) I am guessing the technician is measuring evaporator superheat and not total superheat as the superheat is much higher than it should be if it was measured at the evaporator outlet.(8-12*) Given all this, I would next pump down the system and check for non-condensables. If there is air or nitrogen or 410a in the system the condensing pressure will be higher following Daltons law of partial pressure, indicating subcooling when in reality there may be none. Close the liquid line valve and pump all of the refrigerant into the condenser. Next disconnect the compressor and allow the condenser fan to run until the air temperature in and out of the condenser are the same. The refrigerant is then saturated, and the temperature pressure relationship will hold true. If the air temperature and the saturation temperature do not match, the problem is non-condensables. I would guess air in the system. If the subcooling was 37* the condenser would be flooded, the excess liquid in the condenser would effectively make the condenser smaller, the amp draw would go UP.

A little 410a in a charging hose would cause the same symptoms as air.

If the dryer or liquid line were plugged, the suction pressure would be low because you would be starving the evaporator.

If the superheat was measured at the evaporator outlet, you most likely have non condensables/air and low charge

Thanks
Jim

don_182

?

How can he have non-condensible and it not effect the lowside?

I mean he's got a 21 degree delta T across the coil and as
mention above his saturated temps seem to fall in line with his entering db temps.

I'll say it time to calibate his guages and double check the temps in and out of both hx.

Also if I may add....that it appears he pretty close to capacity or slightly over what the system can do.

One has to keep in mind the system capacity is reduce by 853 btu for every degree below 80.
So in short if or when the system was design and they did not compensate for any setting below 80 degree then I can see why it would be having a hard time with a high load.

Please post new test numbers.

Opps..rated capacity is reduce by 853 btus sensible capacity for every degree below 80 per 1000 cfm.

Eugene Silberstein 3

Yeppers Jim

Excessively high subcooling can be an indication of non-condensables in the system.

In addition, an (evaporator) superheat of 29 degrees is an indication of an underfeeding metering device. As we both mentioned, it all depends on where that suction line temperature reading was taken.

Jim Bergmann_2

Non Condensibles

> How can he have non-condensible and it not effect

> the lowside?

>

> I mean he's got a 21 degree delta

> T across the coil and as mention above his

> saturated temps seem to fall in line with his

> entering db temps.

>

> I'll say it time to

> calibate his guages and double check the temps in

> and out of both hx.

>

> Also if I may add....that

> it appears he pretty close to capacity or

> slightly over what the system can do.

>

> One has

> to keep in mind the system capacity is reduce by

> 853 btu for every degree below 80. So in short

> if or when the system was design and they did not

> compensate for any setting below 80 degree then I

> can see why it would be having a hard time with a

> high load.

>

> Please post new test

> numbers.

>

> Opps..rated capacity is reduce by 853

> btus sensible capacity for every degree below 80

> per 1000 cfm.

Jim Bergmann_2

non condensables

The reason non-condensables will not effect the low side is the TXV; the valve will work to control the load no matter what the high side is doing. As long as there is enough refrigerant in the system for the valve to operate, the valve will do its job and control the load. Non-condensables stay in the condenser during operation and take up condensing space, raising the head pressure. They do not circulate. If you had a fixed metering device the low side would be affected. The TXV will just shut down further as the pressure goes up on the high side of the system.

To calculate capacity you must reference wet bulb temperatures. What you are stating about capacity would have to be at a constant humidity and outdoor air temperature. As the outdoor temperature increases, and the indoor wet bulb decreases the capacity of the system decreases. But if the indoor wet bulb decreases and the outdoor temperature is low, the system can still operate at full capacity.

Here is some data from a Rheem cooling table, RAMC-030 J*Z + RCHA-36A1 TXV coil

Low airflow

75 ODA 63 IWB*F 850 CFM = 31400 BTUH

105 ODA 71 IWB*F 850 CFM = 31400 BTUH

Normal Airflow

90 ODA 67 IWB*F 1000 CFM = 31500 BTUH

115 ODA 71*IWB 1000 CFM = 31500 BTUH

You must use wet bulb because the dry bulb temperature at lets say a 63 IWB from the Psychrometric chart could range from 63* at 100% Rh to 105* at 8% Rh or anywhere in between and have the same total heat content.

Dan Foley

Info

Thanks for the comments, everyone. Sorry for the delay - I was out of town for a couple of days. The suction line temp was taken at the outdoor unit. The line set is approximately 45' long. The condenser coil was clogged with lint but these readings were taken after a chemical cleaning. I'll keep you posted. -DF

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

blackoakbob_2

excessive subcooling....

what would you consider excessive? I've just been working on a system that has been "an exercise in refrigeration basics" each time I've been called out. I'm recalling the following of the top of my head so....
Outdoor temp 89F
head press 290#
suction press 53#
superheat @ txv bulb 49F
TD across condenser 80F
TD across the chiller bundle 8F
The repair that was done prior to startup was a compressor replacement due to broken disc and suction valves causing damage to a piston ( bolt thru the top of the piston) and replacement of Txv power head, liquid line filters changed, and contactors also replaced.
Thanks for your input. Best Regards

Eugene Silberstein 3

Check that Suction line insulation!

Check the suction line insulation Dan.

Even if the TXV is maintaining 15 degrees of superheat in the evaporator, you are still adding 14 degrees to the refrigerant in the suciton line.

This will cause an unnecessary increase in the HOC of the sytsem, which is a MAJOR component of the systems coefficient of performance (NRE/HOC).

The COP is directly related to the EER, and a 0.3
decrease in the COP translates to a full 1-point drop in EER.

Eugene Silberstein 3

Sounds like a nightmare, Bob

Sounds like a nightmare, Bob.

I hope all systems are not like that in MN.

I still ike to shoot for 15 to 20 degrees of subcooling and would call anything over 25 degrees excessive.

Dan Foley

Suction

Eugene,

The suction line runs exposed up an outside wall and 25' across a tar and gravel roof. Most likely, it is picking up considerably heat from this exposed piping. Thank you for the help. -DF

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Eugene Silberstein 3

B-I-N-G-O

BINGO.

I love my job!

Eugene Silberstein 3

How's that System Runing, Dan?

How is that system doing, Dan?