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Are these AC pressures good?

Posted by madtexan (My Page) on
Tue, Aug 30, 11 at 22:09

I have a small 17 yr old 1 1/2 ton Carrier central AC system for our master bedroom. IT's never really had any repair work until this past summer when the condenser fan went out. Not a big deal and I fixed it with an OEM replacement. It worked fine for a week and then I noticed that the unit wasn't keeping up in mid afternoon heat with thermostat set on 79. ONce the sun goes down it cools fine again. I know I could try to cover it with s sun screen but it was fine before. I checked the pressures tonight but it was after the outside temps and sun dropped. The low side is 65 psi and the high side is 240 psi. Would this indicate I need to add a pound or two of R22 or is the compressor getting weak?


Follow-Up Postings:

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RE: Are these AC pressures good?

sounds low on freon probably needs a good cleaning inside and out.


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RE: Are these AC pressures good?

I know the outside coil is very clean since I cleaned it when I replaced the fan motor. I'll check the inside as well tonight. I'm also going to measure the temp coming out of the AC vent.

Regarding adding more freon..... I'm afraid it would push my low side pressure up to high. My limited knowledge on this is that it whould be in the 45-50 psi range.


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RE: Are these AC pressures good?

Charging a system is best left to a trained person because there is a lot more involved than just adding gas, not to mention you must be certified (very steep fine if caught). However, I know you are going to try anyhow so I'll make a few suggestions to make sure you don't damage your system or hurt yourself, just be safe and at least wear safety glasses. I'm going to assume you purged your lines before connecting to system to check the pressures. If you didn't, you already may have a serious problem with the system and no need to go further until that gets corrected. After you have made sure the coils are clean, you are ready to charge the system.

That sized unit most likely has a cap tube metering device. That means the refrigerant charge is critical. AN OVERCHARGE WILL DO MORE HARM THAN GOOD. Your unit won't cool and you may damage the compressor reed valves and also cause an electrical overload so, caution must be taken not to add too much refrigerant. The pressures you show indicate a low charge, not much but, low enough if indeed the unit has a cap tube. Your pressures will also vary according to temperature. So when charging the unit, the outside temperature will be one factor to take into consideration. With a thermometer check the temperature differential at the evaporator coil as you are charging, you want a 15-20 degree temperature differential.
Manufacturers provide a temperature/pressure chart and most indicate if the outside temperature is between 85 and 90 degrees your suction pressure on that unit should be around 68 psig, if using R22. This is very general and without all the other considerations a trained technician would employ, I think you'll be satisfied with the outcome if you don't go over 68psig. Keep in mind, this is a long distance assessment and the unit may not be at it's full efficiency but it will cool for you.

So you'll be adding 3psig, NOT 3 pounds by weight, of R22, very little. This is only a few squirts of gas, not much but enough to make a difference. It must be added on the low side as a gas, NEVER TURN THE CAN UPSIDE DOWN because that will introduce the refrigerant as a liquid, causing damage. Not knowing your abilities, I suggest you not connect to the high side while charging. If done incorrectly, you can really hurt yourself.


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RE: Are these AC pressures good?

Thanks Maryland,

That's exactly what I was looking for. I understand how to charge a system and have done my cars from time to time over the years. I'm doing some extra leg work on this unit because I'm not ready to replace the system and I just wanted to be informed before I call someone out in case they tell me I NEED a new system.... and I don't have any R22 so I couldn't charge it myself. So I will hire a pro :)

Something I didn't mention is that when the unit falls behind is when the outside temp is about 105 and add the sun is hitting the condensor. I just measured the temp coming out at a vent within 6ft of evap and it's 68 at 10PM, outside temp is still 90, intake temp is at 80. 12 degree drop which is weak.

SO just for my knowledge, if the air temps are higher, does the expected pressures increase or decrease?


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RE: Are these AC pressures good?

The higher the temp, the higher the pressures. Still, in your case, if a professional does it at 90-100 degrees, you may see the suction pressure around 70 psig. After that, any lack of cooling will be due to the unit not being able to keep up with the extreme temperatures. That would be due to design temperatures, insulation and unit sizing along with a few other conditions (duct sizing, fan speed, etc).

I do have a concern. If you don't have any R22, how did you purge your gauges? This is a most important step and if not done, you have already caused problems. The unit will need to be evacuated and filter dryers added to resolve the problem. The most minute amount of air, such as those from un-purged gauges will also introduce moisture into the system. This will cause a blocked cap tube (from ice). In addition, the longer you run the system, the air and moisture will react with the oil developing acids that will destroy the compressor. Due to the age of your unit, the insulation on the compressor is aged. Refrigerant by it's nature is a solvent.. Combined with moisture and oxygen, it is very caustic. These acids can clean the insulation off the windings and short the compressor causing what is known as a burn out. Correcting a burn out is a very lengthy and expensive process and on your unit, it would certainly be most cost effective to replace everything, lines and all.

I don't want to be an alarmist but this is a very real issue so, I hope you did properly purge your gauges. If you did it properly, and from what I have read so far, your unit most likely only needs a small refrigerant charge. That's not to say something else (due to age) won't go wrong but as it stands, the unit shouldn't need replacement, as long as there is no issue regarding the purging.


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RE: Are these AC pressures good?

I see the concern and need to purge the lines when adding coolant. My understanding is that for contaminants to enter the system they have to be introduced with a higher pressure than what is in the system... thus causing an inward flow of whatever is pressurized. This would mean the low side would have to have a pressure over the 65psi for it to push into the system. With nothing connected except gauges the only pressure is outward from the system, not inward from an external pressurized source like a pressurized tank of R22. Am I missing something here?


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RE: Are these AC pressures good?

Yes, you can still contaminate the system. Another thing in your case, air in your manifold set would also falsify your pressure reading. Your charge pressures could have been lower than your gauge readings. I think you will be more careful next time. I've never done this because I would normally have the refrigerant I need to purge my lines but, I know some have connected through the high side and purged their gauges that way before connecting to the low side. I discourage homeowners from ever connecting to the high side. The pressures are high enough to inject refrigerant and oil through the skin. A good Technician will generally have up to par equipment, especially hoses that won't burst.


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RE: Are these AC pressures good?

You did set that fan blade at the same position it was in relative to the fan shroud as the original? I am just asking because if you set that blade too high or too low on the blower shaft you will alter the air flow across the condensor. With a fixed metering device that will throw both your pressures off.
I know it has worked fine in the past according to what you have said. I will point out that if you are moving too much air across the evap coil you will not get much drop. An extremely high probability with a 1 1/2 ton unit considering most furnaces are sized for 3 tons or more. With great duct work even low speed often delivers more air than 1 1 /2 tons can bring down.


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RE: Are these AC pressures good?

The shaft on the new motor wasn't quite as long as the original. With that being said it might be sitting about half inch higher. The fan now sits closer to the motor body than the original. I guess I could put some spacers on the motor mount to lower it a bit and see what happens. It is stull pullin plenty of air. I noticed that the high pressure line isn't very hot and the low pressure line isn't very cold. There isn't even any condensation on the cold line. Can the fan cause this?


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RE: Are these AC pressures good?

Threecats brings up a good point, I was just concentrating on the pressures and didn't think to give the fan replacement a consideration. Although that will affect your pressures and ultimate cooling efficiency, you should still get some condensation. Something else that happens with those smaller sized units.....If you have to open the casing to get to the service valve, sufficient air is not being pulled through the condenser coil while it is open. I remember a situation I had some years back with that. While I was charging the system, I noticed the pressure was rising and I wasn't even introducing refrigerant. When I closed off the service panel as best I could with my gauges still connected, I noticed the pressures dropping. A gas and go guy might get the pressures he's looking for and close up, thinking he is finished. Then he has to come back because of a customer complaint and see's he has to add more refrigerant, assuming there is a leak, without actually looking for one.


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RE: Are these AC pressures good?

OK, I finally hired a pro and $145 later it is fixed. ($69 service call, $75 R22) It turned out that the reason for the weak cooling was a few things. The new fan is 150 rpm faster than the original. The position of the fan is perfect even though it is slightly higher than the original. He added less than 1 lb of R22 and now I get a 20 deg drop in temp at the vent. He said the faster RPM fan is pulling more air over the coils, not a bad thing but it knocks it off the optimum balance with the amount of coolant that was in it. He said it didn't have a pressure cut off switch so when the fan died the compressor just kept building up pressure and It may have bled off some coolant at one of the seals or valves but we couldn't tell.

Thanks for your help guys.


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