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charging question on house a/c system

engatwork on Fri June 18, 2004 8:55 PM User is offlineView users profile

I had to replace my indoor/outdoor heat pump a/c system and was watching the tech when he charged it up this evening. The compressor came with a charge of about 6-1/2 lbs (r22) and then he added about 1 lb, 4 ounces while watching the superheat temperature (sensor on the low pressure side at the outdoor unit if I remember correctly). In 50 words or less can someone tell me what he was looking for by charging this way. I believe the superheat temp was indicating in the high 60's when he shut off the charge. He indicated that he will come back by next week after it has run in for a few days and finish charging it. In the meantime it is cooling good.
thanks

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Mechanical Engineer in a paper mill.

MrBillPro on Fri June 18, 2004 11:28 PM User is offlineView users profile

Superheat, is basically the term used to describe the difference between the vapor point or temperature at which the refrigerant evaporates at a given pressure and the actual temperature of the refrigerant exiting the evaporator coil. I try and shoot for anywhere between 10 and 13 degree difference. he probably needs to come back and re-check when the temp. stabilizes in your home that would be my guess. Hope this help you some.

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Don't take life seriously... Its not permanent.

NickD on Sat June 19, 2004 9:41 AM User is offline

Superheat is highly dependent on blower speed and the kind of ductwork that is normally customized for each different home. Plus you have all of those registers and duct valves. I don't claim to have the experience in HVAC like Bill has, I just fool around with my own system. I use the P-T curves more as a guide for the charging pressures with known ambient conditions and try to keep in mind the two worse case extreme conditions. If the system is undercharged, the compressor runs nice and quiet, but you can get evaporator freeze up, but you can also get evaporator freeze up if you evaporator fins are plugged with debris, blower is running too slow, or your filter is filthy. The other extreme is when the compressor kicks on, it darn near jumps off the pad and the power pole bends way over. Ha, that is the way they set up my neighbors system, way overcharged. Unlike MVAC, they seem to lack any fail safe devices, my unit just has a mickey mouse low pressure cutoff switch that will kill the compressor I believe with a total loss of refrigerant, the manual does not say where this cutoff pressure is.

I try to charge somewhere in between these two extremes watching both pressures and listening to the strain on the compressor, I feel it's better to be a tad undercharged than overcharged, but interested in reading other comments. I see no reason to check it again in a week unless you suspect leaks, but I use an electronic leak detector the first time. Normally Joe Weakarm has tightened the fittings that was a cause of a leak when I first bought this house, previous to this, I have always installed my own systems and never had a leak. The schrader valves are also a source of a leak.

engatwork on Sat June 19, 2004 9:42 PM User is offlineView users profile

Quote
he probably needs to come back and re-check when the temp. stabilizes in your home

That was exactly what he said. Keep in mind it was around 90dF when he put the unit in service.

He also indicated that you will rarely see people monitor superheat on these systems anymore.
thanks


-------------------------
Mechanical Engineer in a paper mill.

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