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07-19-2010, 04:43 PM   #16
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that is what is so wierd the liquid line is prety much outside tmep. I checked it several times and it never got hot like I was expection. outdoor temp was 80 when I started and 85 when I was at the 8 pound mark. now the outside temp is still 85 the suction temp is 40.8 @ 68 psi the liquid line is 280 and at 96 degrees. the air blowing out off the condenser is only 88.
6 pounds of freon is not a lot for a 3 ton system is it? maybe it is mislabled or something.

 07-19-2010, 04:58 PM #17 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 34,584 Rewards Points: 8,154 Your jumping around TOO much. How did you get back to 6 pounds. or was that just a general question. 285 at 85 outdoor temp is too high. Take out that extra 1 pound. So your back at 8 pounds of charge. Clean/reclean your indoor coil. Take the indoor temp again. Then if your using an insertion temp probe. Use a small piece of shoe string to cover it. make the shoe string wet. So its a little more then damp, but not dripping water. Insert it in the return at the air handler/furnace while the blower is running. Let it in with the fan running for a minute or 2. Record the lowest temp it reads. That will be your wet bulb. Then restart the A/C. let it run for 15 minutes at least. Then take outdoor temp. Along with your head and vapor pressure. And vapor and liquid line temp. Use ((wetbulb temp X 3)-80-outdoor temp)/2= target SH. EG: Indoor wetbulb 64 Outdoor temp 85 64 X 3= 192-80=112-85=27/2=13.5 High subcool is from overcharge, or restriction in the line set or evap coil. Low SH is from poor air flow, dirty indoor coil, or overcharge.
 07-19-2010, 06:01 PM #18 Member   Join Date: Jun 2009 Posts: 199 Rewards Points: 150 no the factory charge is 6.83 I was just thinking that was really low. All I have is a ir thermometer, but it does have a termcouple on the other end so I could stick that on a shoe string right? At eight pounds the suction line was only 46 psi and 200on the high side. does that sound about right? Last edited by wetbar; 07-19-2010 at 06:04 PM.

 07-19-2010, 06:05 PM #19 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 34,584 Rewards Points: 8,154 Yes. You can stick the shoe string on the thermalcouple. The 6.38 is fine for the first 15 foot of line set. but will make you low on charge because of the additional footage of the line set. IR's are not accurate on line sets.
 07-19-2010, 06:56 PM #20 Member   Join Date: Oct 2009 Location: Nebraska Posts: 2,362 Rewards Points: 1,000 Beenthere, is that formula good for all refrigerants and seer ratings? Thanks in advance
07-19-2010, 07:02 PM   #21

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Quote:
 Originally Posted by Marty S. Beenthere, is that formula good for all refrigerants and seer ratings? Thanks in advance
Yes. R410A follows the same SH as R22(Slide charts I have are both the same SH readings). As far as I know, so does R134A, and R407C also.

Its also based on 400 CFM per ton air flow.

 07-20-2010, 05:45 PM #22 Member   Join Date: Jun 2009 Posts: 199 Rewards Points: 150 Thanks for the superheat formula, I tried to use it today here is what I had and how I got it. This unit was working fine before I started, but wasnt on. The inside temp was 81 degrees. The vapor side was 92 PSI and the liquid side was 305 PSI- right off the bat I thought that was high. The outside temp was a steamy 90 degrees. I stuck my fluke on the low side and measured 57.1 degrees at service port. The high side was 113 degrees. Inside - If I did the wetbuld correct it was 75. to get this I cut off part of my shoe string and ran water over it, then stuck my thermalcouple inside the string. I place it inside the air handler on the intake side about 1/2 inch from the coil. Did I do that correctly? So using your formula from above Use ((wetbulb temp X 3)-80-outdoor temp)/2= target SH. Indoor wetbulb 75 Outdoor temp 90 75 X 3= 225-80=145-90=55/2=27.5 Right? Seems high! using the pt chart - 92 psi is 54degrees so I am at -3.7 superheat. Right? But once again this was all working yesterday, and I left it like I found it, and it appeared to be working. What am I doign wrong?
 07-20-2010, 06:40 PM #23 Member   Join Date: Oct 2009 Location: Nebraska Posts: 2,362 Rewards Points: 1,000 Think about it a bit and reread the posts. Going to be multiple issues so correct one and move on from there. Head pressure too high, subcool too high, superheat too low. What do we do to correct those first?
 07-20-2010, 07:00 PM #24 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 34,584 Rewards Points: 8,154 As Marty points out. Need to look at a system as a whole. And not just a few readings/measurements. Shooting for just superheat. Without watching, and having correct air flow, and subcooling can lead to overcharging, and under charging. How long did you leave your probe in the return. If you don't wait long enough, you get a higher wetbuld reading then it is. The readings you posted indicate a 76%RH in the house/building before you turned the system on. So the condensate drain should have had a large amount of water pouring out of it. Next, If that system is working. Its working overcharged. See my other post for most common reasons systems are overcharged. Even with your subcool. I'd be willing to bet, that the condenser is slightly dirty also.
 07-20-2010, 07:25 PM #25 Member   Join Date: Jun 2009 Posts: 199 Rewards Points: 150 Yeah I thought about that. The house was 80 when I got there. I left it in the air handler for about 10 min, unitl it was stable. I have tried this on 3 systems that seem to be working before I start, and can never get the math work. Im gonne test mine at the house maybe tongiht just to see what SH I come up with. Using the formula what is the highest superheat I would want? I would think below 20? Once again thanks for all you help.
 07-20-2010, 07:38 PM #26 Newbie   Join Date: Jul 2010 Posts: 3 Rewards Points: 10 Go to your local rsc and get a slide chart. carriers chart is easy and works with all brands. It has step by step info and will be easier than what you are doing now.
07-20-2010, 07:54 PM   #27

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Quote:
 Originally Posted by wetbar Yeah I thought about that. The house was 80 when I got there. I left it in the air handler for about 10 min, unitl it was stable. I have tried this on 3 systems that seem to be working before I start, and can never get the math work. Im gonne test mine at the house maybe tongiht just to see what SH I come up with. Using the formula what is the highest superheat I would want? I would think below 20? Once again thanks for all you help.
Had some above 30, more then once. The higher the wetbulb, the higher the SH. And as you see, the outdoor temp has a great effect on what your SH should be also.

Remember, you need to have good indoor air flow. And just because you feel it blowing air hard against your hand. Doesn't mean that the air flow is enough.

The formula, and all SH slide charts are based on 400 CFM per ton.

07-20-2010, 10:01 PM   #28
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Quote:
 Originally Posted by beenthere Its common to find gross over charges. Many times its caused by techs doing PMs/spring clean and tunes while the outdoor temp is cool. They see a low suction pressure and add gas to get it up. After a few years of this. You have twice as much charge in the system as it should have. Other times, its from tech seeing low suction pressure, and they just assume it has a leak, and add gas. When its a low air flow problem, that is causing the low suction pressure.
Had a new customer with that problem today. Unit had not been serviced or cleaned for 5 years. Got everything cleaned up and the subcool was double what the charging chart showed. Reclaimed a bit and the suction pressure went too low even with a high subcool. Did a little snooping around and saw 8x18 return duct for a 4 ton system. Hmm wonder where the problem is there.

07-20-2010, 10:14 PM   #29

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Quote:
 Originally Posted by Marty S. Had a new customer with that problem today. Unit had not been serviced or cleaned for 5 years. Got everything cleaned up and the subcool was double what the charging chart showed. Reclaimed a bit and the suction pressure went too low even with a high subcool. Did a little snooping around and saw 8x18 return duct for a 4 ton system. Hmm wonder where the problem is there.
LOL...

Lots of that crap out there.

And someone always seems to get called out first. And just adds gas until teh vapor pressure comes up. Who cares how high the head pressure is.

07-21-2010, 09:54 AM   #30
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Hey is there some way to measue cfm with the fp H36? Im going to order that today from johnstones. I thought I saw somewhere that it could measure cfm.

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