why is the suction line so cold? - long... - http://www.hvacmechanic.com/ Forums
Original message
| stevescivic
 |  "why is the suction line so cold? - long..."
, posted Sun 6 Jul 04:56  
okay guys, so this thread IS directly related to a thread that I started about my dad's home a/c system.
Now I have not screwed around anymore with taking any more measurements on his a/c system as I'm sure I've already obtained as much data that I can personally record.
I was thinking all day yesterday why the superheat on his system is so low compared to the Trane charge chart. I've looked all over the Internet and it would appear that my suction line temps could potentially be out of whack. Why do I say this you all might ask?
The data again gathered:
All pressures and line temps are measured at the service valves on the condensing unit. System stabilized after running for 20 minutes.
System size: 36, 000 BTU
Age: ~4 years
Fixed orifice size .073
Air handler: 18 year old Lennox G8 on high speed. My best guesstimate on CFM is probably 1000 CFM
Evap coil is CLEAN
Condenser coil is CLEAN
Air filters are CLEAN
Indoor ambient dry bulb: 75F
Indoor RH: 39%
Wetbulb (using online converter): ~59F
Outdoor ambient: 85F
Low side suction pressure: 63PSI (that's about 36F evap temperature)
High side discharge pressure: 220PSI (108F on the gauge's temp scale)
Low side suction line temperature: 43F
High side discharge line temperature: 93F
Based on the trane superheat chart it clearly states a minimum limit of 5F superheat. Upper limit of superheat is in the realm of about 12F. No mention of subcool in the context of fixed orifice application.
Based on the info listed above it would seem like there is roughly 7F of superheat and a subcool of about 15F.
The consensus that I got from the original post that I wrote a few days ago suggests that the system is either charged correctly or possibly overcharged a little.
My own curiosity is clearly getting the best of me and the things that I want to know (for the sake of learning now) is:
1. If the superheat is 7F and so close to the minimum limit then could the evap coil freeze up fairly easily b/c the evap temp are so close to freezing?
2. Although superheat does vary with heat load would superheat EVER approach zero in this situation? now I would assume that as suction pressure rises (due to heat load) that the suction line temps also proportionately increase to maintain a fairly steady range of superheat?
3. How does one determine what a reasonable subcool figure? Even though the system I was examining is a fixed orifice system there wasn't any clear documentation on the Trane charging papers to tell you what subcool should be. I've ready on some sites that 20F is ideal and other places I've seen say 5-10F is ideal.
4. The air handler is clearly very old but well maintained however, the actual CFM rating of the handler is something that I haven't the slightest clue on figuring out. Is there a tool out there or some means of being able to calculate CFM? Some say that BTU rating of furnace as well as motor HP rating can determine CFM however, I do not understand how that can determine CFM flow. What i can say is that the furnace filter on the furnace is a 16x24" filter so its clearly not very big so presumably a unit that doesn't flow a heck of lot of air. In fact when the A-coil was installed on the furnace it was about 1/2" too long and a custom coil box had to be made up to fit the furnace's small top.
5. When should a/c NOT be used? The consortium of when the a/c shouldn't run is when outdoor ambient is less than 65F. I'm assuming that the evap coil can and will ice up at that temperature.
6. I measured temperatures and pressures on various days with various outdoor temps ranging between 75F -85F and strangely the suction pressures and temps seemed to be the same. The only differences was the head differed based on outdoor temp variances. For example head pressures ranged between 190-250 PSI whereas suction always seemed to hover at around 60-63 PSI. Why is this a concern? Read point #2 above. If suction pressure ever went up with heat load then superheat theoretically could go below 5F (if suction line temp remained at 43F all the time) which is REALLY bad for the compressor.
Forgive me for all the questions. I ask b/c I want to learn so that next time a HVAC contractor installs A/C in my house that I know what kind of good questions to ask rather than asking a bunch of questions that I could have figured out on my own. Remember in my area there *appears* to be no truly qualified person that can answer simple question such as superheat/subcool and Manual J calculations. Residential a/c contractors only up until a few years ago were rare (it never gets hot where I live but b/c of global warming many have popped up over night). I have little trust in anyone installing or service anything that has to do with a/c.
Everyone's feedback as always is appreciated,
Thanks
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Replies:
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| mississippi
| "Re(1):why is the suction line so cold? - long" , posted Sun 6 Jul 17:46
you have a mismatched system so the trane charging chart not going to be of much use.the unit seems to be working fine by the info you gave.the lower the indoor wetbulb is the lower your SH will be.with a fixed orifice you charge by SH only.
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| | stevescivic
| "Re(2):why is the suction line so cold? - long" , posted Sun 6 Jul 18:28
hmm so low wetbulb means dry air/low humidity meaning that there is not a lot of latent heat in the air hence why the suction line is so cold resulting in a low SH?
So lets say for example that the humidity goes up to say 55% inside the home and its really hot out. Would this mean that the suction pressure would remain relatively the same but the suction line temp at the compressor would rise as more heat load (in this case latent heat) is put on the evap coil?
As I mentioned in the original post it seems like no matter what the temp/humidity level is the suction consistenly hovers around 60-64 PSI.
Thanks
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| | mississippi
| "Re(3):why is the suction line so cold? - long" , posted Sun 6 Jul 21:06
according to copeland charging chart if ambient is 85 degrees and you have a wetbulb of 59 degrees the charge on that system couldnt be any better.
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| | stevescivic
| "Re(4):why is the suction line so cold? - long" , posted Mon 7 Jul 18:32
hmmmm. I thought charge charts were very specfic to each unit.
Oh well, Trane or copeland they all do the same function!
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