superheat v.s. temp difference

[khc]

Which is more important to achieve when charging a central air conditioner temp difference or superheat. I was always taught to achieve a 20 F difference. But recently I seem to be achieving my superheat but only getting a 14 to 16 F temperature difference.

[BaldLoonie]

Get your superheat right on a fixed meter. 20 degree drop is great but hard to get sometimes. If it is humid, it may be impossible to get. I have readouts for mine so easy to check. I've seen as low as 11 when the house has been open in humid weather to 22 in very dry weather. If your charge is right and drop low, if the house isn't a sauna, check blower speeds. Most installers leave cool tap on high regardless of whether it is right or not.

[docholiday]

Temperature drop is overrated as a troubleshooting method and should not even be considered in charging. You need to know the airflow before you can check either and unless you are going to perform an accurate capacity check, you are chasing your tail with the temp drop thing.

[derrick1623]

in this day and age, i feel that superheat and subcooling are the only accurate way to charge a system.

correct me if i am wrong, but most of the tech lines i call when i get stumped do this as well. i believe that within a few years, the manufacturers will weed out the would be "hacks" by making it more accurate charging, as is the process with 410a.

[MikeJ]

For what it is worth, I like to take the TD on a residential sometime during my visit, preferable before putting on guages. Air flow, turbulence, filter, taking in the wrong spot, wrong motor speed etc can all affect the reading as can a wet coil. How about taking the TD just after start up? That would not be accurate. As I said, I like to take it but I don't judge the system by the reading. If the TD is abnormally high or low it can tell me something tho. I'm down at the furnace anyway checking the condition of the filter, type of, and whether it is a cap tube or not and if the sensing bulb is insulated and mounted correctly.

SH and SC as stated tells you the most.

[bornriding]

I use temp. diff. as one of the factors to look at to determine proper charge...
Only I use the differance in temp. between the return air temp. and the suction line temp ( taken at evap,if possible). This gives me a delta 't' that takes into account humidity. ( no mater what humidity - the suction line temp should be the same ).
I use this along with SH & SC to isure that I have charged as best I could.

[faith]

In our shop we use the Manufactors specifications for the particular unit in question. Since we are a TRANE shop we follow TRANE's data. Our local distributor holds two day training sessions on the aforementioned that covers the entire process in depth. If you have an opportunity to attend vendor training GO FOR IT! You'll be glad you did...

Again, use the method that your unit's vendor publishes, make sense? BTW: It come packaged with the unit <grin>

Also take the time to review

http://www.hvacloads.com/talkpdf/tro..._superheat.pdf

It is short and cuts to the chase...

Hope this helps...

[khc]

Installed a 2 1/2 12seer unit today, got the superheat where I wanted it and the compressor head started to sweat. Danger....Danger. Checked for correct orifice. Thought about valves .....scroll compressor. High side pressure seems low but I haven't had any experience with these higher seer units yet. 60/150 about 34 C here.
Thanks for any help.

[khc]

huh

[mdman]

Originally posted by docholiday
Temperature drop is overrated as a troubleshooting method and should not even be considered in charging. You need to know the airflow before you can check either and unless you are going to perform an accurate capacity check, you are chasing your tail with the temp drop thing.

What's your method for "capacity check"?

Find CFM then, Ht=(4.5)(CFM)(delta H)??

md