View Full Version : super heat indoor dry or indoor wet which 1
06-10-2012, 12:43 PM
There's a lot of talk about super heat charging and the way I charge a unit is by taking the indoor wet bulb and outside dry bulb but there's a lot of talk on relative humidity being a factor. On my charging chart it says " If under 20% RH or Over 70% RH use wet bulb ok but my friends at one of the largest A/C Heating companys in the mid west says he always uses indoor wet bulb. But if this is the case then why do charging charts have an option of using indoor dry bulb? when do i use the indoor dry bulb instead of the wet bulb? And Whats the deal with the Humidity 20% or over 70% on my chart. I think its going to take a true Tech to answer this because knowing the proper way is key to understanding what is supposed to be and figuring out how to understand the system for your self. I use wet bulb all the time, i do have an RH meter so if I knew when to use it and how it applies to my super heat method would be highly appreciated. Thanks HVAC-GUYS!!!
06-10-2012, 12:51 PM
Put thread in Tech to tech chat
06-10-2012, 03:04 PM
Indoor wet bulb is most accurate Trane does have a drybulb chart
06-10-2012, 03:19 PM
One thing to keep in mind about the various manufacturers charging charts is that the ones that use indoor DB temperature also are assuming that the indoor temperature is in the normal comfort range the system will be operated at.
They use the DB temperature as a way to dumb things down because they know the sad truth that the majority of "technicians" don't regularly take WB readings, don't take them in the right place, or don't even own psychometer.
I basically ignore those charts and go by a method that uses the WB temperature.
06-10-2012, 03:22 PM
I always use the indoor wet bulb temp when charging by the SH method. Remember though that charging by the SH method is only truly accurate when charging a system that is orfice / or capillary / or piston controlled. If and when there is a TXV valve in the system the SC method is much more accurate. I always check the SC and SH after installing a new system because even though the mfg. states what the initial charge is on the label and is is supposed to be adequate for the matching coil and 15 ft of line, it usually isn't. Most of the residential systems that I install use a 3/4 suction line and a 3/8 liquid line so I always add about 9oz or .64 lbs of gas for each additional foot of line set and my SC / SH is usually very close regardless of the humidity in the conditioned air space when using the IDWB in my calculations.
06-10-2012, 03:33 PM
I agree that most techs don't take an IDWB temp. These days why wouldn't they though. At least three tools in my tool bag are capable of taking an IDWB reading with very little effort. I usually either just take my SMAN 3 inside and hook up my WB sensor and grab it or take my Fieldpiece SC77 inside and grab it and manually input the info into my SMAN 3 outside. The later is my preference when installing a new system because that way I can just leave it hooked up inside and keep referenceing it because the IDWB changes so fast, or at least it should when you install a new system. I still reference my PT Charts on my iPhone occasionally and usually they are withing 1 degree of what my SMAN 3 is showing the SH and SC to be. Although about anyone can make a unit make cold air the unit it obviously not running at neither peak performance or optimal effiency if either the SC or the SH is off by 3 degrees or more +/- .
06-10-2012, 04:05 PM
You can use this to calculate your target superheat.
Enter Indoor Wet Bulb Temp
Enter Indoor Dry Bulb Temp
Enter Outdoor Ambient Temp
Press SH Key
You can also get corresponding line temps or actual SH or SC occurring.
P/Ts for 28 refrigerants.
Read Instructions provided.
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