On May 9, 7:17 pm, Joe wrote:
OK, I've gotten a better handle on this (better doesn't mean a whole
lot in my case, though).

Here are my stats:

Suction line - 63 psi, line temp is 53.5 deg F
Liquid line - 215 psi, line temp is 163 deg F.
Ambient air at bottom of condenser is 81 deg. F.

(This is after adding about 2 lbs of R22.)

If I get it, the subcool is now about 15 deg, and the superheat is
about 60 deg. But, I don't know how this relates to what I need. How
do I figure that out?

BTW, my system is a 5 ton unit, which is oversized for my house, but
that's what the idiot installed 11 years ago. I have been told that a
3.5 ton unit would be better for my place. I plan on replacing the
whole thing as soon as the money's there; preferably with a
ground-source heat pump.

Joe

On Sat, 09 May 2009 18:30:25 -0400, Joe wrote:
I have a small leak in my evaporator. So, here comes the cooling
season again, and until I have the money to get the system repaired, I
need to add a charge. I've got the gauges hooked up, and measured the
ambient temperature at 82 deg F.

I remember that I used to just subtract x deg. from the ambient, and
add refrigerant until I reached that value on the low-side gauge.
However, I don't remember what that value is. IIRC, it was 35 or 40,
but the synapses don't fire like they used to.

Can anyone help me out here? I fail to understand the
superheat/subcool stuff, but the rule-of-thumb has served me well in
the past.

Joe

Not enough information Joe. What type of high side to cold side
regulation device does this system have; is is a capillary tube, or
thermal expansion valve system?

From your data I would assume capillary tube control, you need to look
up the temp. vs pressure tables for R22. You want to shoot for, at
minimum thermal load, and saturated suction pressure that is near or
above freezing to prevent your evaporator from freezing up. The
problem is if that is the operating condition. With this type of
"expansion regultor" slightly undercharged systems can freeze the
evaporator, overcharged systems will not be cold enough to remove heat
quickly. Measure the suction line temperature at the air handling
unit, and try to shoot for 38-42 degrees F operating temp. with light
load. If it frosts up, add more refrigerant. If too warm, dump some
back into your tank.
One question would be how low on charge was this, it may have sucked
air into the system if too low and as you indicate the leak on the low
side, this could be a problem if ANY air has gotten into the system.
Along with is water vapor, and this would be a very bad thing if you
don't have a dryer installed.
ignator