Help from HVAC guys ,please

[beedees]

Hersr th situation... 4 ton AC/ Heat pump.  High pressure line from condenser to expansion valve not unreasonably hot, returrn line  from coil barely cool. Doesn't cool worth a flip , and I don't anything about heat pumps .Had it checked twice by a guy ( finally remembered he told me he didn't know squat about  heat pumps, ) and was told everything was fine. At about 95-96 degrees and around 24% rel humidity what would I be looking for as far as pressures go (assuming I can find my gauges)? Thanks Dan

[Todd Wilson]

Should be the specs on the inside cover somewhere to tell you what you need to know with the gauges.


Todd

[tommymac]

75-85 lbs on the low side 200-225 or so on the high side. Should have a 15-20 degree TD on supply air vrs  return air. May be low on R-22 -Are the coils clean,air filters clean? Is the reversing valve working all the way? should be powered for cooling. 24V and check chart for #s

[400/6/PAC]

Sounds like it's low on charge.
Bring the low side up to at least 68 psi, this will put the evap temp around 40 degrees.
some older units will require a little more low side pressure.
check the return air temp and the supply air temp, 17 degree differance is about right.
the suction line at the condenser should be sweating and cold.

[aifilaw]

Oh my....

I'll try real hard not to write a book here, but I can certainly help you out.
First of all its R-22, I'm guessing 13 seer at the most on this system, but will need more information
With a proper charge you should be getting 20+ degree's delta-t, but that depends on the age of the system, which means the difference between your supply and return. Don't measure at the registers for that, or if you do, add 2-3 degrees for losses.

If we are going to deal with ballparks, then yes, your liquid line should not be hot to the touch, ballpark for it on a properly charged and functioning system should be as such. Ballpark on the suction line should be beer can cold. (that's actually a technical term in the A/C field)
Since its not you have a problem, and that problem is in my experience most often referring to the fact that your indoor ambient is much too high, it is impossible to properly set a charge without your indoor return air temperature in the low 70's. If it is in the low 80's, then you are going to get the situation you described.

The pressures are completely dependent on your ambient conditions and humidity conditions, as well as the operation of your system as a whole. If you have a clogged indoor coil then you will get low suction pressures, poor cooling, and may freeze your indoor coil. If you have a clogged outdoor condensing coil then you will get higher head pressures and your liquid line will be hotter since it won't be condensing as much.
Your fans not running at full speed or properly can simulate the same occurance, so can bleed-by on a compressor. All of these things will fake the system out regarding charge, so lets get down to how to charge a system correctly.

Once again, I can't give you a superheat/subcooling calculator that shows what your setpoints should be based on the seer of the unit and the ambient temperatures with everything else being clean and spotless, but its a better ballpark than any of the other things you have mentioned.

Once you have ensured that your coils are not dirty, your fans are running well and you still are having problems satisfying your latent heat and sensible heat, and your suction line is not "cold" then the problem may be charge.
Superheat is the method of setting the charge if the system contains a thermal expansion valve at the air handler
Subcooling is the method of setting the charge if the system contains an orifice at the air handler.

They make bi-flow of both systems, so you will have to check, and some manufacturers even pipe around the darn things with check valves.

A TXV is the object just before the distributer, it has a head on it and a bulb that comes off a capillary tube which gets clamped down on the suction line. an orifice will look like a slightly larger piece of pipe just before the distributer or ASC to most people.

With your indoor temperature satisfied (70-74), and your outdoor air temp above that (80+) you will be wanting to achieve 8-12 degrees of subcooling (if you have an orifice system), OR (not and) 12-18 degree's of superheat (if you have a TXV).

To check your subcooling use your high pressure gauge and read off the inner dial that is labelled R-22, this is a condensed version of a PT-chart for that refrigerant, now put a thermister, thermocouple, hell even a mercury thermometer strapped down as tight as you can on the liquid line to get a good reading.
Subtract those two numbers and that is your subcooling.

To check your superheat, use your low pressure gauge and do the same, but physically measure the suction line.

[beedees]

Well, surprise, surprise, surprise!!!... to quote the immortal Gomer Pyle. Painting the inside of the house now and GUESS what I found when when I took the return air vents off.  Thanks for all your help, guys.

[aifilaw]

If I had to guess, I'd say your leaking air into your attic/under house in your venting/ducting system

[tommymac]

was it plugged?

[1BAD68]

really good info!
but dont you have this backwards?
"With your indoor temperature satisfied (70-74), and your outdoor air temp above that (80+) you will be wanting to achieve 8-12 degrees of subcooling (if you have an orifice system), OR (not and) 12-18 degree's of superheat (if you have a TXV)."

[aifilaw]

No, those are correct

what most people don't realize is that sub cooling although it says "cooling" in the word is the effective difference betwene the pressure-temperature relationship of the gas/liquid mixture entering the condensing coil, and the actual temperature of the liquid (or should be pure liquid) coming out of the condensing coil because you are cooling it below its pressure-temperature.

Just like most people confuse S/A, R/A, O/A and E/A
Supply air is the air you are supplying via the vents/registers to the people, not what you are supplying the air handler unit. Return air is what is returned to the unit, not what is returned to the people.

[1BAD68]

I know waht you are saying but, you adjust subcooling on all TXV systems and superheat on all orifice systems.
Unless I'm reading it wrong, you said it backwards.

[aifilaw]

Did I?
I'm too lazy to read...but it should be superheat on a orifice, subcooling an TXV.

Setting charge on an EXV is a real PITA.

A TXV (and EXV for that matter), or thermal expansion valve, has the job of attempting to maintain a set superheat. if it cannot maintain it in a system that is free of other problems, then there are two reasons.
1. TXV needs to be adjusted (alan wrench on th bottom, or the head needs to be replaced)
2. the charge is incorrect and it cannot meet the superheat because it is at its limit.