Delta T, do I have a concern?

HVACDave · 05-17-2012, 04:52 PM

You have a saturated suction pressure of 55 PSI, which equates to 30 F on your PT chart for R22. You measured 70F for a suction temp which gives you a superheat of 40F.

Charging a critically charged unit (orfice, bullet, cap tube - metering device) is typically done using suction pressure, which is directly affected by outdoor ambient, as that makes your head pressure move up or down depending on temps. That in turn pushes more or less liquid into your evap. so it is really an experienced based charging method.

Your suction needs to ideally get up over 58 psi (using R22) to ensure that you aren't going to freeze the coil up, but not too high or you lose your temp difference. So over a few years of service you come up with some pretty good charging knowledge that is difficult to put a handle on.

I would expect to see your current 205/55 if the outdoor temp was say 65-70F (head may be a little lower 175-200 maybe) depends how clean your condenser coil is. As the outdoor temp aproaches 80F I would expect 62-67/200-220, and as you move up from there 86-90F like you are now I would think 65-70/225-270.

But really this is just based on experience and some basic rules of thumb. Discharge press. should be approx. 30F above ambient converted on PT chart to psi (again depends on efficiency of unit and cleanliness of coil/airflow.

If you have the ability to weigh in the charge, it does take a lot of guess work out of the process and gives you a great base line to work from, but many don't have access to the recovery equipment storage tanks etc. that it requires.

viper · 05-17-2012, 05:16 PM

You obviously are much more experienced so I am just trying to learn a bit. Thanks for taking the time to explain your data a bit.

Re SH calcs. The way I understood it is the suction line pressure will convert with the R22 PT chart thus the 30F we agree on. The way I understood the SH adder was to refer to a SH chart that relates the wetbulb indoor temp to ambient outdoor temp. That number will then add to the 30F. I think I was about 12 on that chart which would make SH about 42*. Did I do that wrong?

It was my understanding that target suction line temp should roughly equal the SH calc thus about 42*?

Regarding weighing in a charge, it was my understanding when adding that you only need an accurate scale to put the charge tank on? Tare the tank before you start and watch that value change as charge is added?

HVACDave · 05-17-2012, 05:30 PM

I beleive the method you are refering to is charging by aproach temperature. It uses wet bulb of indoor air and discharge pressure and liquid line temps. I have used it before, but it's been a few years.

You can usually get the info off the inside panel of the corner of the unit where your wiring ties in if that is the preffered method for the equip. manufacturer you are dealing with.

There may be other techs here that are more familiar with that method than i am. Maybe one will try and help on that front.

As far as weighing in the charge goes, you would have to remove all the refrigerant from the system, check the nameplate charge and then tare your tank as you state and refill with appropriate charge, then fine tune from there depending on lineset length, coil size etc.

**beenthere** · 05-17-2012, 05:32 PM

Accurate charging by Sh would require the indoor wetbulb and outdoor drybulb. The vapor pressure you get will vary slightly with the amount of indoor air flow. But should never be under 58PSIG/32°F. At 86°f outdoor temp and 70°f by 50%RH indoor conditions, a SH as low as 7 to as high as 12 is acceptable. lower then 7 would tend to indicate a low indoor air flow, dirty evap coil, or over charge. higher then 12 would tend to indicate a low charge, high air flow(exceeding 425CFM per ton), or restriction in the refrigerant circuit.

**beenthere** · 05-17-2012, 05:35 PM

Lennox uses approach. Approach is comparing the liquid line temp to the outdoor temp. Its very similar to SC.

HVACDave · 05-17-2012, 05:38 PM

Only been using subcooling on new units and charging by suction on old units for years. Not as familiar with some other methods. It's also early in the season here in Canada, and it takes a couple weeks to get back into the AC mindset.

viper · 05-17-2012, 06:57 PM

Dave - When you say "charge by suction", are you referring to the relation of suction pressure to ambient? Or is this charging by suction temp?

Regarding SC, I was not sure if this was truly useful info on a capillary system or not? From what I understood, it should be used only with TXV systems?

Also, maybe to get a better handle on the SH calcs, I was indeed referring to a charge calc in which the suction temp would be the variable in the charge as I understand it. I would like to learn what methods would be best and most accurate within reason for charging this system? The only SH calc I was aware of used indoor wetbulb.

**beenthere** · 05-17-2012, 07:38 PM

Wetbulb is total enthalpy. And using total enthalpy the amount of SH needed to both protect the compressor, and get the highest capacity the system can have under varying conditions can be determined.

A fixed metering device should be charged by SH. But SC should also be checked. As it can indicate a problem in the system that checking SH alone won't tell you.

viper · 05-20-2012, 04:35 PM

As a re-up to this thread, since I could not find any vid files that would indicate the type of noise I am getting from my unit, I thought I would add some vids for discussion. The first one is of the unit just starting. Typical noises of the compressor, then smooths out and purrs. Second vid is of the same cycle but about 20min later. It seems to get a little louder as it runs. I have been considering this vibrating noise "slugging" but it is a highly oscillating noise, making me wonder.
I KNOW the system is low on charge and I am working to resolve that issue. Ambient is 80F, pressures are 185/50right now. I am just debating whether we are wasting time messing with this old system? I know the system is still running good pressures and will pull a 20* dT with a little charge. This is a tempstar unit with copeland scroll and no blanket. There is no cold bybass kit, crankcase heater, etc. Air handler is on main level so any liquid would easily migrate back to compressor. I am not sure if adding some sage in the line set to accumulate liquid would help, or if I am not he wrong track altogether.

I should note that the noise in the second vid is HIGHLY amplified through the mic. This is certainly not a noise that would make you run for the shutoff but make you take notice. It will get louder if operated in cooler weather.

Thoughts?

http://youtu.be/ltAyLsciWX8
http://youtu.be/cvpJWj3eyKw

**beenthere** · 05-20-2012, 06:02 PM

A unit low on charge like your is can make a lot of noise.

viper · 05-20-2012, 09:33 PM

I had a friend out to look at this with me. He thought the noise was more a vibration from the comp from the mounts or something. He also did not get to hear it when it was really talking either.

After adding about 1.5lbs, pressures were running about 205/66 at 75* ambient. DT across the A coil is now at 18*. Not the 20* it was but I realize there are other variables. RH was 53% earlier.