R22 2.5 Ton AC LOW SIDE PRESSURE Fluctuating

[SouthernCA]

The charge was low so I charged the system. Here are the numbers

R22 2.5 Ton AC with Piston (NOT TXV)
1. The unit was low in charge and so I charged the unit. Here is the data
2. Low pressure moving between 58 and 74 (1 5 min cycle)
3. Low tube temperature; 54 F
4. Wetbulb at return: 65.2F
5. Superheat: 9.6 ro 19.6 F (moving)
6. High pressure: moving between 200 to 215 (1.5 min cycle)
7. Liquid tube temperature: not measured but about 80F
8. Subcooling : 22 to 26 F

Question: how can I find out why both pressures are fluctuating?

 

[argile_tile]

fluctuating how often? it should fluctate when starting and stopping and vary a little during the cycle due to indoor temperature dropping

need the frequency of fluctuation to answer: it's not a question without a time sheet.

don't do superhead subcool without your brain installed. my unit has freon inlet/outlet lines laying on each other where subcool is usually measured: it changes the result by several degrees. also many chinese handheld units have awful accuracy - almost may as well not even look at the number they give you.

 

[SouthernCA]

fluctuating how often? it should fluctate when starting and stopping and vary a little during the cycle due to indoor temperature dropping

need the frequency of fluctuation to answer: it's not a question without a time sheet.

don't do superhead subcool without your brain installed. my unit has freon inlet/outlet lines laying on each other where subcool is usually measured: it changes the result by several degrees. also many chinese handheld units have awful accuracy - almost may as well not even look at the number they give you.

Thank you for your response.

The fluctuation cycle is about 90 sec.

I could not understand the rest of your comments. Yes, agree that Chinese made manifolds may be inaccurate.

In my unit the low pressure side is insulated and does not touch the high pressure side.

Not sure what you meant by "don't do superhead (SIC) subcool without your brain installed"

 

[argile_tile]

> Not sure what you meant by "don't do superhead (SIC) subcool without your brain installed"

(it means if the freon is weighed in (rule #1) and the AC unit is cooling properly that "widely wrong sc/sb readings" should be viewed with skepticism)

> fluctuation during / after 90 second horizon

ok 90 seconds the sc sb changes. well. when freon begins to evaporate there is no airflow and indoor temperature A-fram. then the fan turns on and warms the A-Frame. during all this dynamic change things happen.

i believe "hvac school" on youtube says you must let the system settle before taking sc/sb reading. am i right?

> fluctuation happens cyclically every 90 sec despite compressor always on

well this i'd never heard of.

1) is your fan speed on high the whole time (indoor and outdoor variable speed fans). let me assume they are. also that it isn't a breezy day where every 90 seconds the wind speed changes by 10mph

2) you know liquid freon should enter the A-Frame, evaporate, at such a rate it is gas by the time it exits (in other words too little freon and there's no liquid in the A-Frame tubes, too much and liquid exists the A-Frame and slams the compressor)

so then you have a possibility that the liquid level entering the A-Frame is "catching up" and "falling behind". unusual. you'd of course like to know if more freon would take up this gap so that the sc/sb never falls short (sufficient liquid is always in the A-Frame for supercooling)

(supercooling: expansion of gas cools to "beer bottle cold", then also the state change from liquid to vapor adds allot/most of the total cooling)

but perhaps you have a flakey sensor which "quits producing liquid freon". but idk, depends on what unit heavily. for some units to stop producing liquid it has to shut off (and if it didn't, the high side pressure which is easier to read than sc/sb would be too high or low during the 90 second period, then change).

 

[argile_tile]

ok capacitors. i assume you checked their uF (MFD) and they are ok. capacitors can cause motors to run weak or wonky and do so over a time period. (note i didn't say run out and buy new ones without checking them). (i assume since you know sc/sb that you knew that)

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the sheets that came with my Trane unit say: weigh in the freon. "Subcooling (in cooling mode) is the only recommended* method". the table shows that with default line lengths no change is needed. (one makes alterations due to conditions requiring alternate line lengths due to installation)

a 200 to 215 move in high side pressure is allot but not damaging or alarming. (a variance in outdoor temperature by 5 deg. higher would/may change it more than that)

i'll say: if it is working (you charged it, so you know the temp differential between indoor temperature and vent temperature should be roughly 20 degrees) then don't mess with it too much - but do check your sensors. beyond that IDK. 15 deg psi isn't allot i know that. what is your max psi? you need a photo of the label for me to know more.

R22? what is the model and sticker. that's a really old unit you may need someone who had a career with older units and knows their quirks.

 

[SouthernCA]

ok capacitors. i assume you checked their uF (MFD) and they are ok. capacitors can cause motors to run weak or wonky and do so over a time period. (note i didn't say run out and buy new ones without checking them). (i assume since you know sc/sb that you knew that)

----------------------------

the sheets that came with my Trane unit say: weigh in the freon. "Subcooling (in cooling mode) is the only recommended* method". the table shows that with default line lengths no change is needed. (one makes alterations due to conditions requiring alternate line lengths due to installation)

a 200 to 215 move in high side pressure is allot but not damaging or alarming. (a variance in outdoor temperature by 5 deg. higher would/may change it more than that)

i'll say: if it is working (you charged it, so you know the temp differential between indoor temperature and vent temperature should be roughly 20 degrees) then don't mess with it too much - but do check your sensors. beyond that IDK. 15 deg psi isn't allot i know that. what is your max psi? you need a photo of the label for me to know more.

R22? what is the model and sticker. that's a really old unit you may need someone who had a career with older units and knows their quirks.

Thank you for such a detailed reply.

1. The capacitor has been replaced. No difference.

2. The fan and compressor are in and in constant speed.

3. It is a roof evaporator unit. Label below.