What's up with this AC

don_185

You are on the right track Steve.It sure sounds like you have a restriction.

High superheat and high subcooling would be a good indication of a restriction.That mean that your indoor coil is starve and your outdoor coil is stacking freon.

A bad compressor would show no change in pressure or a weak comp would have high suction pressure and low head pressure.

What type of metering device did she have?

Steve Ebels_3

Old Amana AC

Went to look at a 20+ year old Amana AC today that the owner described as "not doing the job". All the numbers are unreadable on the stupid foil data "plate" but I would guess it's 3.5 to 4T capacity.

Here's what I found:

The condensing unit would start and run for about a minute and then shut off. After about a five minute delay it would repeat, shutting down again after about a minutes worth of run time. Suspected a low pressure switch and/or low R-22 charge.
The gauges showed some really weird numbers though, Just before shutdown, the vapor line would get down to 15-20 PSI while the liquid pressure appeared high if anything, reading over 250PSI. Outdoor ambient conditions were in the mid 80's and indoor was the same.

I went to the basement to check the evap for ice buildup or other blockage of airflow and found everything clear. Based on that, I assumed that the charge was low.

When I started charging (vapor) I noticed that the high side pressure was REALLY coming up quickly while the low side was barely moving. Even with the tank on it would not keep the pressure switch from tripping out. With the high side reading 320# and the low side only 45# the unit would stay running. Attempting to add more refrigerant only succeeded in raising the high side to nearly 375 while the low side didn't even hit 50. Superheat readings from the Testo 523 were so far out of whack they were meaningless. The liquid line was ambient temp at the most and at those pressures it should have been to hot to touch.

I noticed that about half of the condensing coil seemed to be working based on an abnormal difference in discharge air temp from one side to the other. In addition to that the amp draw which would normally be in the 15-25A range for a unit that size, was only reading about 8 under those conditions. Seems like the compressor wasn't seeing much actual load despite the high pressure reading.

My gut tells me that the compressor has a bad valve or that there's a restriction somewhere in the condensing coil.

What do you guys think?

clammy

how about this

Steve i ran into a simalar promblem many yeasr ago it ended up being a piece of rubber in the liquid line that would get caught at the expansion valve inlet .i guess i got lucky it was also on a older carrier system doing a a coil on a furnace which had been in service for years also ,peace and good luckc clammy

mtfallsmikey

Something this old

Probably has cap tubes. I've got the same problem with a water-source heat pump serving an elevator machine room in one of my buildings, but the exp. valve is next to impossibe to access...might have to disconnect and drop the whole shebang...and yes, it is a Trane!

don_185

I hear ya

I hear ya Mikey.No one ever worrys about service.Just stick it over there it will fit.

I'm also thinking at twenty years this equipment owes no one anything,replace it.
Used to be I would play tech and try to fix it.Now days on
something this old I breakout the sales hat and sell them
a new one.

Steve Ebels_3

Doesn't matter at this point

Told the customer the old unit wasn't worth putting a lot of $$ into and he wound up buying a nice Carrier VS 2 stage furnace with their Infinity matched AC and control. He'll be way better off with that setup any how.

Thanks for the advice guys!

don_185

You Rock.

You're the man Mr Ebels.That is a excellent choice.

Share some pics if you can.I'm sure you put just as much pride in your forceair install as you do the wetstuff.

I'm jealous.

Eugene Silberstein 3

Three Possible Causes

There are three possible causes for the sytem failure. They are:

1 - A liquid line restriction

2 - A liquid line restriction

3 - A liquid line restriction

Ask me how I know...

Glad you asked...

Let me digress for a moment...

It is a cold winters night and you have in your hand a nice hot (120 degrees)cup of (add your favorite hot beverage here). You walk out onto your patio to briefly admire the newly falling snow. You glance up at the sky to see the snowflakes falling gracefully from the sky. You then re-enter your nice warm cozy home. The temperature of your liquid refreshment is now a toasty 118 degrees.

Now let's say that, while you are outside you set the cup down on the railing of the patio. You forget all about your drink and re-enter your home. About 30 minutes later, you remember that you had left your drink outside. You rush outside and bring in your drink. The outisde temperature is 38 degrees and your drink is now at a temperature of... you guessed it... 38 degrees.

Makes sense?

I hope so.

No back to air conditioning. If you have refrigerant that exits the condenser coil and returns to the metering device quickly, the amount of condenser subcooling will be low. Now, let's say that the refrigerant remains outside for a long period of time... The temperature of the refrigerant will be equal to the temperature of the outside ambient temperature. Sooooo, if your condenser outlet temperature is the same as the outside ambient temperature, the refrigerant is remaining outside far too long. This is a classic indication that there is a liquid line restriction.

Under the umbrella "liquid line restriction" I include blocked or clogged liquid line filter driers, dmaged refrigerant lines, metering devices stuck closed and other reasons for line closure on the liquid line.

Unknown

Notta

What causes high head pressure? 3 items. 1) The person that overcharged the unit. 2) Condensing coil issues like(dirty coils and fans not running). 3) An obstruction in the discharge line between the compressor and the mid section of the condensing coil...

You want to know the biggest secret ever with refrigeration and high head pressure, it's almost never a restriction!!! Why because you can't pump vapor that the compressor is not receiving... Period...

don_192

True that

Eugene Silberstein 3

Yes, but...

You are correct about the high pressure issue but, given the liquid line temperature issue, all else is moot.

Even if you have a system overcharge, no matter ho great, you will always a temperature difference between the liquid line temperature and the outside ambient.

If there is no difference between these two temperature measurements, the refrigerant is not moving.

Unknown

Commercial Refrigeration is very often mis-understood at best.

As far as liquid line temp. goes, if a system is designed on the ragged edge of the limits of piping, expected loads, and actual weather conditions it is very easy to see a fully operational system have a liquid line near or below ambient temp. parts of the year. By near I mean with-in 0*F to 5*F of ambient and running normally, with a 10% to 20% overcharge.

If you are lucky enough to have a condensing coil that is a little oversize to accommodate for coil fouling and adverse weather conditions you can easily see liquid line temps. at near ambient temps. year-round. Every btu taken out of the refrigerant at the condenser is more power to utilize at the evap. sections.

One thing you can always count on with refrigeration is that the system is always in a state of flux. You mess with one item and several others are going to change... Fact

Massive sub cooling of the refrigerant is were the so called high- efficiency units are making the gains.

Eugene Silberstein 3

Back to basics

Here are a few refrigeration and non-refrigeration basics upon which I hope that we can all agree.

1 - The condenser is responsible for rejecting system heat.

2 - On a properly operating system, the refrigerant will be condensing at a temperature that is above outside ambient temperature. (if the condenser saturation temperature was lower than ambient, the refrigerant in the condenser would be absorbing, not rejecting, heat)

3 - Once condensed, the liquid refrigerant subcools to a temperature below the condenser saturation temperature as it transfers heat to the lower temperature outside ambient.

4 - Since there is no change of state taking place at the outlet of the condenser or in the liquid line, the heat transfer is a sensible one and, in order for a heat transfer to occur, the liquid in the liquid line must be warmer than the temperature of the surrounding air.

5 - Unless there is a system malfunction that causes the head pressure and corresponding saturation temperature to fall below the aoutside ambient temperature, there is no way that the liquid line temperature will fall below that of the outside ambient, without absorbing heat from the outside air.

The original post indicated that the high side pressure was way above normal indicating that the refrigerant is condensing at a temperature far above the outside ambient temperature. If the liquid line is at a temperature that is lower than the ambient temperature, where is the liquid refrigerant transferring its heat to? What is the medium that is lower than the new low liquid line temperature that is absorbing this heat that the liquid is attempting to reject.

Riddle me this...

Steve Ebels_3

Exactly

The statement in the last paragraph is the part that has me puzzled.

Your quote........."If the liquid line is at a temperature that is lower than the ambient temperature, where is the liquid refrigerant transferring its heat to? What is the medium that is lower than the new low liquid line temperature that is absorbing this heat that the liquid is attempting to reject."


I'm not enough of a refrigeration expert to noodle that out on my own.

Unknown

I think,,,

he means you are cooling the refrigerant in the condenser and liquid line with air and therefore the refrigerant could or should never be able to be cooled BELOW the temperature of the air cooling it.

mtfallsmikey

OK, but...

See if I can make sense...could the condenser temps be lower in a case where we are using water / cooling towers for condenser cooling instead of air?

don_192

No

it will still be higher then your water temps.I know if I
had 0 degree of subcooling I would be worry about sending gas bubbles to my metering device.
We are talking about subcool temp are we not?

I measure some today at ambient temps of 74 degrees and I was seeing 87 and 90 degree liquid line temps.

Unknown

Hmmm,,,

Good question. Now you're tied to the temp of the water. You'll never get it lower than the temp of the water BUT if the water temp is below outdoor air temp you WILL (or may)be able to get your LL temp lower than outdoor air temp. I haven't worked on any water towers in a while. is it possible for a water tower to get the water temp supplied to the condenser lower than outdoor air temp?

mtfallsmikey

No way that would happen here, MPF

I cannot use the mechanical mode (compressors running)when CW temps are below 54 deg. Forgot to mention that I have waterside economizers on my self-contained units. My buildings have tremendous heat loads, so it will come out of economizing at roughly 40 deg. outside...interesting discussion!

Unknown

Interesting setup...

Unknown

The whole numbers are Eugene's qoutes

1 - The condenser is responsible for rejecting system heat.

1.5 - Most of the time, sometimes as in the heat pump the outside condensing unit absorbs heat.


2 - On a properly operating system, the refrigerant will be condensing at a temperature that is above outside ambient temperature. (if the condenser saturation temperature was lower than ambient, the refrigerant in the condenser would be absorbing, not rejecting, heat)

2.5 - Properly operating is the key word. A grossly overcharged system can surge in the condenser and cause rapid pressure drops in the liquid line causing flashing. Over charged and a cool liquid line it happens.


3 - Once condensed, the liquid refrigerant subcools to a temperature below the condenser saturation temperature as it transfers heat to the lower temperature outside ambient.

3.5 - Yes, the premise of the higher seer units.


4 - Since there is no change of state taking place at the outlet of the condenser or in the liquid line, the heat transfer is a sensible one and, in order for a heat transfer to occur, the liquid in the liquid line must be warmer than the temperature of the surrounding air.


4.5 – Depending on the state of charge, evaporator load, outside ambient temperature, condition of outside coils, condition of the compressor, and ratio between available condensing coil surface area for condensing vs. subcooling. With this in mind the temperature of the liquid could be from 0*F to smoking hot above ambient temperature.



5 - Unless there is a system malfunction that causes the head pressure and corresponding saturation temperature to fall below the outside ambient temperature, there is no way that the liquid line temperature will fall below that of the outside ambient, without absorbing heat from the outside air.


5.5 – The definition of malfunction is the key word. Can a system run normal liquid line times sometimes of the year and not others? Yes quite easily. Can the liquid line fall below ambient temps. Yes it happens. Is it ideal? No. But in refrigeration there are a lot of conditions that are not ideal. But things happen.