A/C coil freezing up...

[mrsilv04]

I've got a 7 1/2 ton Trane unit that has froze up the inside coil twice in the past month.

The inside coil is 20" high high by 52" wide. Both times it has froze up the bottom 8" of the coil solid, with ice on both sides... plus any exposed supply lines are coated with ice.

System has already been checked out once. Tech found the pressure on the high side right at 300, so he removed 5 pounds of freon to get it down to 120. Up to that point, the system hadn't had freon added or removed in at least 4-5 years. He also washed out the outside unit as well.

Filters inside were fine. We've verified that the blower is running when it freezes up.

There is some suction on the inside coil access door when I pull it off when the system is running, but it is nothing that I would consider to be excessive. There haven't been any modifications or changes made to the air return system in many years. It is one large return, only about 20 feet from the unit.

I'm at a loss on this one.

 

[SrDriver]

I am assuming your are using F-22 in the system.

You can find several pressure temperature charts on the internet.

Head pressure of around 300 PSIG on a hot day is about right.

I did HVAC work for years before retirement both commercial and residential and would bet it is low on refrigerant!

You need to get it checked ASAP!

 

[440Magnum]

Why would the tech REMOVE refrigerant?!? 300 is a perfectly normal high-side pressure! Its now low on charge and can no longer "flood" the evaporator coil with boiling refrigerant, so the only boiling is happening at the bottom of the coil and the rest is just gas returning to the compressor, resulting in freeze-up.

 

[mrsilv04]

Originally Posted By: 440Magnum
Why would the tech REMOVE refrigerant?!? 300 is a perfectly normal high-side pressure! Its now low on charge and can no longer "flood" the evaporator coil with boiling refrigerant, so the only boiling is happening at the bottom of the coil and the rest is just gas returning to the compressor, resulting in freeze-up.



I may have not made the time line clear.

It froze up for the first time (and I can't remember this happening before). I thawed it out and called the tech in. He removed the 5 pounds of refrigerant and lowered the high side pressure from 300 to 120. Tech couldn't find any other cause for the freeze up.

Now, it has frozen up a second time, and the coil was frozen up the same way both times, the bottom 8" of the coil was completely iced up.

I was told that 300 was on the high borderline of damaging the compressor, that any more than 300, up to 350 would be bad. That, and the lack of any other obvious cause for the ice up, was the stated reasoning for removing the refrigerant.

 

[SrDriver]

Sounds like you need another serviceman to take a look at it for a second opinion.

 

[mechtech2]

Going from 300 to 120 on the high side is a mystery to me, too.

Do you have enough airflow ? Is your drain working properly?

 

[Steve S]

Thermostatic valve?

 

[Chris142]

I don't know much about non automotive units so I may be off base here. When an automotive evap freezes up it's because the cycling switch didn't let the system thaw out when it should.

Does this system use a cycling switch.

300 psi sounds pretty close to me. Maybe a tad too high but 120 sounds too low.

But I'm only familiar with car AC.

 

[440Magnum]

Originally Posted By: SrDriver
Sounds like you need another serviceman to take a look at it for a second opinion.


I agree on that point.

Home HVACs have some type of expansion valve (usually a simple orifice tube, although heat pumps always have a thermal expansion valve and other types may also). If that's clogged then it could be the cause of the problem too.

 

[brianl703]

Originally Posted By: Chris142

Does this system use a cycling switch.


These systems do not use a cycling switch unless one has been added to allow operation in low outdoor temperatures.

 

[Solo2driver]

You should have suction on the access door, that means the blower is moving air

At school I have a 7.5 ton rooftop package unit that is doing the same thing. The blower is on and running but much slower than spec....it's not drawing enough air over the coil for the temp/pressure and the condensation is freezing instead of just dripping down into the pan.

You can switch the unit off to let it defrost, and then fire it back up. Set the thermostat just a few degrees below ambient so you are not running both stages (I am assuming your unit is a dual-stage for that size you listed) and let the temp come down slowly....then drop it down a few degrees more until you get back into normal range. If it still freezes up, you may have not enough air movement across the coil (check filter, check blower/squirrel cage, blower belt tightness, etc). In my case it needs a new blower motor, been running for 25 years I think it's probably time for a rest

Second thing to try, which I have found a couple bad thermostats that would turn on the compressor but not the air handler blower/fan, immediately freezing up the coil solid. Switching the t-stat fan to "On" instead of "Auto" was a quick fix until the t-stat was replaced.

Just my $0.02

Cell phone pic of said frozen coil on 7.5 ton unit above a classroom building--

 

[mrsilv04]

I caught it all froze up again... this is what the coil in the air handler looked like the first time it froze up as well.

Only the bottom third of the coil is froze up.

So... here's a summary and timeline.

1st freeze up, July 17.

Found coil in air handler froze up as in picture.

Thawed system out, then called HVAC tech. Described how I found it froze up to tech. He finds nothing obviously wrong with system, except somewhat high pressure on high side (300 pounds). Removed 5 pounds of refrigerant, which lowered the pressure on the high side, and cleaned outside unit with water.

Filters in the system were not dirty and blower fan was running.


2nd freeze up, July 30.

I learn that the system isn't cooling properly again. Find unit froze up again, take picture (above). Thaw unit out and restart. Filters and blower fan OK.


August 4th

Call HVAC tech. Same firm sends a different tech and he shows up on site. I show him the above picture. He diagnoses it as an obstruction somewhere inside the coil, blocking proper flow of refrigerant. Recommends replacement of coil in air handler.

Checks refrigerant while he's there to be sure. High side is at 250.

I ask about a possibly blocked orifice. He states that system has a variable orifice, which is automatically compensating the flow of refrigerant, because of the partial obstruction in the coil.


Today, I showed the picture to a guy who was an HVAC tech years ago, and told him the story. He's skeptical of the diagnosis, and his response was to show that same picture to his ex-boss, whom I also know, who is a retired HVAC tech.

System was also lightly froze up again this morning, although it was still working fine. I shut it down and let it thaw again.

Tomorrow (Monday afternoon), I am to meet with the guy who does estimates on major repairs, as I understand it he is a technician as well, for him to look at the system and measure the coil, to determine the cost to replace the coil.

I'm going to show him the above picture as well, and see what kind of reaction I get from him.


Thoughts?

 

[mrsilv04]

Edit for above... the stupid short time limit for editing posts on this site apparently ran out....


Tomorrow (Monday afternoon), I am to meet with the guy who does estimates on major repairs, as I understand it he is a technician as well, for him to look at the system and measure the coil, to determine the cost (parts and labor) to replace the coil.... IF one can be located (R22 system).

Since he hasn't seen any of this before, I'm going to show him the above picture as well, and see what kind of reaction I get from him... to see if he supports the second tech's diagnosis.

 

[StevieC]

Defrost cycle timer is not long enough for a defrost cycle. The valve is set too far open, or the refrigeration set point is way too cold causing it to constantly run. The humidity is above 75% in the ambient air.

 

[mrsilv04]

Originally Posted By: StevieC
Defrost cycle timer is not long enough for a defrost cycle. The valve is set too far open, or the refrigeration set point is way too cold causing it to constantly run. The humidity is above 75% in the ambient air.


It doesn't have a defrost cycle.


I'm going to knock out a few other questions...

It is not a rooftop unit.

It is set up like an ordinary home AC unit. A standard wall thermostat kicks it on, it cools down the area, thermostat turns it off. Repeat as necessary.

It is a single stage unit.

It is not set up for, nor is it used in cool outdoor temperatures.

 

[StevieC]

Ok then the valve isn't set right or there is a restriction somewhere.

 

[mrsilv04]

Originally Posted By: StevieC
Ok then the valve isn't set right or there is a restriction somewhere.


Yes. I'm being told that the problem is that there is a restriction in the coil. Here's a picture of the coil.

 

[StevieC]

If the coil is flooded with refrigerant and the compressor is strong enough to keep the flow it will freeze up it will also do that if there is a restriction creating not enough flow.

There could be junk stuck in the valve or it could be the filter/dryer.

If this is the case it usually means the compressor might be "done" because normal wear/tear metal pieces don't usually cause a blockage/restriction. So depending on what they find you might need a new compressor as well.

Try opening up the valve all the way, counting how many turns you turn the adjustment screw/head, run the compressor and see if the blockage clears and then close it back up by turning the screw/head in reverse the amount of times you turned it forward to the open position.

Good Luck.

 

[mrsilv04]

I've been watching the system more and more, and today, I kept checking it over a 4 hour period, and literally watched it slowly freeze up. Now I have literally watched it happen over time.

It seems like the problem starts with the expansion valve, circled in red. That's where the icing starts, and then it spreads into the bottom third of the coil.

The supply line, which is coming in from the bottom, is OK. But once it hits the expansion valve, then the freeze up begins.

Is it possible that I have a faulty expansion valve?

 

[StevieC]

If it starts at the valve the valve is plugged up with something or someone touched it and adjusted it wrong if it's that type.

Steve