Freezing suction line

[Milk man]

Suction line freezes at the condensing unit at 28 degrees.

The outlet of the evaporator was 34 degrees.

Evaporator was in an attic with about a 30 foot drop.

It has two ton drive with a three ton condensing unit. Fixed restrictor.

The only thing I can figure out is the coil is over feeding and flashing close/at the condensing unit.

EXCEPT, for the obvious install a three ton drive.

[dnt]

Fixed orfice should be charged using the superheat method. You stated that the suction line at the evap was 28f now is that saturation temp derived from low side pressure or just a thermometer stuck on the line? Fixed orfice systems are very sensitive to proper charge level, be sure you are reading total superheat (temp at condensor/compressor end of line set) Do you have a superheat charging sheet and know how to use it? Do you have a accurate thermocouple or thermistor type thermometer? You can probably adjust the charge and make it work but it will probably never be very efficeint and may suffer early compressor failure if the required charge level cannot support sufficient oil return to the compressor. 1. What type of refrigerant are we using here R22? 2. What is your outside ambient temp dry bulb 3. What is your evap inlet air temp wet bulb (hopefully you have a psychrometer right?)? 4. What is your low side pressure reading? 5. What is your suction line temp at the condensor/compressor? 6. What is your high side pressure? 7. What is your liguid line temp at the condensing unit/compressor? It is always best to start with the basics Check filters if dirty change out/check dampers and grilles for blockage/check condensor and evap coils for dirt and clean as required/adjust charge only after all other items have been cleared as being the problem.

[Milk man]

Thanks for the help, but I have all the bases covered. The furnace has a two ton drive and a three ton condensing unit. Which is low airflow. It won't work right until I get enough air accross the evaporator.

It is a scroll and fortunatly has an accumulator.

I guess what I was stating is that I've never seen a temprature drop in a suction line. Ten years doing this. About a 6 degree temprature drop from the outlet of the evaporator and the inlet of the accumulator.

[dnt]

I saw what I really thought was a temp drop like that just the other day, Refrigerant was contaminated some type of crap like R134a in a R22 system I was getting some real crazy readings. I recovered and recharged with virgin r22 and got perfect readings, no doubt it was contaminated with something, are you sure about your refrigerant?, your saturation temp could be way off from what you are thinking it is ausing you to be way undercharged which would make the evap starve and freeze? Just a thought. Do you think a TEV added to the evap might be able to control the superheat enough to make it usable? I guess now I am curious as to how we got this thing mismatched?

[Milk man]

Someone sold them a furnace with a too small blower. Furnace is a year or two old and the CU is 1992.

[Milk man]

I still think liquid is flashing in the suction line because of low airflow.

[Milk man]

An undercharged system takes a while to freeze. Low airflow will freeze it up a lot faster. No blower will frost it almost imediatly. This thing was frosting fast.

[Milk man]

now I'm just driving up my total posts.

[markco]

Has it got a bypass humidifier? If so, is the bypass closed?

[ct2]

Lynn, I had a system that cools a machine cabinet the other day that froze due to a low heat load. The machine shut itself down so over the weekend the electronics were'nt producing the heat that it normally would , it was overcast and humid in sac that weekend and when I came in everything was frozen

[Milk man]

Originally posted by markco
Has it got a bypass humidifier? If so, is the bypass closed?

No humidifier.

Again, the blower in the furnace is not big enough to move the amount of air a three ton system requires. The blower was built to push enough air for a two ton system. I am about 400 cfm short. This is a low airflow situation.

I think not all the liquid is boiling off in the evaporator. I think it is boiling in the suction line, therefore the suction line is colder at the end (by the CU) than at the begining (by the evap. outlet). Does anyone else think this is correct?

Air delivery is by far my weakest point. But the way I understand it is we need about 400 cfm per ton. I need 1200 cfm for the AC to work correctly. The furnace will only push around 800 cfm.

[bbk]

I'm ass u me ing the blower mtr is hooked up on high speed and that's all there is!?

[electrotech]

if there is a restriction in the suction line, (kink, undersized, clogged filter-drier, etc.) it will cause a pressure and temp drop between the a.h. and cond. unit, this can lead to freezing of the line at the condenser, and not at the a.h.

[beenthere]

Lynn, Your right, its boiling all the way to the condenser.

If the furnace is 2 years old, did the a/c work last summer.

If so, then what changed, coil might be just dirty enough to cause this. Since it was already low on air flow, it won't take much.

If its the first a/c season for this set up. Get the 3 ton blower out of your hip pocket and install it.


Some manufacturers use 1 size blower housings for 2 or 3 different drives, just the wheel, and motor are different, check to see if thats the case with this furnace, and you might be able to change just the wheel and motor.

[Milk man]

It was the bosses gig. BL (Before Lynn) I guess they had some noise issues so a smaller drive and some duct work change. When the CU craps out they will get a 2 ton unit.

[stephy]

A three ton condensor on a two A/H...good luck..
Peace,
Stephen

[docholiday]

If you can verify the load (airflow and temps) to be correct, and you feel as though you are near the correct system charge, then you have a restricted suction line. Either an oil trap, kink, undersized or something. If the evap is frozen solid you could theoreticly be boiling off in the suction line.

Your suction line should never drop more than a couple degrees from inside to out.

[Milk man]

Not my deal anymore. Boss man "explained" it to me. I put a frost stat on the suction line at the evaporator because I didn't want it to ice in the attic drip on the ceiling when it thawed.