Defroster is spitting out frost


#1

My 09 Subaru Forrester has been spitting small chunks of frost at me in the mornings, why? frost has been flying out of the defrost vents ~10 min into my moring commute for the last few weeks. The outside temp has been around 50 degrees (+/- 5) in the mornings, and the air has been humid. I rarely use my AC, maybe once a year. I did take the suggestion from a friend to maybe turn on the AC and see what happens, so i ran it for a day and the next couple of days no frost but it has now started again. Any suggestions?


#2

Your r134a may be low. When it’s low it will freeze the coil.


#3

In my forty years of mobile AC repair, I’ve never seen an under-charged AC system freeze over the evaporator. An over-charged system? Yes. An under-charged system? No.

On some vehicles, when the vent system is set to defrost it automatically runs the AC. If it’s cool and damp frost can build on the evaporator.

Tester


#4

I have to agree with Tester on this one. I’ve also seen a partially clogged orifice cause this.


#5

I ve never worked on car ac, but I did hvac for 5-6 yrs after high school, I ve seen the evaporator coil freeze many time because of dirty air filters.


#6

I have seen evaporators freeze with a low refrigerant charge. A lot depends on the environmental conditions and assuming the state of charge is not excessively low.

My opinion is that the gauges need to be put on the car and find out what the high and low side pressures are at elevated RPMs.


#7

When you operate the defroster, the A/C will also kick in to act as a dehumidifier… Try moving the temperature control to the midway position to blend in a little warm air and keep the evaporator from freezing…


#8

My home ac was low on charge and the condenser and evaporator coil froze up. Being all ac systems work on the same principals, I mentioned the low refrigerant.


#9

yes. low charge OR low air flow can cause this. it may just be an air filter clogged up. but low charge is more likely. I doubt that moving the temp control will do anything but possibly mask a problem


#10

Most posters here seem to agree that a low charge can cause a frosting evaporator. I’m really amazed. It seems to defy the laws of physics. Can anybody explain why this is true. I’m not doubting anybody because if it happens, it happens. I’m really curious.


#11

My understanding is that a low charge can partially freeze the evaporator. The low pressure side gets too low and the boiling point of the refrigerant is consequently too low. Most of the evaporator is still frost-free so the temperature sensing bulb doesn’t see the too-low temperature. I’ve seen this on my heat pump in winter. Evaporator (outside) 1/3 frozen and not much heat coming in. Tech added refrigerant; problem solved.


#12

Thank you. It’s starting to sink in as I think about it. I’ve just never seen it happen.


#13

OK - I’m still having trouble with this. As I think about this I’m thinking if you have a low charge, the high side would also be low . It’s the difference in pressure between the low side and the high side that causes the temperature drop which occurs after the refrigerant leaves the orifice tube (or the expansion valve). I’m thinking the pressure difference wouldn’t be as great with an overall low charge.

In my original post I stated that I once had a partially clogged orifice tube which caused the low side to cool excessively due to excessively low pressure on the low side. Even the low side tubes under the hood frosted up. A new orifice tube fixed everything.


#14

that’s the condenser outside,nunless you have a self contained unit, the evaporator would be in your air handler inside.

I ve really only noticed the evaporator freezing because of low air flow, condensation forms and freezes.

I would check the cabin air filter or the ducts for squirrel nests as well as the charge.

I m not saying the evap won t freeze because of low charge I ve just seen the condenser do it more often.

its been over 20 yrs since I did heat and air work, but just last year a friend had a prob with her evap freezing. a really dirty filter was the cause.

the orifice makes sense… two cents


#15

I realize the condenser is on the outside and the evaporator is on the inside. Believe it or not the orifice tube on this car is located up front right next to the condenser, not at the entrance to the evaporator like they used to be. That’s why the under hood tube frosted up.


#16

that was for insightful, some units are self contained with both outside, but most are set up as I said


#17

After I clicked Post Comment I realized it probably was meant for someone else. It’s getting a little confusing.


#18

“Most posters here seem to agree that a low charge can cause a frosting evaporator. I’m really amazed. It seems to defy the laws of physics. Can anybody explain why this is true. I’m not doubting anybody because if it happens, it happens. I’m really curious.”

Insight has it…With the refrigerant a little low, the “low side” gets way to low and the “spread” between high and low is too much…Result, ice forms…

Enjoy it while you can…Because when a little more freon leaks out, the high-side falters and cooling stops…


#19

OK. I’m convinced. Thanks to Insight & Caddyman.


#20

“that was for insightful, some units are self contained with both outside, but most are set up as I said”

For a heat pump, during the winter, the evaporation takes place outside and the condensation takes place inside.