HVAC question - evap coil freezing up

Factual Questions
1

This saga continues.

Short version - I nicked the copper U-tube in an evap coil last fall, and found someone to repair it (vs replacing the whole coil, which one outfit wanted me to do at a tune of $1500). About 2 months ago I had someone reconnect the coil to the rest of the system and recharge with R-22.

Now, the system runs for about an hour or so before freezing up, with frost buildup on the fins - which ultimately leads to little to no cooling ability.

I’ve looked into common causes:

  1. poor air flow / dirty filter
  2. lack of sufficient regriferant

I first looked at the filter and found it was filthy. Ah ha! I thought - so I replaced it. Not wanting to mess around, I put in a spun glass filter to maximize air flow. Still freezes up.

That leaves insufficient refrigerant. When the HVAC person was out to recharge it, we had a brief period of time when we thought the repair to the copper U tube was causing a blockage in the evap coil. It looked like the pressures on the high and low side were not acting quite right. Eventually, things settled out and the pressures, according to the tech, looked perfect. He was satisfied that he ‘blew through’ the blockage and things looked A-OK.

I am suspicious of two things:

  • the repair that was done may be impeding the refrigerant flow
  • although the pressures looked ‘perfect’ at the time the tech left, perhaps the system is still undercharged.

So what I’m trying to find are other symptoms of either condition.

If the repair in the tube had restricted flow to some degree (but not fully), could that cause the evap coil to ice over? I know there is a nozzle in the evap coil that changes the refrigerant to a gas form (thus lowering its temperature and giving the A/C its cooling properties) - so my initial thought was that as long as gas can pass through the partial blockage, it should be fine. However that may be incorrect.

When it freezes up, I notice it freezes from the bottom of the coil and works its way up. If the partial blockage was responsible for it, I’d suspect that the freezing up would be a little more a-symmetrical.

Is it possible that after running for a few hours, the refrigerant completely balanced out, and although the pressures looked fine as of when the tech left, that maybe it took more then ~15 minutes for the system to reach a steady state where pressures should be verified? The system is a 2.5 ton unit, with the evap coil about 25-30 line feet away from the condensor - and it was charged with about 8 lbs of R-22 (in the event that information can be useful)

Thanks for any assistance - like I said, just looking for other symptoms of either suspicion to try and narrow down the problem.

2

There is a good chance the humidity in the system was not evacuated correctly and frost forms inside the system and blocks the circuit.

3

Had this problem with our house A/C a couple of years ago. Diagnosis was a leak in the system that had reduced the total refrigerant quantity. Seems paradoxical, but yes, this makes the evaporator, but only a small portion of it (because of reduced refrigerant flow rate). Condensate freezes on the exterior of that portion of the evaporator, blocking airflow; now the still-cool refrigerant survives to cool the next portion of the evaporator, which freezes over, and so and on and so forth until the entire evaporator is covered with frost that completely blocks the airflow.

The solution is to eliminate the leak and recharge with more refrigerant. Rather than replace the 13-year-old evaporator, the tech squirted some sealant into the system that would circulate along with the refrigerant and glob up any pinhole leaks. Two years later, all is well.

Call a tech in to check the system for leaks. If no leaks (or once he has repaired them), recharge with refrigerant to proper levels and you should be good to go.

4

So am I to interpret that the lack of someone saying ‘HELL YEAH A PARTIAL BLOCKAGE COULD CAUSE ICING UP!’ is an indication that there are other more likely scenarios?

When the tech was here to recharge, I know he pressurized the system with N2 in order to make sure there was no leak and that it held pressure. I dont recall that a vacuum was applied to the system to evacuate and cause any potential condensed moisture to boil off - at least he never asked where he could plug in an electrical connection.

At the time, I half wondered whether that step would be necessary - as the two tube ends from the condensor outside were located in the basement, which has a dehumidifier that keeps things dry. Although the ends were taped to try and keep dust out, its a poor moisture seal. The tech saw the taped up ends, said that was about all I could have done, and proceeded with the recharge. I assumed he would have applied the vacuum if he felt it necessary - so I didn’t say anything, figuring its not my place to tell him how to do his job.

I just dont want to pay for another whole recharge of the system, because it was not cheap. If a required or best practice step was skipped, then I can have an argument that I am not going to pay again because they messed up - but I suspect that they will point to the coil repair and say that there must be a blockage causing the problem.

5

The system ABSOLUTELY needs to be evacuated before filling. If he didn’t do this, he needs to be fired.

6

ok, I’ll call the place back and try to get this sorted out. Thanks.

Is there any reasonable chance that he could have used a vacuum pump that doesn’t plug into a wall outlet - like one that is battery operated? I wouldn’t think so, just because my guess is that the power draw required to operate a pump would require a battery pack much bigger than anything he hauled out of the truck to the back yard where the condensor coil is.

7

When I worked HVAC a few years ago in high school we had a little dongle that we could plug directly into the cutoff at the unit and run the vacuum pump from there without needing an actual outlet.

That said, some of the guys at this shop were a little shady and did things like blow out the lines with R22 rather than wait the (sometimes excruciatingly long) time for a good vacuum. If they didn’t pull a good vacuum this could definitely leave some moisture in the lines and cause the coil to freeze. We had one or two callbacks about this.

8

From the other thread:

Sometimes it is worth it to simply bite the bullet and pay for the new part. It’s fine when you can do all of the work yourself, but if you have to pay a pro for a portion of the job, you might want to hand it all off to him. Sorry you are having problems.

9

Yeah this is all part of the gamble that is taken by doing a repair as opposed to a full out replacement. However, in this case there appears to be a pretty significant step that was missed - so I intend to call them out on it and see what can be done.

Worst case scenario is that they have their story (potentially bad repair that is out of their hands) and I have mine (they knew there was a repair and still chose not to do all the CYA best practices they possibly could have done), and they maintain that they will not do anything without charging for another service call or whatever.

In that scenario, I’ll look to try and figure out a way to automatically shut off the compressor once/hour for 10 minutes or something when the A/C is going - a relay timer or something. Not the ideal scenario - but could be good enough to last until it comes time to replace the furnace with something newer than a 1960’s model - at which time the whole central A/C system can be replaced with one using something other than R-22 and using the services of a different outfit.

10

I work stationary rather then resisential so I have more time but here is how it should have gone.

Your system was taped over that means it was open and had some moisture in it. While soddering the evap he should have N2 flowing through the lines. Also he should add a filter dryer to the unit. After he closed up the hole he should have put a trace amount of R22 in system and then pressurized the system with N2 and did a leak test and checked if the pressure would drop over time.

If no pressure leaks the using an 120 vac electrric vacumm pump evacuate down to 300 microns and see if the vacumn will hold over time. If not there is either a leak or moisture in system.

If you evap is freezing from the bottom that is an idication of a starved evap. Follow the liquid line (the smaller one) from your condencing unit to the evap. If at any point the temperature drops before the metering device then there is a blockage in the system.

NO one is perfect and you may still have a small leak that he missed, but if he did he should cover it.