A-coil repaired, system charged, not as cool now

Problems related to residential installations.

A-coil repaired, system charged, not as cool now

Postby badge767 » Wed Jul 27, 2016 2:34 pm

Greetings all!

Let me apologize in advance for this likely becoming a somewhat long post, but the backstory is important as it relates to my question(s)-

Last week I cleaned my evap coil by removing the coil cover below the plenum and vacuuming the interior sides with a shop vac and soft bristle brush. There was quite a bit of lint build up, so it needed to be done. So far so good... until I'm replacing the sheet metal screws that hold the "triangle" piece of trim in one end of the coil. There are 4 pairs of holes that line up with the framework of the coil (which has numerous holes for the screws to engage). The upper set of holes is somewhat obscured by the plenum duct work and can only be accessed with a nut driver with long extension. (Anyone see where this is going?) The last screw accidentally hits the wrong hole in the trim, and upon tightening punctures a small pin-hole in an elbow on the a-coil (Hissssssss....) Well snap! I immediately know what I've done, so off to call a contractor to braze the coil.

Of course this happens during this recent heat wave we're having in the mid-west! I manage to reach a contractor that can respond almost immediately!

The techs show up, braze the coil using MAPP gas and a fill rod, charge the system with nitrogen to test for leaks, after confirming no leaks, they recharge with R-22 (this is an older system), and upon restarting the compressor and furnace blower motor, we're getting cool air from the registers. OK...so far, so good again...

Here's where some issues later pop -

Later that same evening (about 4 hours post-repair), the house simply isn't cooling down, and now the air from the registers doesn't feel as cool. I know enough about HVAC to check a few - The evap coil is freezing up about halfway up from the bottom on both sides. That seems to indicate to me a low R-22 charge. Fast forward to following -

I can't reach anyone at the first company that came out (and honestly, at this point my confidence in them has eroded somewhat) so I bite the bullet and reach a second contractor who can respond that same day to come out and check. When he shows up, I explain the back story to him, and tell him my suspicions of a low Freon level. He first checks the braze repair job using a spray-on leak detector, finds it is good and comments that it looks like the first guys did a nice job brazing the elbow (luckily an older coil with heavier copper fittings). As soon as he puts his gauges on the compressor outside, he confirms the level is low (about 40 - 45 PSI on the low side instead of the desired ~70 PSI). He proceeds to charge to the correct pressure, and while he's at it, hoses off the fins on the compressor coil outside. (He also commented on how nice the clean a-coil inside looked.)

So far so good again. System is blowing cool air at registers and house begins to almost immediately feel better (although I know it will take 12+ hours to really cool the now ~85F house back down to normal).

Now (finally) to my question(s)-
- Although the air coming from all registers continues to feel pretty cool, and the plenum above the a-coil is ICE cold to the touch, along with good air volume from the registers, the house just isn't getting quite as cool as it was before this entire fiasco began. I called contractor #2 to ask his opinion, and asked if there was such a thing as needing to "fine tune" the pressure on my particular A/C to +/- a PSI or two to find the "sweet spot". He advises 70 PSI on the low side is right where it should be, and it may be that now that the evap coils inside are clean, that the air flow has increased enough to not allow the air to cool quite as much as the previously "slower" air going through the dirty coil.

That seems (at first) pretty counter-intuitive to me, but after using the analogy of my car A/C blower being set to a slower speed and the air coming out colder (which, in fact, seems to be the case in my vehicle), it sort of made sense that maybe the same thing is happening on my home unit.

With all things being equal (assuming 70# was exactly what was in my system prior to the puncture), and the only variable that is now different is the fact my evap coil is clean, as well as the compressor coils outside, is it plausible that the increased airflow is now actually preventing my system from getting the house as cool as before?

For comparison - We're still having really hot weather, and before all of this, the A/C would maintain the house around 77F even when it was stupid-hot outside. (My attic insulation isn't the best, but that's a job for later this fall when it cools down.) Now, in comparable hot weather, the house will get up to around 80F with the unit continuously running. It is still fairly comfortable in the house, as the unit is doing a good job of removing humidity in the air (as evidenced by the water exiting the drain tube from the a-coil), but I'm at a loss for why the system, after all the cleaning to improve efficiency, now has lost about 3F "capacity".

Any pros out there want to help me solve this mystery, and possibly confirm or dispute the theory that the increased airflow through the clean a-coil has actually resulted in the house not becoming as cool??
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- A-coil repaired, system charged, not as cool now

Postby Freon » Thu Jul 28, 2016 3:15 pm

Your system should be operating more efficiently now due to the increased air flow through the evaporator coil. However you have not done a thorough job with determining the accuracy of the refrigerant charge. Simply measuring the low side and saying it's where it should be isn't the best method by any stretch of the imagination.

If you pull the disconnect to the condenser unit and then open the electrical box on the side you should find a paper for the technician to use for proper charging. You need to know both high and low side pressures as well as the high side temperature. Any addition of refrigerant will need about 10 minutes to stabilize. It's a slow process but then you will know your system is correctly charged.

Another possibility is there is moisture or other non-condensables in the system due to a poor evacuation with the vacuum pump. Did the tech add a new filter/dryer? It is also possible some tiny piece of debris is interfering with the orifice that meters the flow of refrigerant into the evaporator coil. A good tech will see this problem from the high/low pressure measurements.

One last point involves the heat transfer when a substance changes state. It's very possible the improvement in your system is lowering the humidity and not the temperature. The latent heat of condensation of water vapor (humidity) is what you should research.
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