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A/C does not work!

posted Jun 24, 2009, 1:05 PM by Quoc-Anh Vuong   [ updated Jun 26, 2009, 7:33 AM ]
It's June, and the A/C does not work. This is the worse time for the A/C to break down. I did not know about the issue until when I was cleaning in the backyard and notice that the compressor is "short-cycling". This is a trade term, indicating that the compressor and condenser fan is on for few seconds, then shut down. Few seconds later, the compressor & condenser fan is on, and then off again. The whole cycle repeats ... until one of the components burned out! I did not want to wait, so I shut the A/C down, and that was a week ago. Last couple days, the temperature in the area is climbing to 85+, and the inside the house is just ... unbearable. So, it's time to "fix it".

Understand the service cost

Quick surf on the net, here is the plain field of DIY vs. Professional Services
  • $24/gal to charge Freon or R22. I believe our Lennox A/C requires about 5gal. This requires licensed technician
  • $105 - $150 to clean the condenser coil
  • $150 - $175 to clean evaporator coil
  • $120+ to replace running capacitor
  • $75+ to replace starting capacitor
  • $900+ to replace compressor. This requires licensed technician.
  • $200+ to replace condenser fan (motor)
  • $230+ to replace blower (motor)
  • $80 diagnostic
So, as you can see, I want to do all the tasks, which do not require license.

Clean the condenser

On Sunday morning (yeah, the Father's day), with the help of Bảo & Thuỵ, we clean the condenser coil.
  1. We remove the top, fan motor, the guard grill
  2. Scoop all the leaves, debris, branches, dirt from the inside chamber. We use a gutter scooper to do this
  3. Connect a soap (dish washing soap) dispenser to the garden hose, we spray the outside, and then the inside. Spraying from bottom up, side to side.
  4. Thoroughly rinse with water. Spray from the inside, until no more bubble remains on the condenser. I make sure to rinse two more time, to get rid of all the soap on the aluminum fins.
  5. Re-connect the fan motor, put back the guard grill, and the top.
  6. Remove the capacitors, inspect. No bulging, a bit of rust, but they're look ok. Well, any way, I took down the numbers and went online order them anyway. They are cheap enough. Perhaps, for the future.
  7. On Tuesday early evening: Put back everything, and tested. Same thing. Short-cycling!
  8. Went inside, to the air-handling system. I usually use two cheap EZ-Flow fiberglass air filters (20x25) instead of an expensive paper pleaded kind, ie. Filcret. My argument is that the cheap fiberglass filter allows much better airflow, but does not trap that much hair & dust; so I use two & change them regularly. Remove one (the dirty one) so that we have more airflow, just to test it out. Same thing: short-cycling.


This evening (Wednesday), I want to test:
  • On the way home, buy a box of 20x25 filters. We ran out. Done.
  • Make sure we have 24VAC on the contactor. Yes.
  • Make sure we have 220VAC on the compressor and fan motor terminals. Yes.
  • Safety sensors: HPS & LPS, high-pressure switch (condenser side) & low-pressure switch (evaporator side), to make sure they are both closed (not open).
    Alright here is the problem:
    • After running for 10sec, the 24VAC to the contactor no longer available.
    • Trace back to the Compressor Delay, then to the HPS and LPS.
    • Looks like these switches are in series, the 24VAC comes in to the LPS first, then to the Compressor Relay, then to the HPS and finally to the Contactor.
    • Trace back to the LPS seems to cut the 24VAC. So, what's wrong with the LPS, and why does it open after running for 10sec?
  • Make a copy of the circuit diagram, to identify other components. Done. Look at here
  • Check the RC (cooling) & Y terminals on the control board, to make sure we do not have a weird behavior which may shut off the compressor pre-maturely. Reference this article.
    Yes we have a solid 24VAC coming out to the Condenser.
  • Check the voltage on the thermostat, the cool to see if the thermostat is working correctly. We should see solid 24VAC on the cool (Y) terminal, activating A/C. Yes, we do have solid 24VAC here as well.
  • Also, check the Compressor Delay, making sure that it work accordingly! Because I hear a click about 10sec later, after the compressor/fan being shut-off. Then 5sec later the compressor/fan is activated again. The timer or delay should be 5min. instead of 5-10sec! So, that click must be from the HPS/LPS or some over-heat protection switch.
Conclusion: This is a low refrigerant pressure switch, which shuts down the compressor and motor fan to protect the system.
Action: call AAAnnandale Heating & Cooling to recharge the refrigerant. Ready for $100 service call.



  • The HVAC guy showed up last night to charge the refrigerant. I asked him how much per gallon, he said  that he charges flat rate of $160. I & Huong argued that other HVAC guy charges only $24/gal, and I know our A/C can only handle maximum of 5gal. The LPS trips that mean our refridgerant is low, not completely empty; perhaps it only needs 3-4 gals max. At the end, he agrees to charge only $120.
  • He start charging and the fan quit. So he said the fan motor is out, and that needs replace, which is another $350 (on top of $120). I told Huong to ... pay him the $120 and let him go, so that I can go out & troubleshoot the A/C myself.
  • Huong paid him $130 instead, he was happy leaving. Promised to call me back with a quote for replacing the whole A/C system.
  • I went out & measured the voltage across the Contactor, the fan motor, the capacitors ...etc. Nothing! No voltage to the fan motor at all. Yes, we have 24VAC but not the high voltage.
  • I replaced the running capacitor, and the starting capacitor; which arrived today. They were not the culprit, but since they are here, might as well replace them.
  • I then reset the HPS, because the fan was not running so the HPS tripped and shut the compressor down as well. Reset the HPS allows the 24VAC to pull in the Contactor, but still the compressor & fan did not run.
  • Could not figure out what's wrong, still no 230VAC for the compressor & fan.
  • Put everything back, and went inside, it's around 10pm, and hot.
  • Research on the net ... suddently it dawn on me: I did not check the circuit breaker!
  • Ran down, sure enough the circuit breaker in the panel tripped. I reset that, started the thermostat and the A/C begun to run. Yes, we now have A/C.
I could not be happier! We save $350, and make A/C to live a bit longer, it's more than 14 years old now. I know we should change it, but ... hey, we don't have the budget at this point. Replacing the A/C system (3-4ton) at this point would cost us another $6-$7K easily.

Woke up this morning in a cool house! Having coffee in the kitchen, with working A/C, is a beautiful moment. The coffee sure tastes better, perhaps it was my imagination.
Quoc-Anh Vuong,
Jun 24, 2009, 2:50 PM
Quoc-Anh Vuong,
Jun 24, 2009, 2:50 PM
Quoc-Anh Vuong,
Jun 24, 2009, 2:15 PM
Quoc-Anh Vuong,
Jun 24, 2009, 2:35 PM