Blank Thermostat: cause and fix

Over the years, I had AC issues almost every year due to the aging AC unit we had. The most common issue is blown capacitor to the outdoor condensers. Rarely but costly, AC units reached the end of service life.  For AC issues, I typically call AC service company, but I did solve a few issues myself, including a) short life of my second floor AC condenser, b) one bedroom really hot, no cold air from vent.  Friday last week, I had blank thermostat , tried to DIY without success and had to call AC service.

Here is the story.

Friday afternoon last week, our second floor AC unit thermostat went blank, and the  2nd floor AC stopped running. It was hot there. But the first unit worked just fine, worked much harder than usual because the single unit had to cool the whole house.

 

The first thing came to my mind, the breaker to AC unit tripped, it did not. I searched internet, for this brand it might have a battery, and battery died, it did not have batteries. 

I called my AC installer, a technician came. The technician was pretty sure what happened,  a clogged AC drainage line made the thermostat blank. He checked the AC unit drainage, it was backed up! 

Here is the AC drainage set up. The drainage lines from the two units merged at a vertical line, and the drainage goes to a bath room sink drainage. 

drainage lines merges at a single vertical PVC pipe

The black hose connects the AC drainage line to the sink's drainage

He was trying to unclog it for me. While he was working, I asked quite a few questions. 

He took off the hose first, the hose had some debris in it, but was not clogged. I cleaned the hose, he put it back on. The 2nd floor AC drainage line was still backed up.

There is a block valve to the 2nd floor AC unit drainage line. He installed a block valve to the 1st floor AC unit drainage line. So he could use nitrogen compress air to blow the blockage. He closed the block valves, applied pressure from the dysfunction side, a couple times, the drainage did not clear. He then applied pressure from 1st floor unit AC line. He opened the block valves on both lines, did not see drainage clear. He told me that I had to get a plumber to clear the drainage line. Before he packed up his tools, he applied the pressure one more time from the 2nd floor unit AC line, the dysfunctional one, water shot out of the 1st floor AC unit emergence drainage line.

Then the the water in clogged AC line for 1st floor unit cleared! Both units were working. Before he left he told me that if it backed up again I had to call plumber.

The vertical open pipe is the emergency relieve 

On the early morning of Saturday, around 3 am, I was awaken by the un-idling first floor AC, it was running but not cooling. I went to attic, checked the 1st floor AC drainage, it was backed up. I drained it manually, to a large bucket. The AC ran, and started to cool again, in about 30 minutes the unit idled. I raised AC set temperature 1 degree, and went to back to sleep.

But I did not solve the root cause of the problem, the clog in AC drainage line(s) remained.

Lily and I analyzed the situation, and decided that the clog must be above the junction of the two drainage lines because condensate backed up in only one AC  drainage line. 

I called the AC service during the day on Saturday.  I talked to the technician on duty, and convinced him to come to my house to unclog the drainage lines in attic, above the junction, for me. I also requested that the service visit should be free since the problem was not solved from the first visit Friday evening. He  got the approval to make the service call free of additional charge.

The second technician came Saturday evening. I did not drain the 1st floor AC line before he arrived so I showed him the backup. He checked the AC units, drainage lines, one worked, one did not. 

He applied pressure from 1st floor AC line, and then 2nd floor AC unit. One time he applied really high pressure, the drainage lines shook with pretty loud sound. Both AC drainage lines were clear. We could see that 2nd floor AC drainage line clear through the transparent U-pipe. He poured some water in the bucket into the 1st floor AC emergency drainage pipe, it drained quickly, he poured more water, it drained again. We were pretty confident that he unclogged the AC drainage lines. 

The worry now was that the vibration from the high pressure could break some pipe couplings, and make the drainage lines leak. We looked and could not see any issue, we decided that I had to monitor the situation.

U-pipe indicated proper drainage 

A week later, everything still works, no leak anywhere. 

Summary

1) Blank thermostat can be caused by a) tripped breaker, 2) clogged AC drainage line

2) If only one AC unit drainage backed up, it means the clog should be above the junction of the two drainage lines, AC service is responsible.

3) If both AC unit drainage backup, it is likely 

        a) both lines above junction clogged,  still AC service's responsibility

        b) the clog occur below the junction, say close to the black hose near the bathroom sink, then a 

            plumber is needed

4) To prevent future clog or mold in drainage lines, we need to pour some bleach or vinegar into the drainage lines. Note one must NOT mix bleach and vinegar. Mixing these two produce chlorine gas!which is harmful to one's health and may cause damage to your drainage lines.