Debugging my HVAC system

It's about that time of year in North Carolina where my Yankee Blood compels me to engage the modern marvel known as central air conditioning. High temperatures average in the mid 80's fahrenheit (~29C)  , but can sometime go above 90F(32C). It's also the time of year that you tend to discover something has gone wrong with your air conditioning hardware. Nothing stresses me more, it's one of the more expensive maintenance items facing a homeowner.

Last weekend, while my son was using one of our space heaters Xbox I noticed that the air conditioner didn't seem like it was keeping up. Heart-pounding, I sprinted through the 2nd floor of the house checking the air vents. Sure enough, they all seemed barely cool. We have independent air conditioning units, one cools our first floor, and the other cools our second floor, and the room on our third floor that serves as Anibit's central office.

The "evaporator" and blower part of the AC unit lives in an unfinished, uncooled, part of our third floor. You can usually feel that the ductwork leading from it is cold when running. It was warn. Bad news. Time to let the wife in on it so she can join me in the journey on the potential repair-bill coping process.

I went outside to where the compressor unit lives, and noticed that it was silent. Neither the fan nor the compressor motor was running. The coolant lines where exposed were slightly cool (everything had been working 30 minutes prior). Hmmm. This unit was relatively new. We had it installed about 4 years ago. The prior uint had, once or twice had its starter-capacitor die. The capacitor is only needed to handle the large current draw when the compressor motor first turns on. Last time we had it replaced by a service technician, it ran us about $250. I searched online and found the model capacitor used in my unit. $26 with express shipping.

I'm a genius! I saved ~90% (even more probably, since prices on service have probably grown since the last time). So we sweated it out for a couple days. Luckily, the weather changed mostly in our favor, going slightly below average. It did rain a lot, so the open windows and fans were bringing in a nice sticky-humidity you only really know about if you live in The South.

The capacitor eventually came, and I set upon swapping it out. The old one did have some rust on it, but not excessively. I love electronics, as you are well aware if you read this blog. But I avoid wall mains power as much as possible, because I'm clumsy, and make a mistake or two every once in while. The AC unit runs off the 220V appliance line, so I'm even more paranoid. There was a lot of voltage testing, and ground checking. I wore gloves and goggles, and used plastic-handled tools.

Then came the moment of truth. I had my wife on the phone, inside "Ok, go ahead and set it to 'cool'".

"Ok, it's flashing 'cool', flashing 'cool', ok it turned on"

CLICK! The relay in the outdoor unit engaged.

Nothing.

"Sonofa, I was so sure that was it! Hmm, wait keep it on, let me take some more measurements."


The fan and compressor motors are controlled via a large relay. Measuring the voltage on the output side of the relay revealed that it was not conducting.  "Ah, the relay is bad!'

Not wanting to wait for another part to ship, and having the luck of a little free time in my schedule, I tracked down a local supplier a half hour away for a new relay.

I installed the new relay, still with as much caution, but not quite as much pomp as the capacitor. I turned the unit on, and raced out to check the compressor(my wife wasn't around this time).

Success!

Mechanical relays are relatively simple devices, and this one wasn't very old. I wondered to myself about why it might have died, and why they don't use a solid state relay. Maybe the high current makes it unsuitable, maybe AC solid state relays are prohibitively expensive or complex.

I decided to take apart the old relay and take a gander. The root cause became obvious at once:

Do you see it?

How about a closer look at the contacts (click on the image for a larger view):

Oh the antmanity!

Encrusted at each contact was a mass of dead ants. I have heard that ants were attracted to electricity, especially fire ants. We have fire ants in our area, but I have never noticed them on my property. At the same time, we have ant's in our house 365 days a year. This winter was the first time in 10 years I went more than a week without finding an ant in my kitchen, but they were back in rotation by february. I'm not sure if these are fire ants. If so, they are a little smaller than I typically see them. I'm not a biologist, but it is possible that 220V might cause a drop in ant stature.

The ants were all clustered right at the contact. There were no ants elsewhere in the relay.  One thing I'm not sure is if the ants collected there in one event, eg they picked that fateful day last weekend to throw an electrofest, or, if it was a build up of individual ants that happened to by passing by right when the relay kicked in crushing them. Either way, it's hard to muster up any sympathy for them.

--P