Debugging In The Dark


So what do you do when an ice storm knocks out your power? Well, this time I was prepared, sort of. I live in eastern Pennsylvania north of Philadelphia. Our house is now without power as are a lot of neighbors. We lost a couple trees (Fig. 1) but luckily we had the large ones in the front yard trimmed earlier in the year. We were also the lucky ones that happen to have a gas generator. I took a cue from a couple neighbors that had them last year.

I picked up a Generac GP6500 last year after we lost power for over a week. That trashed the contents of two refrigerators and made it just nasty because of the cold and dark. I had been meaning to get a generator because we lose power for at least a day a couple of times per year. It is why I have more than half a dozen uninterruptable power supplies but they only help when the power is out for a few minutes.

I had done some testing with the set up that includes a manual transfer switch. That swaps out the circuit devices are connected to the generator instead of the regular circuit breakers. I had half a dozen circuits that include important things like refrigerators and the garage door openers (so we can get the cars out). Of course, other important things like lights, the HDTV and all the computers including my servers were on other circuits.

And then there is the heating system. It is a little convoluted because there are three zones. Two are hot water basedboard heat and one is hot water plus forced air. The first two worked just fine and I thought the latter worked when I tested the system before but it did not want to cooperate when the power went out.

The starting using Nest thermostats (Fig. 2) when I reviewed them (see An Elegant Thermostat Designed For The Internet). The baseboard zones with the Nests worked but the Nest that controlled the other zone was out to lunch. Its battery was running down and nothing worked.

The one that did not work also controls the air conditioner in the summer and there is a separate fan. I started by trying to debug the fan connection after charging the Nest by plugging it into the USB port of my laptop. I discovered the Nest’s debug capabilities because it knows what wires are connected and tracks the power connections so it was able to tell me there was no power for the fan. It does not sound like much but it takes awhile to walk around in the dark to find this stuff.

Unfortunately this Nest is in a hallway where it was dark since I didn’t hook up all the lights in the house. It is rather large with dozens of breakers so there is a lot the generator does not handle. I verified the lack of power using a multimeter and then went down to the basement to try to figure out what was up. Tracking the wires using a flashlight was not much fun but I eventually found where the wiring for the thermostat split. A couple wires went to the hot water system and they were hot, electrically speaking. The other wires for the fan and air conditioner were not.

What I finally figured out was the fan is on the 220 volt circuit pair for the air conditioner. My initial test of the system worked because I did not take the air conditioner off line. Of course, with the power out, that circuit was dead. So here we are with hot water but no fan.

Unfortunately this is going to take some outside support. I could probably rewire things so they would work but since I normally have someone else service the system they might like to know what is going on. I am not sure if the fan can be moved to the circuit that the heater is on but that would fix the problem.

I am now out of the dark where the lights, HDTV and laptops are running. Luckily this area is in the zone where we have heat. It is likely to be a couple of days before we get power back. Hopefully it will not be another week but there are thousands of people without power so it takes time.  On the plus side, it was warm today so the ice was melting but more bad weather is on the way. I am just glad I will not have to debug my alternate power system in the dark with a flashlight.

If you are nearby I hope you have power or get it back very soon. I’ll tweet when it does.

Discuss this Blog Entry 2

on Feb 5, 2014

I hope Mr. Wong stays safe and unharmed. We readers wouldn't want to lose any more dear writers of Electronic Design.

on Feb 11, 2014

Seems a bit strange for us with forced air furnaces to have a system that might use 220vac blower or fan control relay. 120v systems have 500-1400 cfm blowers that run on 120vac and might need 300-500w to operate. Not familiar enough with the nest to know how it senses power, but I would suspect that it is only verifying the 24Vac typically used in thermostat controls. All of my furnace connections with fan control are all 120v, the 24vac is supplied through a 120/24vac furnace transformer which should last through an EMP pulse. The only 220vac needed is for the compressor and exhaust fan within the condenser unit (outside). Most of these devices have schematics on the removable panels so a field tech can see what goes where. It sounded like maybe there was an additional 120v circuit that feeds the control for the A/C that wasn't migrated to your transfer switch setup.

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William Wong

Bill Wong covers Digital, Embedded, Systems and Software topics at Electronic Design. He writes a number of columns, including Lab Bench and alt.embedded, plus Bill's Workbench hands-on column....
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