In the summer of 1992 we moved from Arizona to Michigan. In late September we closed on our new house. The following summer we learned that the house did not have air conditioning. (Coming from Arizona, and spending the amount of money on the home that we did, we never dreamed that the question needed to be asked.) After a couple years, we needed to take the plunge. We had two attic AC units installed.
The original house had baseboard hot water heat with four zones and four thermostats. The original SPST single-function mechanical thermostats were replaced with programmable ones that supported both heat and AC. Two of the four where brought into play for the two new AC units. One had been added with a new room addition and with a little foresight, all the needed wiring had been run, blunt cut, and left in the attic. However, the thermostats used a common connection for the two transformer power sources from the heat and AC systems. The other had an existing pair of wires going through the attic to the thermostat. The AC installers pulled down two more wires for the AC and fan returns, but made the common connection in the attic to a single wire. Needless to say, sneak paths were ubiquitous.
The AC worked fine that summer, but in the fall when I turned on the heat, without first shutting down the two AC units, the heat failed to work. The furnaceís electronic switch module (ESM) gave off that unmistakable electrical burn odor. The furnace (probably original to the house, which was built in 1964) needed the ESM to sense water temperature and signals from the zone control valves, and then safely to turn on the gas valve and the re-circulator pump. Its main relay had burned, and I figured its time was up. I could not find a replacement for what I considered an affordable price and so, with parts I had lying around the basement, I made a solid state relay (SSR) replacement.
There was ample room for it in the ESM enclosure. The decision to make it was fortuitous since it forced me to take some voltage readings to pick parts for the SSR design. Finding 58 V AC in the ESM was a real eye opener. The old furnace wiring made prodigious use of the ground offered by all the copper baseboard plumbing and the connection to the house water pipes. Given the above-mentioned sneak circuits, I ended up getting two transformers in series. This provided overvoltage to the relay. I would have burned up another one had I found one and installed it without discovering the root cause.
Needless to say the AC installers should have known better and should have checked these things at the time of the install. Knowing now that they did not, I was not about to get them involved with this repair. At this point I drew up the circuits to see where the issue originated and realized that I actually had three transformers in the complete system and all were interconnected at the single furnace. I needed to rewire the thermostat in the room addition to get its transformer in parallel with the other AC unit (the polarity on this other one cannot be changed due to the missing wire from the attic to the thermostat), and then change the polarity on the furnace transformer to get it in parallel with the other two.
The heat ran fine that winter, but during the following summer, the AC on the single-wire thermostat would not come on. This I traced to a blown fuse in the attic control unit. Again not thinking root cause, I replaced the fuse and lost the new one just as quickly. By now the family was complaining about the heat, so I killed the furnace circuit breaker and disconnected its transformer from the thermostats. Now, with another new fuse, we enjoyed AC for the rest of the summer.
With a little more time for reflection, I went back to my circuit drawings and eventually realized that there was a short to ground through the furnace that should have blown the fuses in both AC units. To this day I do not know why the other did not go. The fix was to change the polarity on all three transformers. Logic told me that doing so should not really change anything, so I took another approach. Toward the end of the summer when I was home without the family, I bought a new thermostat to replace the one in the new room. This new one had a separate connection for each of the heat and AC transformers, effectively isolating its AC unit from the rest of the system.
I rechecked the voltage on the other two transformers and found that they were not connected in series and now the voltage to the plumbing ground was only 24 V AC. The heat came on that winter and now the only time I have trouble is if I have to replace a zone control valve or break into the electrical for any other reason without being able to kill the circuit breakers on all three systems. (This is rare but it happens.) If I am not very careful I can short something to the plumbing and lose a fuse.
One thing I have not done is look at the polarity of the three units as wired into my fuse box. The two AC units are 220 V AC and the two control units are on a single 15 amp line (no issue here). What might need to be changed is the house phase that the 110 V AC furnace is connected to. This will have to wait for now since things are working fine.
This entry was submitted by Bernard Smith and edited by Rob Spiegel.
After nine years in the Air Force, flying the F-4 Phantom (WSO), Bernie Smith got an MS in Electrical Engineering at Arizona State. He then went to work in the Military Aerospace (McDonnell Douglas) and Automotive (GM and Chrysler) industries.
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