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Simple Schematic, Challenging Installation

I recently helped a friend install a “smart” Ecobee 3 thermostat in the second floor of his home, see “HVAC Upgrade Shows Interconnect Complexity of These “Simple” Systems”. He was so pleased with its performance and the Internet-connected functionality that he decided to install another, identical one on the first floor, which is a separate zone of his heating system and he asked for my help again.

Hey, I figured, this will be easy. We had worked out the many issues of this smart unit and his system controller, and I had fully documented the final three-wire interconnect with a physical and schematic hand-sketched diagram, Figure 1 . To keep it easy, we’d use the same make/model of DPDT electromechanical relay and 24 VAC power supply, and just duplicate what already worked.

Figure 1

This hand-drawn schematic and physical interconnection diagram of the first installation made me confident that the duplicate one would be quick and easy.

This hand-drawn schematic and physical interconnection diagram of the first installation made me confident that the duplicate one would be quick and easy.

Even better, when I checked the existing thermostat wiring, there was good news. Although the installed thermostat was a basic, dumb, two-wire unit, the previous owner or builder of the house had run three wires from the basement (where the heating system was located) to the first-floor zone. Thus, there would be no need to “snake” a wire from the basement which can be a major challenge due to the vagaries of construction, in-wall fire stops and insulation, and other obstacles.

I figured that this project would be about an hour from start to finish, and I packed my basic tools – wire stripper, pliers, crimping tool and crimp connectors, screwdrivers and similar – since I generally like to use my own, even though he had a fully equipped toolbox. Then, just as I was leaving my house, and probably out of long-term habit, I grabbed my soldering iron kit (iron, solder, some pieces of #20 and #24 AWG solid and stranded interconnect hook-up wires, and heat shrink tubing) and headed out.

When I arrived, Installed and wired the relay socket (another wonderful convenience) for this new zone, and ran the basic wires in the basement between the AC transformer, socket, and heating-system control, while leaving the old unit still working. I had to extend the existing wires, which I did using my crimper and extra wire, crimped spade lugs to the ends, connected them to the relay socket screw terminals — hey, it was all so easy, Figure 2 . Then we went up to the first floor, documented the existing wiring wires (hey, don’t laugh – even two wires can cause headaches), removed the old units, and declared we’d be done soon.

Figure 2

Relay #2 installed and wired next to its predecessor: so far, so good.

Relay #2 installed and wired next to its predecessor: so far, so good.

That was when the reality of old house wires became obvious. The three-wire run between basement and first floor was stranded #18 or #20, and it was old. So old, in fact, that the insulation one each wire, as well as the trio, was not easy-to-strip plastic, but some sort of woven, waxed, sticky fabric. It was hard to strip without nicking the strands, which is a sure set-up for failure at some random time in the future. So I used my X-Acto knife (another item I always pack) to cut the insulation from the inside outward to avoid making those nicks.

But that was actually the easy part of the wiring problem. The stranded wires themselves were not clean but had some of that waxy insulation substance had diffused onto each wire (it was not corrosion); I assume this is a normal occurrence after many years. Inserting these striped wires into the spring-loaded contacts of the smart-thermostat connector would not assure a solid copper wire-to-contact finger connection. Even more frustrating, the small openings for each contact in the connector were far too small for the #18/#20 stranded wire, Figure 3 .

Figure 3

As this Ecobee 3 installation manual photo implies, the push-in contact of the Ecobee 3 connectors are sized and intended for #22 and thinner-gauge solid wire (in my assessment); they certainly can't accommodate #18 stranded wire. (image source: Ecobee)

As this Ecobee 3 installation manual photo implies, the push-in contact of the Ecobee 3 connectors are sized and intended for #22 and thinner-gauge solid wire (in my assessment); they certainly can’t accommodate #18 stranded wire. (image source: Ecobee)

In the end, the solution was to add wire extenders. Much as I dislike burying a connection in a wall (it’s OK here with respect to the code, since it is only 24 VAC), I didn’t see an alternative. But I couldn’t solder right away to the stranded wire, as the waxy “whatever” on each of the constituent strands violated rule #1 of soldering: the copper has to be clean. I splayed out the strands of the stripped end of each of the wires, cleaned them with alcohol, and used some brown paper from a bag as a gentle form of sandpaper to get that crud off each strand. I then soldered a piece of #24 solid wire to each stranded wire, covered each connection with heat-shrink tubing, and inserted the new wire ends into the contacts of the connector. We inserted the relay, powered up, and the Ecobee thermostat recognized the connections were in place (it does that, which is very nice), we did the set-up and initialization, and all was good from there on.

In the end, this simple “no-brainer” one-hour project took a lot more time, effort, and thinking than we expected. Key to pushing this project to finish line were the soldering iron and the thinner-gauge hookup wire, along with the realization that we had to clean those existing wire strands to avoid cold-solder joints. All this effort was for an interconnect of three non-critical wires supported by full documentation of an identical installation!

It’s another reminder of that eternal lesson: ‘Sometimes things look like they are going to be easy, but they are not’ (but in rare cases, it’s the other way around). We’ve all had this experience: many years ago, I worked several levels under a department head whose real engineering experience was near zero. He was under the delusion that if it was easy to say, it would be easy to do. On one project, there was excessive ringing causing false triggering on a data line. The first “cure” was well known: insert a 100-Ω resistor in series to damp the ringing. We tried it on a pilot-run unit (we were past the breadboards and early evaluation-unit stage) and the fix worked fine. But as soon as he heard that the solution was “just stick a resistor in series” he thought formally implementing it would take just a few keystrokes.

That was one of my introductions to the realities of production. We had to update the schematic, do a re-spin of the PC board layout to accommodate that resistor, verify that the new layout worked, update the BOM, notify purchasing of the update to the BOM, change the assembly drawings and sequence, and do a bunch of other little things. While it was not an enormous task, it took a lot more than the simple nature of the change indicated to someone who had little concept of the leap from prototype to real production. I learned a good lesson, but unfortunately (but not surprisingly) that department head did not!

Have you ever had a project which looked to be simple or trivial to implement, yet ended up taking a lot more time and energy, and perhaps improvisation, than you expected? Did those around you understand, or were they skeptical of your reality and explanation?

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