My heating system is divided into two zones. There are very clever motorized valves that open and close based on the heat demands of the thermostat in each zone.
Near the end of last winter, I noticed that one of these valves was not working right. It seemed to be taking a very long time to open and close. If I helped it along by hand it would rotate over the way it should and stop, but it seemed to be taking forever on its own. Then it was literally taking forever: it just wouldn’t do it.
I detached the motor head from the actual valve, turned the valve to “on” by hand, and left it. Its motor was shot, but still had enough oomph to turn itself and the rotary switches; just not enough to turn the actual valve. So for the remainder of last season if Zone A wanted heat, Zone A got heat, and if zone B wanted heat, both zones got heat.
This fall I took the motor unit out of the wiring (labeling the 8 wires attached to it with he screw numbers they came off of; the wires were color-coded actively wrong) to a heating supply company where I know a guy. My guy wasn’t there but one of his minions took what I had, went into the back, and brought out something that looked the same but had a slightly different part number. I assumed it was a works-alike replacement. When I installed it I discovered it had an extra screw terminal in a spot where mine had had an unpopulated spot for one. I was unconcerned. I installed it, I started the system, and I went to bed.
In the morning the house was 95º in that zone (which does not contain my bedroom; that’s how I was able to sleep through it). Turns out that the one I took out was a “two wire” model made for “two wire” style thermostats, which have a single “need heat” connection: if it needs heat, that gets set high. When the thermostat is satisfied it sets that wire low and the valve shuts. The one I put in, on the other hand, was for “three wire” systems, which have a second “heat satisfied” connection (that extra screw). When the thermostat is satisfied it’s supposed to set that one high instead. So when I started the system it started up fine…and never stopped.
I shut down the system and took the valve back to the heating guy. He saw his error immediately, apologized profusely, and gave me the right unit, which was exactly the same part number as the original broken one.
When I got to the basement, I discovered that in my haste, several of my labels had come off the wires. So I had to actually learn what they all did and figure it all out to put the thing in. Which I did. Started the system up and babysat it through a complete start-heat-stop cycle.
In figuring out the wiring, I discovered that the person who had put in the valves in the first place had used whatever wire he had at hand instead of using some of the multicolored wire designed for this application, with the colors that are specific and standard for modern thermostats. And worse: what he had was red/white two conductor cable. What he needed was red/white/yellow/blue/orange/green. What he used was the red wire from three pieces of the red/white and the white wire from a forth piece. So there were four cables where there should have been one, four spare conductors neatly twisted back and wrapped out of the way, and it was a big confusing mess.
This was roughly concurrent with my formulating the plan to replace the thermostats. So when I pulled in the new wires for the thermostats, I used some of that wire to replace all that mismatched confusing wire with correctly-coded wire.
On the electrical box that is attached to and feeds the boiler, there is a 24VAC transformer. This was connected to the valves’ wiring and appeared to be powering the thermostat system. At the end of the thermostat-valves-etc chain is of course the wire that goes into the boiler control to signal it to turn on. This wire I did not touch, because I didn’t want to open up the control box. According to what I could tell, the boiler was expecting voltage on that pair for “demand heat”. When there was a demand for heat I could hear a relay in there clicking over.
It turns out that the boiler control box was also supplying 24VAC on that cable. It was expecting simple contact closure. It was not expecting another voltage source on those wires. Why was there an external transformer at all? have no idea. Why was the wire connected to the boiler control color-coded backward? I have no idea except perhaps that the same yabbo who used all red wire to do the valve wiring is the guy who did this. But whatever the reason there were two transformers, and by “lucky” chance I hooked them up hot-to-ground, ground-to-hot. So the first time the thermostats wanted heat, Bad Things were going to happen.
Bad Things happened.
Assume you have two identical transformers. One cost $12 and is right out in the open and simple to replace. The other is part of a $550 unit and is buried behind an unfortunately-placed pipe. Assume further that at least one of them is going to fail.
Of course the one that failed was the one in the control unit.
So the next morning there was no heat.
I went down into the basement. The furnace control box was buzzing. It had never buzzed before. I fought the cover off of it. I saw the transformer in there and swore. The “demand heat” relay was buzzing, but not pulling over. I pushed it over with a screwdriver. The heat started up just fine. I disconnected the thermostat wiring and all that from the unit and tried just shorting the “demand heat” lead. The relay buzzed. It did not pull over. I swore a mighty oath. It took me about half an hour to get the control box removed from the little niche it was in, behind the main radiator return pipe, where I had roughly an eighth inch of clearance. I went to the HVAC place, where my guy was back from vacation. I showed him my unit. He sold me a new unit at cost. $220 is still painful, but its better than $550.
When I got it back into place, I noticed that it was not quite the same. (but unlike the last time, this one really was a works-alike updated unit, clearly marked “replaces older models XXX, YYY, ZZZ”, including mine) The old unit had 8 screw terminals for 4 pairs of wires: 120VAC in, Pump power, gas burner control, and 24VAC out/demand heat.
The new unit has 13 terminals: they’ve separated “demand heat” and “24VAC ground” into separate connections (which is a good update), and it has connections to control and sense the position of an active damper, if my boiler had one (Which it does not. Which is fine; it shipped with those circuits disabled). Also, one of the gas burner control wires (just one) now wants a spade terminal instead of a screw. WTF? So a quick trip back to the hardware store for some crimp-on female spades, got it all hooked up, and started it up. Success!
Tried to put the cover on, and one of the screw ears on the cover shorted to the 120VAC hot wire. Which blew the breaker, damaged the ear, and luckily did no other damage. It’s been running with the cover off ever since then. If this heating season ever ends, I’ll put the cover on again when everything is shut down.
About an hour later there was a BANG as the overpressure valve opened, spewing water onto the floor. The new unit shipped set for 190°F water, and the system was designed for 180°F. Adjusted that, and all was well.