Eh? The puzzle says all the switches are in the basement and all the fans on the top floor, and that it will take him nearly an hour to climb the stairs.
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Quite possibly a furnace switch.
Hmm… can he just swap the polarity on one switch so that fan runs backwards?
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Turn on one fan and go upstairs. Observe which fan is on. Pick one of the fans that’s off, and replace the connection to the fan with a short. Now, when the lazy electrician returns to the basement, one of the remaining switches will blow the breaker when pushed.
I can imagine variations… I suspect the key to the puzzle is finding a way to know whether current flows through the switch or not. Even if there are no breakers, and no multimeter is allowed, you can observe whether a spark is created when lightly touching the bare wire to the switch terminal.
1 - The electrician rewires the fans so they work in the room where they are required, rather than having to walk up set of steps that take an hour to climb.
2 - If they aren’t allowed to do this, they call the person who installed the fans in the first place, after yelling at them and calling them an idiot, they find out which fans are connected to which button.
3 - The electrician re-wires one switch to use a lower voltage and thus will now spin slower. They turn on the rewired on and one more. They should now know which is which. They then call the landlord a moron and add idiot-tax.
4 - The electrician makes up an answer. The landlord naturally won’t ever know and is too foolish to allow them to rewire.
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little known true fact, this puzzle is based on a 2000 year old Roman puzzle called “The Lazy Toga Maker” - in which said toga maker has to carry the correct one of three different color heavy rolls of toga fabric up to three hilltop villages on three separate hills - on a dark and stormy night - through the fig groves - no possible communication - etc - how does he make sure he has the right color without having to carry three rolls up the first hill, etc?
i’ve never found the answer in English, but here it is in Latin:
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
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Except:
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But that takes two trips upstairs.
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These are Internet of Things fans, right? That’s cool.
So the lazy sparks just has to telnet into the router from his laptop in the basement, turn off the firewall, deactivate SSH on one of the active IP-listed devices, and wait five minutes for Russian hackers to snag that fan for a botnet. Then, um, turn on switch A, I guess, go upstairs, and see which fan is blinking a GoDaddy “bandwidth exceeded” warning at him, which one is spinning, and which one @doctorow is smashing to pieces with a copy of The Cathedral and the Bazaar.
Or something.
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Disconnect one, leave one connected, and short-circuit the third one. Go to the top, measure voltage and resistance, match to expected values. Go down and reconnect.
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That is the exact solution I came to first! Only problem is that the electrician does not accept assistance (from the renters in your case) or want to go up the stairs twice (to hang the string himself, like I concluded).
I also think measuring the current and switching around the wires seems too complicated for this puzzle. Even though he is a electrician and he clearly could do this.
My guess requires no trips for our lazy electrician (my explanation resembled code so much I put every condition on a new line):
If it’s winter:
First turn off the heating and open-up the automated windows or activate the AC.
Then turn on one of the fans.
Else if it’s summer:
First turn on the heating, close the automated windows and deactivate the AC
Then turn off one of the fans
Wait for the furious tenants to come down to the basement.
And ask him what his room number is.
Repeat until you got at least two people to make the journey down the stairs.
Optionally you could even go for three to double-check!
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If it is true that residual heat in the motor is detectable ( and it would be at least with a decent thermocouple, or since he is lazy, an inferred thermometer) the logic is one off for a long time.(cold), One on, and one off but warm (recently running).
Goes up to first floor and finds it off or on. If it is off he checks if it is warm. If it was on he walks up to second and it has to be off. All he needs to do is see if second is warm. No need to go higher.
Goes to first and it is off. Checks if it is warm or cool. Goes up to second and it is either on or off no need to go higher.
So if he can make all floors in less than an hour he can make the second floor in less than forty minutes.
This assumes he knew all the fans were off before he started. The statement does not make that entirely clear.
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Sorry, I though one per floor so yeah it will be the full hour either way.
On the contrary, there must be a reason the subject in our puzzle is an electrician and not something like a liberal arts major. 
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All right, let’s try redistribution of laziness.
The electrician is only willing to climb a total of three flights of stairs. It shouldn’t matter how those three flights are distributed.
So (calling the fan on the first floor Fan 1, second floor Fan 2, third floor Fan 3.):
- The electrician presses each button twice (to make sure that each fan is turned off)
- The electrician turns A and B on.
- The electrician climbs one flight of stairs. (Total: 1 flight climbed)
If the fan on the first floor is on:
- Either A or B controls Fan 1.
- The electrician climbs another flight of stairs (Total: 2 flights climbed).
- If the fan on the second floor is on, whichever of A or B that does not control Fan 1 controls Fan 2, and C controls Fan 3.
- Otherwise, whichever of A or B that does not control Fan 1 controls Fan 3, and C controls Fan 2.
- The electrician descends to the basement, and turns off switch A.
- The electrician climbs to the first floor (Total: 3 flights climbed).
- If the fan on the first floor is on, B controls Fan 1, and A controls whichever of the other fans was determined to be on after climbing to the second floor.
If the fan on the first floor is off:
- C controls Fan 1.
- The electrician descends to the basement, and turns off switch A.
- The electrician climbs to the second floor (Total: 3 flights climbed).
- If the fan on the second floor is on, B controls Fan 2, and A controls Fan 3.
- Otherwise, A controls Fan 2, and B controls Fan 3.
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Uhm, it says that all three fans are on the top floor. Of a very tall building, where it will take an hour to climb the stairs.
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Ah. Oops. Misremembered the riddle. Thanks!
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Actually, there is a simpler answer. Light a candle in each room. Turn one on and leave it on. Turn another on, and then turn it off.
Go upstairs and the one you never touched has a lit candle. The one you turned on is still running. The one you turned off and on has a blown out candle.
That still requires climbing to the top floor multiple times.
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Not a bad guess, but that’s not it since I know where the boiler switch is. Given that it’s 3-conductor romex, I’m kind of thinking that it’s one half of a 3-way switch that didn’t get wired up on the other side. But I can’t begin to guess what the other half would be.
