Lights off. Heater on. What?

April 6, 2010 1:39 AM

Why does turning off my lights sometimes cause my heater to turn on?

Setup: I have an apartment with electric heat. Sometimes when I turn off the lights in the living room, the electric heater in the living room (whose thermostat is directly above the lightswitch) turns on instantly thereafter (literally as if the lightswitch itself were controlling the heater). What's going on here? Vibrations from me hitting the light panel? Changes in voltage that I don't understand? Ancient Indian burial grounds?

omgconfirmationbias math: In the absence of a causal relationship, these things should be completely independent. I toggle the lights in the living room 4 times a day, tops, and the heater turns on less than half a second later. Even if the heater cycled 100 times per day and I toggled the lights 16 times per day (both huge over-estimates), I'd still only have a ~1% chance of seeing this in any given day, and this usually happens at least once a week in the winter.

posted by 0xFCAF to science & nature (10 answers total)

Crazy idea: Thermostat reacts to infrared from light source (and overestimates temperature when light is on)? Experimentum crucis: Try shielding the thermostat from radiation from the light source and see if it triggers the effect.

More likely: It's really something in your electrical setup. Experiment: Try inducing similar or greater power dips in other ways and see what happens. If you want more detailed speculation, tell us about your lights and heating setup. Do you have DC lights behind a transformer, regular light bulbs, fluorescent? Where is your thermostat?
posted by themel at 2:02 AM on April 6, 2010

Do you know what kind of thermostat it is, i.e. is it the mechanical kind with a bimetallic coiled strip that tips a mercury switch (or otherwise opens/closes a contact), or is it solid-state? If the former, then we can rule out line voltage variations and focus on heat-motivated theories. If the latter then both are viable.

In theory, the lights being on would cause an ever so slight dip in line RMS voltage such that when they go off the RMS line voltage increases. This effect would depend on the wattage of the lights and the resistance (lengh, guage) of the wiring -- assuming that both the lights and the thermostat are on the same circuit. I can't see how this would really affect the thermostat however, because line voltage varies all the time for this very reason and if it's a solid state thermostat it's likely DC driven which means the line voltage is stepped down and regulated.
posted by Rhomboid at 2:45 AM on April 6, 2010

It could be an electrical thing.

Most thermostats use hysteresis, meaning that the temeprature at which they turn on is different than the temperature at which they turn off. Ie, turn on when less than 18C, turn off once over 22C.

A little jolt of current as the circuit settles could be affecting the temperature sensor somehow to misread the temperature as being lower than it is (17.9 instead of 18.2), causing the heater to turn on, and then it stays on, even though the temperature sensor is back to normal after the jolt, because it has to keep working until it reaches the shutoff temperature.
posted by molecicco at 3:01 AM on April 6, 2010 [1 favorite]

If its an old style thermostat with a mercury switch the act of turning of the light switch, the slight vibration resonating through the wall could cause the mercury switch to activate. try just tapping the wall near the light switch and see if you get the same effect.
posted by jmsta at 4:07 AM on April 6, 2010

Seconding jmsta: Vibration pushing an almost, just not quite tripped mercury capsule or snap-action bimetal strip to its trip point.
posted by TruncatedTiller at 6:31 AM on April 6, 2010

Try plugging a high current device like a toaster into the same circuit and see if it has the same effect.
posted by JackFlash at 11:49 AM on April 6, 2010

Thirding jmsta. Flipping the light switch right next to my thermostat does the same thing. So does tapping the wall, or closing the immediately adjacent door.
posted by Dimpy at 4:29 PM on April 6, 2010

Some (most? all?) thermostats you can take cover off and look at the mechanism inside. If it's vibration that triggers the thermostat, then taking the cover off will trigger it too. But if you leave the cover off for a while, the next time you go to flip the light you can watch what is happening inside.
posted by fantabulous timewaster at 5:50 PM on April 6, 2010

Even if the heater cycled 100 times per day and I toggled the lights 16 times per day (both huge over-estimates), I'd still only have a ~1% chance of seeing this in any given day

If the heater did cycle 100 times per day, out of 2880 half-minute time periods per day, the heater will cycle during 3.47% of them. Observing during 16 randomly chosen half-minutes, we would expect the heater to cycle 0.556 times a day on average. Since I started with your over-estimates, the heater does not cycle so often, and you notice it about once a week.

If I pop by unannounced 16 times a day and stay for half a minute, I will indeed have a less than 1% chance of observing both the lights being toggled and the heater cycling on the same visit (assuming you behave as you normally do -- we'll pretend), but you have been observing every time you toggle the light. It's not a randomly distributed event.

Also, it's possible that the drop in temperature that happens while the heater is off makes it more likely that you might choose to leave the room and turn off the lights, which would make it slightly more likely that you would observe the heater turning on at that time. Or, walking around near the thermostat might move colder air towards it -- not all air in the room is the same temperature.
posted by yohko at 1:13 AM on April 7, 2010

Compulsively tapping the wall like a crazy person every time I walked by eventually reproduced the behavior.
posted by 0xFCAF at 11:22 AM on April 7, 2010