rheostats getting warm/hot ??
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  1. #1
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    Default rheostats getting warm/hot ??

    I am a home inspector. Often lights or fans are controlled by dimmer switches. Since I spend 2 to 3 hours inspecting a home, I don't take a lot of time to recheck, but if the object being controlled is on for awhile, when I go back to turn off the object, sometimes the switch plate covering it gets warm or even hotter.
    Is this normal ?

    Thanks, Buckeye Bobbie
    May 11

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    early dimmers came mounted on finned heat sink cover plates.

    There must have been a reason....

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    rheostats? how old are the homes your inspecting?

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    There is a limit to how hot exposed surfaces can get. I do not recall the exact number, but I can say that "warm" is OK. Hotter than you would want as a shower or bath, is not.

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    Old rule of thumb: if you can keep your hand on it, it is probably OK. If you can't, then definitely investigate why.

    Reality Check: I am 76 years old. I worked in electronics for all my 45+ year career and I have never seen a rheostat used for a light dimmer in a home or commercial building. Rheostats are variable resistors and resistors are basically a device that converts electrical energy to HEAT in a controlled manner. That is basic to the operation of any resistor. So, if you really are observing rheostats, then the heat is probably part of their basic operation. And they should be designed to dissipate that heat: heat sinks or whatever; some means of dealing with that heat.

    BUT, each and every light dimmer that I have ever seen in use in a building's lighting was NOT a rheostat. They use electronic circuits to do this in a more efficient manner. So less heat is produced. Not no heat, but LESS heat.
    Earlier ones, several decades ago, and higher power ones used less efficient circuits and they often did have a heat sink that had external fins. Today the fins are often not needed because much less heat is generated. So the age of the light dimmer can be a factor.

    All that being said, excessive heat in any electric circuit can and often IS a sign of a problem. If you are not qualified to inspect electric circuits and you do not appear to be, then when you encounter a light dimmer that is excessively hot you should call in someone who is. It may be a simple problem, like loose connections. Or it could be something more serious.

    How do you know if the heat is excessive? See my first sentence, above. If you can't put your hand on the heat sink or face plate and keep it there for a minute or so, then it is probably excessive.

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    Yes, as to the "rheostat", I have not seen one for dimming except in very old theaters from the DC power days. Almost certainly any dimmer you run into will be an electronic device, probably employing a "triac" to control power.

    Older ones had fairly large heatsinks exposed on the front, typically newer ones do not. Neither type should get hotter than described.

    It IS possible that a dimmer which gets too hot is being used to control several new type CFL or LED lamps which are not designed for dimming. That is a fault which you should be aware of and check for.

    Some types of CFL or LED are OK for use with a dimmer, others are not appropriate for use with dimmers. Usually, inappropriately applied lamps will either fail, or simply not work well. However, in the process they may overheat the dimmer itself.

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    typical 500w lamp dimmer:

    no appreciable heat comes out of it.
    Attached Thumbnails Attached Thumbnails 0-042.jpg  

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    Quote Originally Posted by bobbie View Post
    I am a home inspector. Often lights or fans are controlled by dimmer switches. Since I spend 2 to 3 hours inspecting a home, I don't take a lot of time to recheck, but if the object being controlled is on for awhile, when I go back to turn off the object, sometimes the switch plate covering it gets warm or even hotter.
    Is this normal ?

    Thanks, Buckeye Bobbie
    May 11
    You are are a trained and licensed home inspector?
    Holy crap! Some customers deserve a full refund.

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    Every one of you guys knew the OP was talking about contemporary dimmers, not rheostats. You just had to display your 'superior knowledge' and blast the guy. Home inspectors' duties vary widely depending on the type. Building inspectors (electric) will be familiar with terminology and code, while a general inspector employed by, say, a mortgage underwriter has no need for such detail. Give the guy a break.

    Jerry did a good job in post #4

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    Quote Originally Posted by Gordon Heaton View Post

    Jerry did a good job in post #4
    Bullshit.
    New Theater dimmer racks with rheostats were still being installed in 1970 and they weren't DC either.
    "Old" style Triac dimmers can function just fine with LED bulbs, no overheating.

    Home Inspector or?? Why post an Electrical question on a machining site?

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    Quote Originally Posted by reggie_obe View Post
    Bullshit.
    New Theater dimmer racks with rheostats were still being installed in 1970 and they weren't DC either.
    "Old" style Triac dimmers can function just fine with LED bulbs, no overheating.

    Home Inspector or?? Why post an Electrical question on a machining site?
    Bullshit yourself. The guy said he was a 'home inspector'. If he was talking about some fancy 'theater rack' he'd have said so. Justify it all you want, it was just a minor dogpile.

    As for the 'electrical question', there are people here from all occupations and vast experience in many fields. An OT would have been in order though.

    I became aware, as an airline pilot for 36 years before getting paid to do machine work, that it is unwise to judge another uneducated or unqualified without due diligence. I worked with many flight attendants who held advanced degrees, even MDs and J.D.s and who had successful careers in those fields. Sometimes they just wanted to get away from the grind and do something more interesting. It is wise to consider someone at or above your own level in life until proven otherwise.

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    Quote Originally Posted by Gordon Heaton View Post
    Bullshit yourself. The guy said he was a 'home inspector'. If he was talking about some fancy 'theater rack' he'd have said so. Justify it all you want, it was just a minor dogpile.
    You really have no clue.

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    Quote Originally Posted by dian View Post
    rheostats? how old are the homes your inspecting?
    Quote Originally Posted by dian View Post
    typical 500w lamp dimmer:

    no appreciable heat comes out of it.
    How do you call the black part with a slider on the right side of the board?

    Who is talking about rheostate/potentiometer installed directly in series with the load? It's in the control circuit of the dimmer. And this type of dimmers is still widely sold and used in the US.

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    Quote Originally Posted by reggie_obe View Post
    You really have no clue.

    That may well be the case, but if so I am man enough to 'own it'. I have no need to ooze conceit in an attempt to prove otherwise.

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    Quote Originally Posted by bobbie View Post
    I am a home inspector. Often lights or fans are controlled by dimmer switches. Since I spend 2 to 3 hours inspecting a home, I don't take a lot of time to recheck, but if the object being controlled is on for awhile, when I go back to turn off the object, sometimes the switch plate covering it gets warm or even hotter.
    Is this normal ?

    Thanks, Buckeye Bobbie
    May 11
    If that was a concern to me then I would look at the final load on the circuit. Such as if it was a six light display on a ceiling fan or just a 100W lamp.
    A dimming control is going to chop power to the load. During the chopping power is spent during the switching. There is your heat.

    A rheostat is just going to heat up from resistive action. The farther the control is turned down (less power to load) the more heat from the rheostat.

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    Quote Originally Posted by MichaelP View Post
    How do you call the black part with a slider on the right side of the board?

    Who is talking about rheostate/potentiometer installed directly in series with the load? It's in the control circuit of the dimmer. And this type of dimmers is still widely sold and used in the US.
    That is a slide potentiometer (variable resistor).

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    Bottom line here is that a "rheostat" for a reasonable lamp load would be large, and in a ventilated enclosure. They waste heat, and are typically rated at a substantial fraction of the load power. They may be several inches to a foot or more in diameter.

    There are other controls for AC such as variacs, which are almost as smooth as a rheostat, that work on AC, and look just about the same as a rheostat.

    None of the above will fit in a wall box. And there is absolutely no reason why not knowing that makes anyone stupid or inferior. Just a matter of your background and specialty.
    Last edited by JST; 05-11-2020 at 06:00 PM.

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    Quote Originally Posted by reggie_obe View Post
    That is a slide potentiometer (variable resistor).
    Yes, I know. It was a reaction to Dian's "rheostats? how old are the homes your inspecting?".

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    Quote Originally Posted by reggie_obe View Post
    Bullshit.
    New Theater dimmer racks with rheostats were still being installed in 1970 and they weren't DC either.
    "Old" style Triac dimmers can function just fine with LED bulbs, no overheating.

    Home Inspector or?? Why post an Electrical question on a machining site?
    Warm and overheating are two different things. I would expect a triac to get warm under normal conditions.

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    Quote Originally Posted by MichaelP View Post
    How do you call the black part with a slider on the right side of the board?

    Who is talking about rheostate/potentiometer installed directly in series with the load? It's in the control circuit of the dimmer. And this type of dimmers is still widely sold and used in the US.
    whatever. the box never gets even warm. what op describes is weird. if my in-wall dimmers would get warm i would call the police and have the electrician arrested.

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