On the other hand, the margins for cooling that manufacturers need to include would make Bernard's explanation a bit tenuous for most situations.
The
extra amount of dust likely to enter (e.g.) a TV set from being above a heater is likely to be small in relation to other effects - it may be a factor if other effects already combine to make things problematic. E.g if you have lots of pets in the house, you live next door to a quarry, several family members work in said quarry and come home covered in dust, no-one in the house is house-proud when it comes to getting the vacuum cleaner out, there's heavy smokers in the house, ...
In real terms, unless you already have a significant dust problem, the extra effect of adding a heat source is likely to be minimal.
I'll add that over the years I've worked on various bits of kit (mostly computers) that's been "a bit dirty".
One I recall (a "clean" sort of dirty) was in a printers, and the floppy drive in the Mac II (that'll date it if you know your Apple history) stopped working because it was full of paper dust. The rest of the machine wasn't much better but still going without problem.
And then there's a machine we got in (at a different employer) owned by a HEAVY smoker. It didn't escape us that the machine was owned by someone campaigning against nuclear power because of the health risks, yet smoked exceedingly heavily
To say the heatsinking would be significantly compromised by the heavy deposits would be an understatement.
Many a time I've taken a PC out back and got the industrial airline on it to clear out the dust bunnies - always outside, and always taking note of wind direction
On a different tack, I was friends with the guy that ran the local Apple dealer back when the Apple II was a current model. One of the customers he had was the local cellophane factory and there was a machine sat down on the factory floor doing some data logging. When the machine failed it was just scrapped - the chips were starting to fall off the board because the hydrogen sulphide (one of the "smells" that came off the process) had eaten the legs away
Now, in terms of making the cooling air for the TV "hot" then I doubt it makes all that much difference for most setups. The actual air temperature rising off most heaters, or at least any that are likely to be on for a significant length of time, isn't that high. Don't forget that in general the hot air will create circulating air currents that will cause the hot air to mix with cooler air that's drawn across the room - thus lowering the mixed air temperature once you get a little distance away from the heat source. And in the case of blown air heaters (whether electric or wet), the air is blown out and turbulence will mix things up.
And then there is a question as to how sensitive electronics actually are to temperature - and actually barring issues like Bernard raises, generally not all that much.
It's quite old now (2007), but Google's "
Failure Trends in a Large Disk Drive Population" report makes interesting reading. On temperature (Section 3.4) it says "
The figure shows that failures do not increase when the average temperature increases. In fact, there is a clear trend showing that lower temperatures are associated with higher failure rates. Only at very high temperatures is there a slight reversal of this trend". That was certainly interesting to many of us in IT, having been told as though it was a fundamental law of nature that "the colder the better" regardless - and I know I've been into server rooms where you put a thick coat on before entering ! Clearly there's a difference between hard disks with mechanical components and TVs without, but I would expect to see vaguely similar results if it were possible to do a similar study with them.
And if you are a hard drive data junkie,
BackBlaze's stats are always interesting to look at.
So the TL;DR version. I don't consider it a problem unless the heat source is particularly powerful/hot and perhaps you have a lot of dust around. A mate of mine has a 55" plasma TV above his fireplace - 2kW fan heater if he puts it on. That's not bad to watch if you sit back and put your feet up - I believe he is generally popular when footie matches are on.
