Re: Warning about cheap TVs currently on sale

DM wrote:
John Rumm wrote:

You seem to be attempting to shift ground a little here. The discussion was whether the TV consuming power at 100W would also be radiating energy to the room at 100W. I take it you now accept that it would once it is warmed up?

Any time delay or lag in response does not change the efficiency, although it lowers the responsiveness. A TV is probably comparable to a normal radiator in terms of response time.

John there is not a shift of ground or anything - however things have inevitably drifted.

There *is* quite a substantial change in ground here if you read some of your earlier contributions to this thread. I could quote them for you, but I am sure you can find them as easily. This is not in itself a bad thing in that it suggests that your understanding is changing (and perhaps understandably you would rather not admit some of your earlier statements were somewhat ungrounded). I get the feeling there are/were a few low level misunderstandings that influenced some of your earlier statements. I am not trying to score points here, just fill some of those gaps if I can.

(I don't intend that to sound condescending - accept my apologies if it does)

The position remains the same- not all sources will heat up a room to the same extent- In this context we are talking about a tmeprature increase- there is no argument regarding energy in = energy out.

All sources dissipating at a given power *will* heat a room to the same extent and to the same temperature once steady state is reached.

[This assumes the heat is being spread about a bit by convection etc]

Let us take an absurb extreme situation - we take a 1kW eletric heater- it raises the temeprature of the room by T1.

ok, getting to the crux of it here... how would you work out T1?

(answer at the end)

Now we stick it in insulated box in the same room- the temperature rise in the room is obviously less. So we have less heat trasnfered from the source to the room.

Ah, ok, I there is another slight gap here. Insulation will indeed change the rate of flow of heat - it increases the "thermal gradient" or the temperature drop across the boundary. This is why it works well the in walls of your house.

However, rate of heat loss is dictated not only by the level of insulation (or the thermal resistance), but also by the temperature differential.

If both sides of a wall are at 20 deg c then there will be no nett flow of heat from one side to the other. If there was a 10 degree differential then there will be, and raising it to a 20 degrees difference would double the rate.

So, if your electric fire had a thermostatic control such that it turned off once the room temperature reached 25 degrees, then sticking it in an insulated box would change the power consumed and passed to the room since the internal box temp would very quickly reach 25 and turn the heater off. Hence reducing the average power consumed from 1kW to a few hundred watts probably.

Now, if the heater was like the one in your experiment and not thermostatically controlled, what happens is different. The temperature in the box will rise very quickly to a much higher level - say 150 deg C. This will increases the temperature difference across the insulation, and hence the rate of heat loss through it. So the heat loss from the insulated box will remain at 1kW. In reality the insulation has made no difference at all.

Yes this is obviously fairly extreme, but as soon as you stick a bunch of electronics inside a plastic box then this condition exists to some extent.

Indeed it does, and as described above it makes no difference to the heating effect.

Where it does make a difference is to the designer of the electronics in the box. Care needs to be taken to ensure that the temperature rise of each component is controlled, and over heating damage does not result (or more likely; cause serious reductions in component lifetimes). Ideally, with electronic items like a TV this would be achieved using convected air flow (the so called chimney effect), and heatsinks on vulnerable components to aid heat loss. In more extreme cases forced air cooling or some other mechanism will be required.

A TV is not as efficient way to heat up a room- it is not a good source of heat in this respect., where heat is indicated by a rise in temperature of the room.

I hope you can see from the above that this is not true. A TV will get to operational temperature and start dissipating heat at a rate equal to its electrical power consumption within in a few minutes. In this respect it is easily as good as most central heating systems in terms of lag.

The issue of lag is also not that important - think of an electric storage heater. These are designed to introduce a massive lag between energy going in, and it being delivered to the room. However they are still intended to be effective heaters.



Answer to question posed above:

You need to know three things (and a fourth in real world cases - but we can ignore that for now):

The thermal conductivity of the rooms construction materials (the "u values" as they are called in the building regs)
The surface area of each material used.
and the rate of power input.

So, we know the power input is 1kW

lets assume the rooms is starting out at the same temperature as its surroundings.

Lets assume it is made from materials all with a u value of 2 w/m/K, and surface area of walls, ceiling, and floor totalling 100m^2.

So equilibrium will be reached when heat out = heat in, so:

1000 = T1 x 100 x 2

therefore T1 = 5 degrees

(which sounds pretty low, but in real life you don't normally have 6 outside walls to a room!)


--
Cheers,

John.

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| John Rumm - john(at)internode(dot)co(dot)uk |
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