We've had a couple of problems with our off-peak hot water recently, and last night, at 2:45 am (don't ask!), I noticed that the switch was still off. So I called Ausgrid's emergency number, and they organised for someone to come out this morning, which they did, at about 6:15 am.
Anyway, the guy simply bridged the switch, and attached a note saying that it needed to be replaced by a time clock because of "ongoing signal issues".
"Ongoing?"
Can they tell that there's a signal issue when the switching signal isn't being sent? Is there a constant pilot signal?
Or did Ausgrid already know that a problem was likely, and just waited for me to complain?
I've got a microcontroller based one with a PROM in a carrier marked "Air Conditioner 283Hz" , (I bought a bunch for $1 each mainly for the enclosures)
490 was from memory of the resonant reed in an an older waterheating one, the latching relay in that one was the mechanism from a telephone bell and a couple of opposing microswitches wired in anti parallell.
It seems they use different frequencies for different purposes, it's probably at the whim of the local power company too.
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?? 100% natural
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** The 1050 Hz signal would be attenuated by many factors - not least the distance from the tone generator to the consumer.
If you scope the AC ( or better the output of an AC plug pack ) you can see the tone super-imposed on the 50 Hz wave - running slowly along it in fact as it is not synchronised.
This thing has actually worked fine for many years. My expectation was that it would simply be replaced. The apparent instant decision to substitute a clock based system makes me suspect that either that's the standard approach to any failure of a signal based system, with no diagnosis involved, or Ausgrid already know of an issue in my area, and just wait for complaints before acting.
I hope the clock has a decent backup for power outages.
I wonder if you'll have as effective water heating too? That is, if a clock makes it come on at night time only, when demand is low, you'd miss on the occasional top up through the day (during brief periods when demand is also low). I think that's the advantage of signalled systems - otherwise we'd all just have clocks?
Last clock one I had was a mechanical one, and it stoped during power outages. Luckily you could remove the face and re-adjust the time. You could also trigger the hot water at any time you wanted to by rotating the dial. I thought that they'd be insisting on the smart meters these days, have three of these in the new place (one per phase).
There's no top-up on the tariff I'm on. In any case, as I understand it, top-ups require a separate element, higher up in the storage tank, which mine doesn't have.
Storage hot-water systems rely on the fact that water is a very poor conductor of heat, so the water at the top can stay hot even though cold water has been added at the bottom as the hot water is used. If you try to heat up the cold water at the bottom, you'll create convection currents that make the water in the tank mix, and the water at the top will be cooled down, after which the only recourse is to heat all the water in the tank to the desired temperature.
Tanks with a separate top-up element lose significantly more heat.
** That ( oft heard) comment always grates heavily with me - cos water sure ain't no thermal insulator !!
Nothing cools something down faster that dumping it into cold water or pouring cold water all over it - fire brigades have relied on this fact for quite a while now ....
Water is a better thermal conductor than the vasty majority of materials - except for metals.
Compared to metals water is a poor thermal conductor PROVIDED that internal circulation is prevented However, in most practical situations, this provision is *NOT* met.
Water simply bubbles and convects like crazy when heated and takes away huge amounts of heat from any hot surface it comes in contact with - likewise pouring hot water over ( or immersing ) an object heats it rapidly.
Also, compared to metals on a weight for weight basis, water is a FAR better absorber of thermal energy - about 5 to 30 times better than common metals.
It takes 11 times more heat to raise the temp of a gram of water ( by a given amount) than a gram of copper.
** Hot water is less dense than cold so rides on top of any cold water introduced at the bottom of a tank, so heat from the top layer passes relatively slowly into the layer underneath PROVIDED there is nothing to cause mixing of the two.
Heat transfered by circulation is not heat transferred by conduction. Usually there is circulation, and the poor conduction is not apparent. Prevent the circulation, and then you see the low level of conduction.
That is to say, in the absence of convection, and because water has low thermal conductivity, the top stays hot and the bottom stays cold.
Yes, because you get convection, which is what I said.
I predict a change in the subject line, and some pointless abuse, now.
I don't mean a separate boost element, just that the main element might come on at any time there is adequate power available. A plumber told me, and it sounds plausible.
Looks to me like you objected to the statement that water is a very poor conductor of heat. One can have a debate about what "poor" and "very poor" mean, but you justified your objection by reference to the effect of circulation, which is a different phenomenon entirely.
But the effect of doing that is to cause convection that mixes the remaining hot water with the cold water, which is undesirable except when restoring the temperature of all the water to "hot". Unless the latter is achieved, users will find that the water is tepid.
The boost systems use a separate element higher up in the tank. The idea is that it only produces convection above it, and so has less water to heat back to "hot".
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