The Shop > Electronics & IC Programing
Water Heater Monitoring
RussellT:
I agree that you will make best savings by adding insulation, but a timeswitch will also produce significant savings. In a lot of households water only needs to be heated for a couple of hours a day if the tank is well enough insulated.
--- Quote ---saving are vanishingly small
--- End quote ---
I'm not sure I'd agree with that either. I was making the point about a tank with no water drawn off to clarify that there must be a saving but in practice things are more complicated. In most domestic hot water systems with a storage tank the hot water sits on top of the incoming cold, so it's quite possible to have the bottom of the tank warming up as the incoming water is cooler than the air surrounding the tank while the top half is losing heat. Interestingly in that situation it's possible to reduce the water temperature at the tap by putting the heater on because the heater starts convection currents and destratifies and mixes the contents of the tank.
There are lots more factors at work here too - for example where is the thermostat? Most gas heating systems have a thermostat about two thirds of the way down the tank and so won't heat more water until a third of the hot water has been drawn. In most electric systems the thermostat is combined with the heater so won't come on until most of the hot water has been used.
As far as losses in the heating process go they are practically zero for electric heating as all the electricity is turned into heat and is surrounded by water - the heat has to go into the water. (I'm assuming cables are correctly sized etc).
In the case of gas heating if you are heating a cold tank the boiler efficiency will be improved compared to a tank that is already hot because the water returned to the boiler will be cooler and heat transfer in the boiler will be more efficient.
Ok. I'll get off my hobby horse now.
Russell
Manxmodder:
Cheers Russell,that last part answers my earlier query about heat transfer efficiency between the heat source and the heated liquid becoming less effective as the 2 temperatures become closer.
That is how I have always understood it to be......OZ.
awemawson:
"Rate of heat transfer is proportional to the absolute temperature difference"
That was the answer to the ONLY technical question I was asked in the interview for University applied physics course :bugeye:
There were lots of other none technical ones and after all they had my exam results but I was rather shocked to say the least - perhaps they were surprised that I got it right :lol:
Manxmodder:
Cheers Andrew,this is evidenced by simple observation of how the cooling system on a car engine works more effectively in cold weather conditions vs heatwave conditions. The only thing that has changed is the heat differential between the engine coolant and ambient air temperature.
It therefore is also clear that a good deal of energy can be saved with a water heating device by having the upper limit on the thermostat set at a temperature that isn't heating the water to a higher temp than needed.
.....OZ.
sparky961:
Who knew such a mundane topic could elicit such spirited response? I'm thrilled. :)
So, I stuck it with a meter tonight just to make sure what I was working with. I currently get 121V and 243V, which I'm sure will vary by a few volts depending on the current grid load. If I go with Andrew's updated math it still gets me back to my original number (which I had yet to double). I'm favouring the hypotheses that the element isn't accurately rated or my makeshift setup isn't accurate enough, but I still have to wonder if it worked out ok for 60W (I tried 3 different bulbs) and 100W why it wouldn't for 3800W. True that incandescent bulbs offer non-linear resistance, but the current is inversely proportional to temperature and certainly stable once warmed up. I was indirectly measuring the current through whatever effective "on" resistance was there at 120V. It would be better to characterize my setup using a known resistance closer to that which I'd like to measure, and some points in between but I don't have that many 100W light bulbs to connect in parallel** to get a few KW....... or do I....? :)
(** Boy, just about put "series" there. That would have caused a RIOT!)
The side debate that's been going on regarding whether putting a timer on it can make it run more efficiently (to really distill it down) is essentially what I'm trying to prove or disprove to myself. The most accurate way I can come up with is to actually measure the power it uses and log the data over time, giving me kWH to compare timer vs. no timer. True, there are commercial meters that do just this but they cost money (why not buy a steam engine kit instead of making all from stratch? - age old question, no good answer) and more importantly I don't like the interface. A simple display doesn't let me process the data as I will inevitably want to.
Bottom line? Geez, I don't know. ;)
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