water heater energy logger

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#1
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I'd like to record the on/off times of an electric water heater with two heating elements.   A couple weeks of monitoring should be enough.  My first thought is a battery-operated pro-mini, with 2 pins monitoring each element.  I'd make a small circuit to connect to the 240V element with a half-wave rectifier and a 1M/10k resistor divider to output a 3.4V peak when the element has power.  I'd log the on/off times to EEPROM, dump the EEPROM when the logging is finished, and write a simple script to convert to CSV.

I tried to think of a non-contact solution, but the wiring into the water heater is BX shielded.

 

I'm going for quick and simple, with no intention of designing a product to sell.

 

 

I have no special talents.  I am only passionately curious. - Albert Einstein

 

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If you just wrap a wire around the live feed - so no actual contact with 240V - you will likely pick up enough "noise" to tell whether it's ON or OFF ...

 

You can also get these current  transformers quite cheap:

 

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awneil wrote:
You can also get these current  transformers quite cheap:

Link please

 

(Possum Lodge oath) Quando omni flunkus, moritati.

"I thought growing old would take longer"

 

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ralphd wrote:
connect to the 240V element with a half-wave rectifier and a 1M/10k resistor divider to output a 3.4V peak when the element has power.

Please don't try that!

 

(Possum Lodge oath) Quando omni flunkus, moritati.

"I thought growing old would take longer"

 

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I tried to think of a non-contact solution, but the wiring into the water heater is BX shielded.

Get a 240v light bulb (such as $1 neon indicator*) & just monitor that with your photodiode...even a butterfly board might do the trick (has a light sensor).

 

 

* I added one to our old dryer many years ago, so I could tell when it was actually heating.

    ..I eventually put it on wheels and took out the drum, installed a shelf behind the door & now use it for my chop saw!

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Wed. Mar 3, 2021 - 05:01 PM
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ralphd wrote:
I tried to think of a non-contact solution, but the wiring into the water heater is BX shielded.
Access is in the breaker box.

The thermostat will vibrate a bit (microphone or etc.)

 

edit :

Low-BOM Microphone Interface Using the Analog Signal Conditioning (OPAMP) Peripheral

 

edit2 :

There "should" be an electrical junction box in the water heater's closet ideally with a switch.

Reason : Plumber can easily disable heater's electrical power without having to ask where's the breaker box.

 

"Dare to be naïve." - Buckminster Fuller

Last Edited: Wed. Mar 3, 2021 - 05:11 PM
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awneil wrote:
You can also get these current  transformers quite cheap:

ki0bk wrote:
Link please

https://www.sparkfun.com/product...

 

https://www.amazon.co.uk/Akozon-...

 

https://uk.banggood.com/1PCS-Non...

 

https://www.ebay.co.uk/itm/19198...

 

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Thanks for the links Andy!

 

(Possum Lodge oath) Quando omni flunkus, moritati.

"I thought growing old would take longer"

 

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awneil wrote:

If you just wrap a wire around the live feed - so no actual contact with 240V - you will likely pick up enough "noise" to tell whether it's ON or OFF ...

 

You can also get these current  transformers quite cheap:

 

 

Several years ago I tried wrapping 28AWG wire about a dozen turns around a 120V/60Hz line, and couldn't detect it with my multimeter.  I don't have any current transformers on hand, and don't want to wait for an order.  I'm also not sure if they are small enough to fit behind the access panel on a water heater.

 

p.s.  I may try repeating the test anyway, but with an op amp or darlington.

 

I have no special talents.  I am only passionately curious. - Albert Einstein

 

Last Edited: Wed. Mar 3, 2021 - 06:43 PM
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avrcandies wrote:

I tried to think of a non-contact solution, but the wiring into the water heater is BX shielded.

Get a 240v light bulb (such as $1 neon indicator*) & just monitor that with your photodiode...even a butterfly board might do the trick (has a light sensor).

 

 

* I added one to our old dryer many years ago, so I could tell when it was actually heating.

    ..I eventually put it on wheels and took out the drum, installed a shelf behind the door & now use it for my chop saw!

 

I like the way you're thinking.  I've got a bag of PC817 optocouplers.  I'll tap off the 240V power from the element, put a 10k resistor in series which would give a peak current of ~34mA.  I'll connect the other side of the opto to a pin on the AVR with the pullup enabled, and the emitter to GND.

https://www.farnell.com/datashee...

 

I have no special talents.  I am only passionately curious. - Albert Einstein

 

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Did something similar using a relay for isolation. Sampled the contacts every 7.5 seconds.

There are no prizes for guessing the reason for that interval.

 

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Don't forget the diode on the input, or put a reversed diode across the opto, so you don't blow it out with the first reverse voltage 1/2 cycle.

 

If you tap into the heater behind the access panel you now have to be concerned with how hot that area gets, in addition to electrical insulation.

 

How accurate does your timing have to be?

(Especially since hot water usage is likely to be somewhat variable.)

 

Remember that you could put a thermistor against the tank wall, under the insulation, and measure the temperature.

When off, it will slowly fall, (think of an RC curve).

When the heater turns on the falling temp will now start to rise.

 

The confounding variable in this approach is the tanks intake of cold water to replace the hot water that is used.

 

You might also consider an AC current Hall Efect Sensor.

That would be the easiest approach.

 

JC

 

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It might be easier to access the heater feed at the switchboard to put a current clamp on the active. Or as the doc suggests, a sensor like an ACS712 or similar can be placed close to the active wire and you should get a healthy reading without having to connect to mains. Due to extra distance, it might not be accurate but you’ll be able to resolve the difference between no Amps and 20 Amps clearly.
I had a board with an acs712 current sensor - it could pick up the magnetic field of a nearby relay. This was not a design feature!

Last Edited: Thu. Mar 4, 2021 - 01:31 AM
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DocJC wrote:

Don't forget the diode on the input, or put a reversed diode across the opto, so you don't blow it out with the first reverse voltage 1/2 cycle.

 

Thanks for the suggestion.  I probably would've figured that out the hard way.  I have some 1N4007's somewhere, but since I have a roll of 1n4148's sitting on my desk, I'll put that across the opto in reverse.

 

I'm not going to put the AVR inside the heater access panel, I'll just put the opto in there, and run the low-voltage wires out to the AVR.

 

I thought about temperature sensing, but that's a lot more complicated.

 

I have no special talents.  I am only passionately curious. - Albert Einstein

 

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That 10k resistor will get warm.
my rough calculations ;
Opto load of 10k at 5V -> 0.5mA collector-current,
a worst case CTR of 50% -> opto diode-current of 1mA
240VAC -> 340V peak, using a threshold of 200V -> feed resistor of 200k at about 150mW (half-wave)
(don't forget to check that your resistor can handle at least 350V)

Last Edited: Thu. Mar 4, 2021 - 04:30 AM
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If you can unhook the power (somewhere) then let it go thru a cheap watt meter.

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mikech wrote:
That 10k resistor will get warm.

You are right, and I think you are being modest with the word "warm".

0.024A * 240V = 5.76W!.  My biggest through-hole resistors look like they are either 0.5 or 1W.  I'm guessing ~100C/W thermal resistance to air, so they'd quickly get to temperatures hot enough to melt solder.

A couple of 47kOhm would make the RMS average current for a half-wave circuit 1.3mA.  Power per resistor would be 156mW, so heating should be < 20C up to 125C/W.

 

I have no special talents.  I am only passionately curious. - Albert Einstein

 

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sparrow2 wrote:

If you can unhook the power (somewhere) then let it go thru a cheap watt meter.

I have 2 standard NEMA 5-15 watt meters, only one of which works.  The other is dead after I used it for a few days on a 240V-powered server.

Most tank-style water heaters in Canada are hardwired circuits with a 30A breaker.

 

I have no special talents.  I am only passionately curious. - Albert Einstein

 

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Perhaps one wire thru a CT coil. 

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that takes us back to #9 ?

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But if we only need current or no current, it's just a bead and some some thin wire (and then one of the AC wires).

(and if it's 10A+ there is a good change it can drive a optocoupler direct)

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I found a similar circuit being used as a zero-crossing detector.  However this one puts 105mA of current through a 4N25, and 24W through a 2k2 resistor!  There goes the magic smoke...

https://circuitdigest.com/sites/...

 

I have no special talents.  I am only passionately curious. - Albert Einstein

 

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I sat down this am to start prototyping the circuit.  My plan is to do initial testing on a 120V circuit.  Then I started thinking how I'd work somewhat safely with a live 120V/60Hz line.  The last time I tried that to test a full-wave bridge rectifier, I had an inadvertent short, fried the rectifier and tripped the breaker.  A 1:1 isolation transformer would be nice, but I don't have one.  After thinking for a while, I remembered I have a 12V DC to 120V AC inverter around somewhere.

Any other ideas based on what a typical hacker would have around home or could pick up at a local hardware store?

 

 

I have no special talents.  I am only passionately curious. - Albert Einstein

 

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 A 1:1 isolation transformer would be nice,

 

There's nice, and then there's safe!

 

Having been a Darwin Award Winner runner up a couple of time, (long ago, in my lots of tinkering but before being an engineer days), I now focus heavily on safety.

 

Just how many days delay in your project would it be for Amazon / Mouser / etc. to deliver an isolation transformer to you?

 

Enough of the lecturing...

 

Except for one more reminder.

 

When working with live Mains circuits you ALWAYS put and keep one hand in your pocket.

 

That prevents you from having a trans-thoracic shock, and perhaps killing yourself.

 

It isn't a perfect approach, but it will help to increase your safety somewhat.

 

Next:

Know that the little 12V to 120 V inverters, the small cheap ones anyways, often put out a square wave or pseudo sine/square wave, not a true sine wave.

The output voltage might also be somewhat load dependant.

And you would, once again, be much safer powering it from an old car battery, NOT from a Wall Wart.

Plons had a nice Thread in the past on wall wart leakage currents.

Bottom line the vast majority of little wall warts have switcher supplies and don't use an isolation / stepdown transformer like they did in the old days.

 

That said, it would certainly be a reasonable approach to use it, assuming you aren't going to wait for a true isolation power transformer.

 

You might find a 120 : 12 V or 120 : 24 V little, low power transformer, (think old style door bells), at the hardware store.

I've read of people using two of them, 120 : 12   connected to 12 : 120 , for a "poor man's", low power, isolation transformer.

I have not used that technique.

 

In the time it takes you to order a decent isoloation transformer, you might just as well order an AC Hall Effect chip / breakout board!

 

Isolation transformers are not known for being inexpensive, and it will likely cost an order of magnitude more than the rest of your entire project.

 

Lastly, another "trick" we used to use, many years ago, when simulating the ringer voltage on POTS telephone lines, was to put a bunch of the little retangular 9V batteries in series.

That was a low cost, low energy, "high" voltage source.

Know that even that, however, can easily kill you if you aren't careful.

 

OK, one more comment.

I've probably posted a dozen times, or more, about the little $20 USD Toy Scope "Kit" from Banggood Electronics, (Arm processor, color GLCD, etc.).

There are multiple postings for it on the site, some old models, some of the newer user interface models, some with an included plastic case, etc.

 

If you don't have a toy scope go buy one.

The "kit" has the arm pre-soldered on the PCB, you add the R's, and C's, and a diode or two...

 

You can power that little scope from a battery, so it does truely FLOAT, and you can see your various voltages and waveforms, etc., without blowing up your real scope, or accidentally providing a Ground Path through your O'scope that you didn't at first appreciate.

 

Be safe!

 

JC

 

Edit: Typo

 

 

 

Last Edited: Thu. Mar 4, 2021 - 06:32 PM
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The photo below shows three of the Banggood Electronics (Toy) O'scope kits.

The one in the white case is my favorite, although there might be other posts with it for a lower price.

Its user interface is much more like that of a "real" O'scope, so it is easy to use.

 

The one to the right has a clear plastic case.  That is an older version, (with the case), and although IIRC it has the same specs, it has a more challenging user interface.

 

The one on the far left is the original version.

Again, no case and a more challenging user interface, but dirt cheap.

It would also likely make a great little ARM / Color GLCD / user switches testbed for tinkering with ARMS.

 

Even a toy O'scope will provide you with a LOT more info than a DMM when dealing with Mains circuits.

 

JC

 

Edit:

My mistake, the newer version, (white case model), has better spec's than the older versions, but it still isn't comparable to a "real" DSO.

Next, be sure to check the  Maximum input voltage on the one you purchase, (assuming you wisely purchase one).

The white case one has a 50 V input voltage limit.

That means that you would need to make your own voltage divider front end, (two resistors), before feeding a Mains level voltage into the input.

 

JC 

 

 

Last Edited: Thu. Mar 4, 2021 - 06:53 PM
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Do not connect ANYTHING between your water heater & your circuit.  Either use a small lamp that can be sensed, NOT an optocoupler, since this would bring HV connections into your board.   There should be zero wires between the heater power connections and your board.

You can also use a pickup transformer or maybe hall effect on the outside of a power conductor (you can often shove the bx metal back to reveal a wire.  Live to tak another shower.

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Wire up a wall wart . With such lack of creativity, how can you call yourself aspy??

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I already suggested the relay in #11. I still believe this is one of the safest and easiest solutions.

Almost any relay with a 120VAC coil could be wired to the heater element terminals and mounted inside the water heater. Then a pair of isolated volt-free contacts can be brought out to your AVR. It will "look" just like a key-switch to the AVR and you already know how to read and denounce those.

 

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N.Winterbottom wrote:
I already suggested the relay in #11. I still believe this is one of the safest and easiest solutions.

+1

 

Almost any relay with a 120VAC coil could be wired to the heater element terminals

He actually said 240V - but the principle remains the same.

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ralphd wrote:

Any other ideas based on what a typical hacker would have around home or could pick up at a local hardware store?

 

Two identical grungy old transformers.  Wire them up back-to-back and pop - an isolation transformer.  Oh, and you'll need a phase splitter - you can't just wrap wire around the whole AC cord and expect anything.  S.

 

ETA:  No,  you don't want little dinky transformers for this, you want great big thumping ones - think 10-20 lbs of iron each - and then be careful you do get the same windings wired up the same way.  If you get them wrong or backwards, you will get serious fire danger.  S.

Last Edited: Fri. Mar 5, 2021 - 12:11 PM
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awneil wrote:
He actually said 240V - but the principle remains the same.

 

Isn't Ralph Canadian ?

 

Do some area of Canada use 240V ?

 

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I think the Americans have 240V for some "heavy" loads (eg, water heaters?) - I think it's some sort of split phase thing?

 

EDIT

 

This: https://en.wikipedia.org/wiki/Split-phase_electric_power

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Last Edited: Fri. Mar 5, 2021 - 12:32 PM
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awneil wrote:

I think the Americans have 240V for some "heavy" loads (eg, water heaters?) - I think it's some sort of split phase thing?

 

EDIT

 

This: https://en.wikipedia.org/wiki/Split-phase_electric_power

 

It is.  I have 240V split phase running from the pole to my house, and in the house I have two 120V phases.  Some things (like the water heater) run on 240.  Basically the idea is that half the house runs on one phase, the other half the other, and when the ground opened I got 170VAC on light bulbs on one phase when the fridge kicked in on the other through imbalance....  I got the power company to fix that ground problem, but I did get bright lights for a bit. 

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Have you tried a HAL sensor? or a magnetometer ( 3D hal sensor) to see what that does when the heater is ON or off. There will be a change in the fields when you put the sensor right next to the cable, you might pick up a change in fields when the heater turns on or off.

all you then need to do is log times. Not sure if the eeprom inside any mega chip will be big enough to log more than one or just a couple of days of data. The heater will be potentially switching on and off more times than you can think of.

Writing this, if you have a large inductor somewhere you might hook that up and use that as indicator.

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meslomp wrote:
HAL sensor?

Do you mean Hall sensor?

 

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I'm sorry Dave, I can't do that! devil

 

(Possum Lodge oath) Quando omni flunkus, moritati.

"I thought growing old would take longer"

 

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25 years ago I played with magnetoresistive sensors the conclusion at that time was they didn't work precise enough over time, but I know they got better and cheaper <$1 .

 

perhaps use one of those phase finder things. (if you hear the phase the heater is on) 

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sparrow2 wrote:
perhaps use one of those phase finder things.

I think that's the principle of my first suggestion in #2:

 

wrap a wire around the live feed - so no actual contact with 240V - you will likely pick up enough "noise" to tell whether it's ON or OFF

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No #2 is a CT transformer.

 

A magnetoresistive sensor is like this 

 

https://www.digikey.com/en/produ...

 

and that can also work on the field from a DC current

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sparrow2 wrote:
No #2 is a CT

I made two suggestions in #2:

  1. The capacitive coupling
  2. The CT

 

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And ?

magnetoresistive sensor is neither of those.

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Even 1Kbyte is 8192 bits...you can sample every 2 minutes & record over 16000 minutes---that's eleven days

 

If you use a relay be careful & use heatshrink.  Our tech just figured out why the switch wasn't working on the control panel...on of the wires broke off & was flopping around inside.

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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N.Winterbottom wrote:

awneil wrote:
He actually said 240V - but the principle remains the same.

 

Isn't Ralph Canadian ?

 

Do some area of Canada use 240V ?

 

 

From the street, every house gets 2 120V 60Hz feeds both 180 degrees out of phase.  Household recepticles are NEMA 5-15p.

http://internationalconfig.com/p...

Newer houses often (at least in these parts) have 20A t-slot plugs in the kitchen.

Wiring is 14AWG NMD-90 2-wire (+bare ground) going back to the panel, connected to one of the 2 phases with a breaker.  One of the two wires is black, and it is the live/hot carrier.  The other is white, and is the neutral which is connected to ground at the panel.

Heavy loads like water heaters, clothes dryers, and electric ovens use 2 hot, which are colored red and black.  Since each is 180 degrees out of phase, the total RMS voltage is 240.

 

I have no special talents.  I am only passionately curious. - Albert Einstein

 

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ralphd wrote:
both 180 degrees out of phase
which results in them being in phase with each other.

Ross McKenzie ValuSoft Melbourne Australia

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meslomp wrote:

Have you tried a HAL sensor? or a magnetometer ( 3D hal sensor) to see what that does when the heater is ON or off. There will be a change in the fields when you put the sensor right next to the cable, you might pick up a change in fields when the heater turns on or off.

all you then need to do is log times. Not sure if the eeprom inside any mega chip will be big enough to log more than one or just a couple of days of data. The heater will be potentially switching on and off more times than you can think of.

Writing this, if you have a large inductor somewhere you might hook that up and use that as indicator.

 

I'm intending this to be a weekend project, using parts I have on hand or can pick up locally.

 

Typical tank-style electric water heaters are usually 180L, or 300L for large families.

https://www.homedepot.ca/product...

The thermostats that control the 2 elements (upper and lower) have a large enough hysteresis (~10C) that they don't short cycle.

 

For data logging, I'll probably go with 1-minute resolution, using 16-bits to store each timestamp.  Even if I use the m168p instead of the m328p (I have both lying around), I'll have more than enough space for 2-3 weeks of logging.

 

I have no special talents.  I am only passionately curious. - Albert Einstein

 

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ralphd wrote:

 

From the street, every house gets 2 120V 60Hz feeds both 180 degrees out of phase.

...

Heavy loads like water heaters, clothes dryers, and electric ovens use 2 hot, which are colored red and black.  Since each is 180 degrees out of phase, the total RMS voltage is 240.

 

That's what I've got, although a few more 240V plugs about the place.  Since there's no gas service, the electric heater(s) run on 240.  S.

 

PS - I've an ~200L electric water heater too, and I discovered the other day that the two breakers (one for each phase) in the fuse box labeled 'water heater' actually are NOT the correct breakers!  There's still power there when they're pulled!  So again, be careful!  We like having you around...

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Access Denied

You don't have permission to access "http://www.homedepot.ca/product/..." on this server.

Reference #18.3fc17a5c.1615029685.1ba98664

 

Do these "heavy loads" get wired into terminal boxes or do you have special 240V plugs ?

 

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Yes and no.  The water heater is directly wired in.  The heater in here has a plug, as does the stove/oven combo.  They are special plugs.  S.

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Be sure to label all of the wires, you don't want to have any mistakes, or lose your electricity into an infinite loop.

 

Don't be scared of using all the wires needed to accomplish the job--do it right the first time!

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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What could possibly go wrong?

Ross McKenzie ValuSoft Melbourne Australia

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I'm sure that the OP will be thinking safety for this project. I don't know what the situation is in his location ( which somebody said was Canada), but I'm sure that there are electrical codes as to how you can tap into AC circuits. Not a problem unless 1) something bad (fire etc) in the area and investigators note some non code device or 2) the house goes up for sale in which case it will likely be inspected.

 

Not being critical, just bringing up some items for consideration.

 

hj