Total newbie question--switching off electric water heater when HVAC comes on

[wright1bby]

I know next to nothing about home automation.  It seems like this should be simple to do yet I am having difficulty finding a solution.
My wife owns a bed and bath that has significant usage only about 3 days per month.  It does not have web access so solutions will have to be local network only.  
I want to be able to have the power to the 2 hot water heaters shut off anytime the HVAC unit kicks on.  The reason why is that the new "smart" meters installed by the utility use any peak 15 minute draw during the month to calculate a demand charge that applies for the whole month.  This has more than quadrupled the electric bill for this facility.
Currently this building is completely dumb.  I assume I will need a couple relays, or simple PLCs, a smart thermostat, and some type of controller but have no idea where to begin looking for this type of solution.  Any guidance would be greatly appreciated.

 

[wright1bby]

I don't necessarily need a thermostat if something that monitors total house current power consumption could trigger switching off of water heaters over a certain KW rating.

 

[rsw686]

The simple solution would be a 24V relay wired to the Y terminal and C terminal on the HVAC unit. The hot water heaters would be wired to the relay NC contacts. When the thermostat calls for AC the relay will open the connection turning off the hot water heaters.

 

[dpilati]

OT? - where are you located? I'm trying to assess trends in demand charges and yours is exactly like mine - ie 15 min. 
 
What I do is run a 1.2kw element on the hot water that shuts down from 6pm-9pm when the solar panels are not generating anything and when the a/c is running. 
 
Obviously having solar panels changes the dynamic a lot. And having renters changes the dynamic a lot. 
 
The hot water will be on 240V which makes your ability to shut it off a little harder. I use a 12V relay fire up a 120V that trips a 240V relay. The 12V off the ELK was not enough juice to trip the 240V relay I bought. 240 relays are expensive but I found an ebay one for $50 when everything else was $150. But it does take a little power to trip. So yes you can just do some relays but it does take some attention to the power requirements of a 240V relay.
 
Of course a/c sometimes runs for several hours and people could be bathing etc. Do you have NG? or even propane if the facility isn't used much?

 

[tmbrown97]

You could also use a current sensor around the wire to the AC and have it disconnect one leg of the water heater supply.  I'm nut sure what's so special about 240V relays as the above poster said - because as long as you disconnect one leg of the supply and the relay is rated appropriately, it wouldn't be different than disconnecting a 120V load.  You can also look at Contactors as that's a term used for electrical high-voltage relays or the RIB product line.
 
Current sensing switch
Relay for water heater (confirm required amps)
 
OR - instead of the RIB perhaps a contactor like this one - pretty affordable.
 
So you'd wire the current sensor over one of the hot wires to the AC - then you'd grab 120V ideally from another circuit that would be running through the current sensor - when tripped, it'd use the 120V to energize the RIB and disconnect one or more of the power legs to the hot water heater.

 

[BraveSirRobbin]

Elk makes a product just for this purpose and is even enclosed in a metal box so you can properly terminate wiring/conduit power from the water heater.
 
Rather than throw out the infinite number of possible ways to accomplish your goal, can you tell us a little about your current infrastructure?
 
For instance, you mention 'local' network.  Are all the water heater locations near a network outlet?  What is the distance/number of water heaters?  Are you concerned about only one water heater/AC Power or a series (in other words if just one AC goes on disconnect ALL heaters?).  What is your technical capability as a DIY'er (wiring, network configuration, etc...)?

 

[tmbrown97]

Forgot about the Elk contactor - though if used I'd still skip adding any additional control and just do this via current sensors unless additional "smarts" are needed.  Provided all three items feed from the same load center, this could all be accomplished with a little rewiring near the breaker box.  
 
Of course we're all automation junkies here so if I were tackling something like this I'd probably have the current sensor trip a zone on the Elk which would activate a UPB outlet to trip the Elk contactor...  but if none of that is in place, might as well keep it simple and self contained.
 
Then again, if the items are NOT centrally located, and you still wanted to accomplish this, it'd cost more but you really could use a UPB or ZWave or Insteon I/O module at a current sensor monitoring the power lead to the A/C unit - then have that send a control command to an appliance module located near the water heaters that controls the Elk contactor to turn off power.
 
I have a current sensor on my AC unit now via Ubiquiti's mFI that I did when beta testing for them - it could also trigger all sorts of stuff if I wanted; currently however it just tracks runtimes and nicely graphs it out so I can tell how often it cycles, which stage it's in, etc.

 

[BraveSirRobbin]

See, I'm thinking of this differently.
 
I'm assuming:
 

1. The OP does not want to create an 'automation' infrastructure (just wants enough hardware to get the job done).
2. Wants minimal learning hassle
3. Has an existing network (all hardware will be near, or have the ability to be plugged into a local LAN)

With these assumptions I think I would suggest looking into a methodology based on the WebRelay components.  These units are a bit pricey (over $100 each), but this really would be the only 'high dollar' items, other than the Elk Relay/Contact power controlling device, that would have to be purchased AND it would solve the problem of the water heaters and air conditioners being in remote locations from each other (again provided a network cable can be run to these locations).
 
You can use a current sensor (my favorite is this one as it provides a variable threshold that will give a contact closure when that threshold has been met, plus it requires no remote power), but you can also do simple methods such as a piece of cardboard in a vent with a magnetic contact (use a very small magnet) so it gives an open when the cardboard moves due to airflow (I have not done this myself, but there are posts here where this was useful).
 
Many ways to get a contact closure based on the air conditioner turning on.
 
So the theory is when one WebRelay device sees the contact closure change on its input, the other one (on a remote network location) will active its output and turn on/off the Elk contact device controlling the water heater.
 

 
Note that if there are only one water heater and one air conditioner, only one WebRelay device would be needed.  If this is the case there are other simpler methods that could be used where the expense of a WebRelay would not be needed! (I would use an Elk-960 timer relay) to control the Elk contactor.
 

 
The only thing I'm not sure of is you can turn off ALL water heaters if ANY AC unit is turned on (have to double check the logic capability of the WebControl units).
 
The OP will also have the advantage of turning on/off any water heater from a web interface as well if needed.
 
Anyway, really didn't want to get into all of these details until we hear more info from the OP as I requested above. 
 
I really don't think he wants to incorporate an automation infrastructure (Z-Wave, etc...) but could be wrong.

 

[dpilati]

Well as far as I can see, both those relays are 120V rated. Even though a 240 is just 2 legs of a 120, the potential difference on any part of the completed circuit is 240V. So I can't see that either of those would be technically adequate. And since you are talking likely the biggest amp appliance in a house, it doesn't seem like a place to skimp.

 

[BraveSirRobbin]

Well as far as I can see, both those relays are 120V rated. Even though a 240 is just 2 legs of a 120, the potential difference on any part of the completed circuit is 240V. So I can't see that either of those would be technically adequate. And since you are talking likely the biggest amp appliance in a house, it doesn't seem like a place to skimp.

I might not be seeing the same concern as you are here (not saying you are wrong)...
 
The coil of the Elk-9200 power controller is 110 VAC at 35 mA.  Its relay contacts can handle  220 VAC at 60 Amps.
 
I'm saying to activate this coil using either an Elk-960 relay (its relay contacts can handle 125 VAC at 10 Amps which is well under the Elk-9200 coil power requirements) or a WebRelay output (its relay contact can handle 240 VAC at 12 Amps which again, is well under the Elk-9200 coil).
 
I would also recommend anything connected to 120 VAC or above be mounted inside the Elk-9200 enclosure (the WebRelay units can operate up to 150 Deg F.).
 
Of course if you are concerned about further isolation, you could then use yet another relay between the controlling device and the Elk-9200.  They also make 24 volt controlled contactors (that latch and unlatch) but they are a bit more expensive.
 
If the concern is isolation of the contacts detecting the air conditioners on/off, a relay could again be used.
 
I didn't bother to show the 'wall wart' or DC power supply for the Elk-960 or WebRelay.  It would also be used as the source to trigger the Elk-960 input (from the contact closure detecting the AC on/off state).
 
Hope this made sense.

 

[tmbrown97]

I was just messing around when getting into the full blown automation techniques ^_^  even though they're what I'd probably do since I have the infrastructure in place to handle them.
 
I actually saw this as an even simpler situation at first - one where ideally the two water heaters and single AC unit (as implied in the OP's details) were powered from a single breaker panel.  In such a scenario, it'd be very simple to keep things very basic, automatic and fully mechanical with zero "smarts" involved - by using a combination of the correct CT or "current sensor" to trip a relay (or pair - one per water heater) that'd kill power to the water heaters.
 
As long as everything is electrically sound and rated as appropriate, this sure seems simple.
 
Of course, until the OP replies back, we're all working from assumptions here - and a change in these assumptions can sure chane a lot of details (as BSR implied in his first response).

 

[wright1bby]

OT? - where are you located? I'm trying to assess trends in demand charges and yours is exactly like mine - ie 15 min. 
 
Of course a/c sometimes runs for several hours and people could be bathing etc. Do you have NG? or even propane if the facility isn't used much?

 
[SIZE=10.5pt]This building is located in Canton Texas about 60 miles east of Dallas.[/SIZE]
[SIZE=10.5pt]It has 2 very well insulated 80 gallon hot water heaters that I am reasonably confident will keep up with demand even if they are cut out whenever the HVAC system runs. If I am wrong then I can't think of a good solution for reducing our monthly peak 15 min usage.  NG is not available and building location makes a propane tank unfeasible.[/SIZE]

 

[tmbrown97]

going along with the water heater keeping up... I can say that the last time work was being done on my gas and my water heater went out, it took 3 days before we realized it was off - and that's through showers for a family of 5.  Within 15 minutes of reigniting it, we were hot again...
 
I'd be curious if there are any other big loads contributing... or what it might be worth to throw up a couple solar panels.

 

[wright1bby]

Elk makes a product just for this purpose and is even enclosed in a metal box so you can properly terminate wiring/conduit power from the water heater.
 
Rather than throw out the infinite number of possible ways to accomplish your goal, can you tell us a little about your current infrastructure?
 
For instance, you mention 'local' network.  Are all the water heater locations near a network outlet?  What is the distance/number of water heaters?  Are you concerned about only one water heater/AC Power or a series (in other words if just one AC goes on disconnect ALL heaters?).   
What is your technical capability as a DIY'er (wiring, network configuration, etc...)?
                          

I mentioned local network because I was looking at smart thermostats and they all seem to emphasize ability to be controlled from the web.  I think I am liking Work2plays simple solutions much better than the path I was considering.  There is only one main HVAC unit for the building and 2 water heaters.  I want to disconnect both water heaters when the HVAC unit comes on.  There is no wired network in the building.  The HVAC supply wiring and the water heater supply wiring do not originate from the same load center.  They are however all on the ground floor and I have good sub floor access.  The HVAC supply is all exterior on the ground floor.  I would like to install the contractors at the water heater because extra electrical boxes on the wall would not be unsightly there.  Wire run from the HVAC supply to the water heaters would be approximately 20 and 30 feet.
 
I have a working knowledge of relays, contactors, PLCs motor strarters etc.  I can use a meter and a soldering iron and run wire.  I am no network expert but have worked on configured and troubleshot Wifi, Ethernet, Interbus, Canbus, thicknet and arcnet networks.  (really I am not that old the equipment had been there forever;-)  I have no experience with the particular protocols that seem to be in use in home automation.  I am not afraid to learn but I am cheap;-)

 

[wright1bby]

Both water heaters are currently set up with a pigtail that plugs into a 4 prong 220V outlet.  My ideal solution would plug into a 220 V outlet and have a 220 V outlet on it for the water heaters to plug into.  I am not a big fan of tape, texture, and painting.
I like the price of the contactors  posted earlier.  I think I have adequate space in my HVAC load center to insert a current sensor and supply it with 120 from its own breaker.  Running 120v control voltage from there over to both water heaters is feasible so I may just go that route.  If I get myself a couple enclosures I can just cut my existing SO cord and use the half with the plug on it to supply the contactor input terminals and wire the other half from the output terminals to the water heaters bringing the control wiring into the back of the enclosure through the wall.  Now where did I last see my fish tape????
I think I could do it for about $140 bucks.  1 breaker, 1 current sensor, 2 contactors, 2 enclosures, and about 50 feet of rope.