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

[wright1bby]

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.

I have gotten rid of the commercial coffee maker.....It could draw almost 2 KW.  The dryer is not used by renters so we can control when it is turned on.  Other than that there are no single large loads.
Solar might help a little in the summer months when the peak usage tends to be in the afternoon but it is unlikely to help any in the winter when the peak probably occurs at night with resistance heating elements in the HVAC unit and all the lights in the place on.  Also with such intermittent usage the return on solar just doesn't pencil out.
Thanks a ton for helping me get my head out of my #$% and see the simple solution I should have seen from the get go.

 

[wright1bby]

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.

Thanks.  The actual HVAC unit and thermostat are upstairs and it would be more difficult to run wiring to the Hot water areas from there than it is from the HVAC breaker box downstairs.

 

[wright1bby]

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.
 

suggestion.jpg
 
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.
 

suggestion2.jpg
 
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.

Thanks for the awesome detailed replies.  Only one of the 2 hot water heaters has a readily available 110 outlet near it so that moves the Elk contactor down my list.  Besides I love the price of the industrial controls contactors on amazon.

 

[dpilati]

If you have 2 80 gallon tanks, you might consider a much simpler solution.
 
A timer on the hot water heater.
 
In my area (and the way these things are usually set up), the demand charge is only for the peak times. So just fire the hot water heater (s) up at night. Then the Kwh charge is usually less, win/win. Again in my area, there is a 3 hour window in the middle of the day that is off peak in the winter (1-4pm) so that helps. Summer time hot water use is usually significantly less (people choose cooler showers and the pipes are warmer to start)
 
I have a 80 gallon solar tank and a 50 tank. In the winter, I run the heating element on the top 1/2 of the solar during off peak times. In the summer, there is no need for that.
 
I would suspect your much bigger problem is the electric heating strips. In my area, in the winter, the peak starts at 6am which is often when the heating strips are used. I have 10 kw in heating strips for my upstairs (I can control that so they are almost never used and certainly not after 6am).

 

[wright1bby]

Dpilati,
Ohh if were only so simple. Unfortunately I live in Texas where energy markets are "deregulated" and this is a small business account... Without consumer "protections".
I have been on the phone and internet for 4 hours this morning and nobody can provide me even an itemization of the bill.
In Texas the "supply" provider is who you sign up for and who bills you. They are called the retail provider and allegedly you pay them for the actual electrons provided and they bill you for everything. Their part of the bill is transparent..670ish kWhr some at weekday rate ...most at nights and weekend rate. Total charge $40 something.... Then we have a line item for transmission, metering, and delivery charges of $140.40 that nether the retail company nor the delivery company can provide me an itemized invoice for. The only thing they say is that it is based on peak kW draw of 10 during a 15 minute cycle during the last month. I don't quite believe that yet.
Then you have taxes..... Ultimately the bill works out to nearly $.30 per kwhr.

 

[wright1bby]

From reading regulatory filings I have learned that the transmission and delivery fee is based on a laundry list of fixed fees related to which type of customer you are considered. A typical small business falls in the category that causes these fixed charges to be about $40 a month.

They add to this another list of distribution fees based on either peak kW usage for the month in any 15 minute period or 80% of the highest kW usage you have had during any 15 minutes in the last 11 months whichever is greater. This ranges from $4.89 to $6.61 per peak KW depending on a few factors.  If your peak for the year is under 20 then the lower rate applies.  If your peak is over 20 the lowest rate applies to annual load factors over 25% with prices increasing as your annual load factor decreases.
Then there are the transmission fees based on either the above number times $3.079 per peak KW.... or if you request a specific type of meter $3.517 per average peak KW you used during the 4 15 minute system peaks that occurred in the most recent June, July, August and September.

 

[tmbrown97]

my average is about $.40/kWh, all day every day, so it could be worse!  our electric bill is around $900/month in the summer - just for cooling primarily.

 

[dpilati]

Both in AZ and NC, the demand charge is based on highest demand during peak hours. While yours could be different, it may not be. Often talking to the reps they have no idea. I've had other customers and reps think that it was highest demand period but reality is that it is during peak hours only.
 
Think of EVs, all power companies (regulated or not) want to encourage charging at night. Demand charges during peak hours is one way to accomplish that.
 
In NC (Duke Progress - largest electricity utility in the US I believe), we have an optional TOU-D rate. We pay 5 cents off peak, 6 cents on and then roughly $5 per kw peak (depending on season). The regular rate is more like 11 cents/kwh. 
 
I pay about $50 a month with 4500 sqft and an EV with solar. Anyone paying $.30 or $.40 should have solar if they can. One must wonder where all that money goes when NG and coal both generate electricity for about 3.5 cents a kwh. 

 

[tmbrown97]

I'm on a straight tiered rate - no TOU or otherwise; Tier 1 is used up in 3 days, Tier 2 shortly after, then the rest of the month I'm in tier 3 or 4 (same rate anyway).  I can go TOU and will next spring when I put the solar panels up; other projects jumped the list putting that one off a little.

 

[wright1bby]

Both in AZ and NC, the demand charge is based on highest demand during peak hours. While yours could be different, it may not be. Often talking to the reps they have no idea. I've had  other customers and reps think that it was highest demand period but reality is that it is during peak hours only.
 
In NC (Duke Progress - largest electricity utility in the US I believe), we have an optional TOU-D rate. We pay 5 cents off peak, 6 cents on and then roughly $5 per kw peak (depending on season). The regular rate is more like 11 cents/kwh. 

In Texas this demand rate is not exactly optional for businesses.   When the local monopoly installs its smart meters you are automatically put in this category if anytime after that you have a 15 minute peak over 10KW.  You stay there unless you pay several hundred dollars to have your smart meter uninstalled and an additional $25 per month meter reading fee.   
Then you can go back to a regular meter and non demand charges, unless you ever use more than 3500kwh in a month at which time they will once again replace your meter with a smart meter and put you back on the demand billing if you have any 15 minute peak over 10KW.
Their non demand transmission and distribution rates are various fixed fees that total around $20 plus about .03 per kwhr.

 

[LarrylLix]

It may cost a little more but I wouldn't fool around with dumb switching and have to spend more later.
 
I would get an ISY994i, some Insteon 240v inline modules and some CT current detectors on your HVAC, contacts into IO Links, or Smart meter I/f devices talking to your meter to detect your current load demand.
 
With two 80 gal water heater I would be alternate switching these for load control during peak times and turning them off for short durations during HVAC times. Turn them both on just before peak TOU times or whatever fits the best usage/bill profile. This takes smarts like ISY or other HA.
 
You pay for these peaks for the next 11 months if your rules are similar to Canada's energy policies. It will take some time for the break-even time-point but energy is slated to get more expensive until some people start paying attention. Save your bitcoins! Solar PV may never break even.

 

[tmbrown97]

You pay for these peaks for the next 11 months if your rules are similar to Canada's energy policies. It will take some time for the break-even time-point but energy is slated to get more expensive until some people start paying attention. Save your bitcoins! Solar PV may never break even.

Depends on where you live; where I'm at, a self-installed PV system's payback is 2.5 years; 5 if I just hire a company to come toss it up.

 

[dpilati]

160 gallons of hot water is a lot. I think dumb switching is not bad.
 
In a hot climate, the a/c may need to run continuously for 8 hours. Not a lot smarts can do about that. If the a/c never shuts off, you can never run the hot water. 
 
The typical 80 gallon tank has 2 4.5 kw elements. They have a stat that prevents both elements from running at once. These can be wired together (pretty sure) so that only 1 of the 4 elements runs at one time. You would have to replace a stat or two. The typical setup is the top stat feeds power to the bottom stat only when the top is not running. So you need 3 "top" stats with power running from 1 tank to another. So a $10 top stat and a wiring change to fix that problem - even if it is a "dumb" fix.
 
PV payback is very regional - was about 8 years for me. $30k installed. $23k in incentives. Saves about $1000 a year.
 
Demand charge for businesses is pretty universal - not just Texas. Demand charges make sense. The kwh are cheap, the delivery/generation costs are based on peak demand and that costs a lot of money - approaching 50% I'm sure. Which only gets more complicated with residential solar. You get cloudy hot days or cloudy really cold days. People with solar still need to pay for all the capability to deliver a peak

 

[LarrylLix]

PV payback is very regional - was about 8 years for me. $30k installed. $23k in incentives. Saves about $1000 a year.

Without somebody else paying for them,  $30k at 5% per annum  would be $1500 per annum LOI, being generous.
 
I have spent about $30K on solar equipment and the payback will be never with solar insolation at about 4 hours/day. I haven't figured it out, recently, at the newer rate inceases but a few years ago it wasn't paying the LOI from the capital outlay. I pay about 23-25 cents/kWh consumed and they pay me 16 cents/kWh for backfed energy.
 
Most people here are finding at the 42.x cents/kWh the payback is many years unless you are selling systems and we have $.99/Watt panels now. Our four PV manufacturers are folding up.
 
 Solar DHW is apparently very lucrative though.

 

[tmbrown97]

We've gone WAY off topic, but do those numbers take into account ToU billing?  Essentially overproducing during peak hours and buying back at night?  Not many people I talked to could run the math correctly but once I dove deeper into that and analyzed my usage for the last year (our utility lets me download hour by hour usage for the last year+) we figured out that a 12.5kw installation would offset the $6K/year I pay for electricity.  Now those come in all different prices from $25K to $50K; less 30% fed rebate and any applicable local rebates (my local rebates have been used up for a long time) - you can figure ROI.  Say I install that myself at a cost of even $30K, I get ~10K back so I'm at $20K (rounding for ease) - that's 3.5 years assuming my usage doesn't go up and/or rates don't go up.  Given our situation, it'll only get better from there.  Without getting into ToU billing I'd have to go considerably larger to net a $0 bill and I don't have the roof space to do that, but with ToU I don't have to produce everything I use - just hit the ratios right so they balance out.
 
No - those numbers don't work for everyone - we get a LOT of heavy sunlight and have the $.40kwh rates, and I'm perfectly capable of taking on a project like this.
 
Now - I talked to most of the solar companies in town, and the solar lease or Power Purchase Agreements are horrific scams.  Anyone who buys into those is just foolish.  A family member did it - and she was bragging about how little her utility bill was - but she didn't add to that any taxes or the actual lease payment to compare how she came out - not to mention, should she wish to sell, she has to deal with the transfer, buyout or removal then.