Tuesday, December 20, 2011

Stupid Water Heater

I just got my power bill for November, 2011.  I had a peak demand number of 4.9....crap !  I sort of knew this was going to happen because one day I saw the hot water heater going during the peak period.  This shouldn't have happened, but:

I use an X10 appliance controller hooked to a contactor to control power to the heating element of my solar water heater.  See, when it's cloudy, the solar collector can't generate enough hot water for the house and there's a helper coil in the heater to take over when that happens.  Usually, I keep the power off to the helper coil using an X10 timer that shuts off the power to it during peak periods.  X10 is notably unreliable and failed to turn the power off that day.  Of course, I didn't know it and crawled in the tub to soak some sore back muscles.  The water I used caused the heater to come on and my demand went up during a peak period.

If it hadn't been overcast and cold, or the X10 timer had worked, or I had manually shut it off before using the water, or I had an alarm that told me the usage was that high, I wouldn't have had this problem.  This probably means a new device for the house.  The obvious solution (from my perspective) is to put another timer in like the one I use for the Acid Pump.  Just set this one to keep the helper element off during peak periods and let it run as it wants during off-peak.  I can get the time from my XBee network and I would be able to include the status of the hot water heater on the House Controller.

But, it's the holiday season and time is at a premium so it'll have to wait until January.

So, how much did this little event cost me?  Looking at my bill there were the following charges related to peak usage:  Demand Charge on Peak - Generation $34.86 and Demand Charge on Peak - Delivery $10.24 for a total of $45.10.  My usual Demand level is less than 2 kW averaging around 1.7 so my expected amount would be about 2/5 the billed amount for around $18.00.  The actual power it used is trivial since it was only 4 kWh at $0.19 per kWh or around 80 cents and I would have used power later anyway. So, a soak in a hot bath tub to loosen some back muscles cost me $27.00 extra last month.

And, that's how they get us folks.  One time, one mistake, and your entire month's bill is screwed up.  Next month, you swear to be more careful and watch it more closely and something different happens; you're screwed again. In my opinion, the only way to overcome this is automation to control what happens in the house during the peak periods.  I just didn't realize how reliable it absolutely has to be to declare success.  So, in January, I'll be back to working on controls.  This time I'll disable the water heater element based on peak, and not rely on a stupid X10 controller.

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