[Greenbuilding] Opinions on electric tankless HW heaters?

Paul Eldridge paul.eldridge at ns.sympatico.ca
Fri Apr 22 08:27:25 CDT 2011


Hi Bob,

You mentioned that your electricity consumption increased by more than a thousand kWh per month when you operated your electric water heater continuously, which suggests a hot water demand in excess of 33.0 kWh per day.  The Energy Factor rating for a newer, conventional model would be in the range of 0.92 to 0.94, which means the standby losses are roughly equal to that of a 40-watt light bulb, or less than 30.0 kWh/month.  [Note that if you were to add an insulating blanket, you could probably cut that by a third or more, but first check with the manufacturer to ensure this won't void your warranty.]  After subtracting these standby losses, some 970+ kWh would be attributable to water heating; assuming an inlet temperature of 10C and a supply temperature of 50C, that's roughly 180 gallons or three and a half tank's worth of hot water demand per day.  Are you sure you were using this amount of hot water?

We use to heat our DHW with oil (there's no natural gas service in our area) and have since switched to electric.  We installed a small, 70-litre, 115-volt, 1.38 kW model that is plugged into a Kill-a-watt power monitor.  Our two person household averages a little less than 5.0 kWh per day, including standby losses, and we both shower daily, wash clothes in hot water and operate the dishwasher twice a week.  We've made a concious decision to wean ourselves off fossil fuels to the greatest extent possible, and as part of this purchase 12,000 kWh/year of 100 per cent renewable energy through Bullfrog Power.  That $20.00 a month effectively offsets 115 per cent of our home's total energy needs, i.e., space heating (ductless heat pumps), DHW, cooking (induction), lighting, appliances and plug loads.  From our perspective, electricity is our best option.

Cheers,
Paul

----- Quote -----
  We put in an off the shelf mid-price electric water heater, and added an on off switch, so most of the time it is off and we turn it on maybe twice a week.  People periodically tell me that that doesn't save any energy, for various reasons, and while it is true that I am not a scientist. . . as an experiment we left it on 24/7 a couple of months in the fall when it wasn't especially hot or cold and our bill went up over a hundred dollars each month (more than a thousand kilowatt hours) so I would say that the simple on off switch does in fact save energy. Not to mention giving my poor frugal heart pains when I opened the bill. The plan when we get more money is to add a solar water heater.

   Bob Waldrop, Oklahoma City







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