Thursday, June 27, 2013

Keeping Hot Water Hot

My wife and sons are off visiting with Grandma and Grandpa this week.  The house is definitely less chaotic without two energetic boys running around and I miss that.  But their absence does allow me to do some projects that might otherwise be impractical.  

We will soon need to replace our water heater.  Ours is showing signs of age and better to address before we end up with a leaky tank and a flooded basement.  Part of my research into replacement options is to determine approximately how much energy our current water tank loses to its surrounding environment – the basement.  The best way to do this is to shut the water heater off (we have an electric heater) after measuring the hot water temperature at a faucet.  

Last night I recorded a water temperature of 120 deg. F at the kitchen faucet.  Once I recorded the temperature I was conscious of not using any more hot water, and after waiting an hour or so just to ensure the replaced water in the tank was heated, I turned off the heater at the breaker.  This evening, the hot water temperature was 98 deg. F:

Change in T = 22 deg. F (12 deg. C)
Tank volume = 50 gallons (0.19 m^3)
Water density = 1,000 kg/m^3
Heat capacity of water = 4.18 kJ/kg_deg. C
Energy loss = (22 deg. C) x (0.19 m^3) x (1,000 kg/m^3) x (4.18 kJ/kg_deg. C) = 9,760 kJ

There are 3,600 kJ in a kWh, and thus over the twenty-one hour observation, my water heater lost 2.7 kWh of heat to the basement.  That is the same as having a 130 watt item plugged in and running all day, every day.  130 watts is not a huge draw, but that accounts for about 15% of our daily electric usage.  And now for the big picture - multiply roughly 3kWhs by almost every house in the nation and that is probably close to the amount of energy we use just to keep our household hot water hot.  

So now the dilemma: purchase another water heater tank and pay close attention to the insulation, or purchase a tankless heater, which poses additional challenges such as proper utility connections.  There are also a lot of good DIY ideas here.  Stay tuned...    



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