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Question for the math junkies

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Member Posts: 991
OK, I have been thinking about my recirculation system I install on tankless heaters... I tell my customers it will use around \$7 of electricity a month, but it may be more after I messed with the math a little...

OK, so a 1440 7 gallon electric mini tank with a 25 watt circulator, that runs on a timer with the aquastat override. Temps usually set at 110* {I know}...

I notice mine hardly runs, since when you use the hot water it leaves the tank full of 120* water, so throughout the day I can't imagine that tank uses much electricity....

Am I far off at \$7 a month????

• Member Posts: 7,572
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The cost to operate the circulator is literally pennies a month. 2hrs X 30 days X 25 watts X .00015 = 22.5 cents. Your heat loss threw the piping is another question all together. How long is the pipe? What type of pipe? What is the flow? How is it insulated? What is the temp of the air?

I lent out my Modern Hydronic Heating book and am unfortunately not smart enough to do it in my head.

Carl
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 991
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I remember back when I designed the systems

before they were in any installation manuals, we were using a 5 gallon tank {GE I believe} 120v and a bronze 006 circ with an insulated 1/2" copper return line {sometimes pex}, with just an a-stat......

Back then it was tough to figure the energy consumption of the tank...

Here is what happens, the initial start up, the tank is filled with hot water from the tankless, then I set the timer for the customers needs {I like the grundfos} which is normally 15 minutes at 7am, then on from 4pm to 10pm with the aquastat over riding the timer when the return reaches 105* {or so}...

So now what did the tank do it maintained 110* water for the entire 24 hours, BUT every time the tankless runs it gets a shot of 120* water, and it only circulates {in my home, I put a timer on my circ and in one week the circ ran for 1 hour and 52 minutes so that is like 16 minutes a day {vs 5 hours per day with the aquastat taken out of the equation}.

I think I need to somehow hook in an electric meter to the mini tank to actually figure this out, and I know the ambient situation is going to greatly change this, my basement is heated, and I know when I install these systems in an uninsulated NE crawl space that sees 40 degrees for a lot of the heating season it uses a lot more energy....

I just hate to say to my customer "you use about \$7 per month" and really I am guessing, don't get me wrong I do my share of guessing but this is the wrose kind of guess, my other guesses are educated, this one I make with one eye closed while grinding my teeth....

Hopefully someone from the wall has a good way to figure this out that my mind hasn't thought of... I know for a real number I will need to do a system by system heatloss and thats not worth the effort.. My issue with this is, I am selling them a tankless water heater to save them money ,I dont want to eat that savings up with a bad design, when I may be able to design it better....
• Member Posts: 9,546
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Kill a watt meter

I have one plugs into an outlet, and the pump plugs into it.

Leave it hooked up for 30 days and see the usage. Makes it easy for a variable usage item.

They are around 30 bucks.

For kicks with three women in the house using curling irons, blow dryers, and straighteners it was an eye opener.
• Member Posts: 991
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Good idea gordy

Are they good for a couple thousand watt loads?
• Member Posts: 9,546
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Rating

15 amps

1875 watts

Limited to a 15 amp circuit

Mine is an older one though.
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