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Calculate cost of running recirculation system


Autocirc is 33 watts each time motor operates
Timer runs 2 half-hour periods per day > yearly cost for re-circulation is $11-17 at Today's Rates

Two factors affect cost:

1) Pump cost: Pump uses approx 30 Watts = .030 Kw
Pump runs 1 hour per day = .030 Kwh per day
365 days x .030 Kwh = 10.95 Kwh per year
10.95 Kwh x Todays Rate 10 per Kwh (expect 30 tomorrow)= $1.95 per year

2) Water heater cost from re-heating water:
Amount of water: 1.2 gallons per 100 ft 1/2'" pipe (see calculation to right)

Amount of wasted heat:
Hot water travels to faucet anyway, so waste equals return length of pipe
Exact calculation would factor insulation, temperature of water, effect of home HVAC, efficiency of water heater, water heater thermostat setting, cost of energy.

Additional water heater Cost: Reheating 1-2 gallons uses .2 Kwh of electricity per event See chart
2 events per day x .2 Kwh  x 365 days per year = 146 Kwh per year
146 Kwh x 10 per Kwh = $15 per year for electric heater, $9 for gas

So $9-15 per year for reheating + $2 to run pump = $11-17 per year for re-circulation system that runs 1 hour per day assuming new equipment and optimal conditions, and today's low energy prices.

Heat loss during transmission is not factored: 100 ft of 1-1/2 in bare copper tubing results in a heat loss of 5300 Btu/h.
Calculate gallons inside 100' pipe:
1 gallon = 231 cu inches
Inner diameter of 1/2 pipe .54" so r radius = .27"
Area = πr

A = 3.14 x .27 x .27 = .229 sq in
100 ft = 1200 in
1200 in x .229 sq in = 274 cu in
274 cu in
231 cu in per gallon = 1.2 gal

3/4" pipe: r radius .37"
A = 3.14 x .37 x .37 = .429 sq in
100 ft = 1200 in
1200 in x .429 sq in = 514 cu in
514
cu in 231 cu in per gallon = 2.2 gal

How much does it cost to operate Autocirc?

Using the timer and running Autocirc 1 hour a day costs between $11 and $17 per year to operate.
Run autocirc 2 hours a day is $22-34 per year.
Run 12 hours a day costs $11-17 per month, or $132-204 per year.
This includes pump operation and additional time that water heater operates.

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