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Calculate cost of running recirculation system |
| 3 factors affect cost: Factor 1) Wattage of
pump. Wattage appears on pump label.
Factor 2) Number of hours pump runs each day. This can be estimated if pump runs via thermostat, or known for certain if a timer is used or a Kwh meter is used. Factor 3) Water heater cost from re-heating water ... cannot be known for sure unless a gas meter or electric meter are installed on heater. Note also that a pipe with lots of elbows vs straight run can cause the pump to work harder. Unknown how much heat loss that slow circulation would add. |
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| Example 30 watt pump a) Pump
runs full time > yearly cost $26 + reheat water $10 = $36 per
year
b) Timer runs 2 half-hour periods per day > yearly cost $2 + reheat water $1 = $3 per year Calculation for Full time
Pump uses approx 30 Watts = .030 Kw (30 watts divided by 1000 = Kw kilowatt) Full time calculation: .03Kw x 24 hours = .72 Kwh per day .72 Kwh per day x 365 days = 262.8 Kwh (kilowatthour) 262.8 Kwh x 10¢ per Kwh = $26.28 per year Calculation for two 1/2 hour periods a day Pump uses approx 30 Watts = .030 Kw (30 watts divided by 1000 = Kw kilowatt) Pump runs 1 hour per day = .030 Kwh per day .030 Kwh x 365 days = 10.95 Kwh per year 10.95 Kwh x Todays Rate 10¢ per Kwh = $1.95 per year |
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| Water heater
cost from re-heating water ...
cannot be known for sure
unless a gas meter or electric meter are installed on water heater. Circulating pump
does not cause new cold water to enter tank via the cold water inlet.
Instead the recirculated water cools down after leaving tank, so when the water arrives back at tank, it is cooler than when it left and needs to be re-heated. Factors that affect
how
much the water cools include pipe insulation, length of pipe, ambient
house temperature, and length of time water spends inside the
recirculation loop.
Factors that affect how much it costs to reheat circulated water include efficiency of heater and cost of energy (gas or electric), with gas costing less than electric. |
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Estimate cost for extra water
heating Let's say circulating loop is 100' of 1/2" pipe and the water temperature drops by 10°F during recirculation. This means 1.2 gallons of water weighing 10 lbs must be reheated to thermostat set point. (2.2 gallons for 3/4" pipe) See calculations to right. It takes 1 BTU to raise 1 lb of water by 1°F If you have gas heater and run pump two 1/2 hour cycles per day 10 lbs of water x
10°F = 100 BTU per day x 365 days = 3650 BTU per year.
Use this figure and estimate from gas bill. Probably 50¢. If you have electric water heater and run pump two 1/2 hour cycles per day It takes .000293 Kwh
to raise 1 pound of water 1° F
It takes .00293 Kwh to raise 10 lbs water 1°F It takes .0293 Kwh to raise 10 lbs water by 10° 10 lbs of water per day must be raise by 10° ... so .0293 Kwh x 365 days = 10.69 Kwh x .10 per Kwh = $1.06 per year if pump runs two 1/2 hour cycles per day For full time pump, we might guess it takes 10 times more energy than two 1/2 hour cycles per day?? maybe $10-15 per year for electric, $6-9 for gas |
Calculate
gallons inside 100' pipe: 1 gallon = 231 cu inches Inner diameter of 1/2 pipe .54" so r radius = .27" Area = πr² 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 1.2 gallons = 8.34 x 1.2 = 10 lbs 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 |
Kevin emails question Dec 2020 ''I
have a heat pump water heater with a 44 watt
recirculation system.My pump plugs into an outlet with a timer so it doesn't run during the night. My plumber thinks that's nuts because he says it just costs pennies a day to operate, and he says that it's harder on the pump to go on/off as opposed to continually on. We have a dedicated return line on the recirculation system that is about 60 ft. long and it is insulated the whole way. It's also in a conditioned basement. Energy Prices: Where we live, the price is about $0.0785 per kWh. So, given all of that, I'm coming up with about $41 per year to run that pump 24/7. Am I missing anything? Resource: Plug in timers |
| Gene answers with calculation 44 watt pump x 24 hours per day = 1056 watts, divided by 1000 = 1.056 Kwh per day ... or .083¢ per day .... less than 1¢ per day 1.056 Kwh per day x 365 days per year = 385.44 Kwh per year 385.44 x $.0785 per Kwh = $30.24 + tax and other charges if applicable Heat pump water heaters are efficient but are slow to reheat ... taking up to 4-6+ hours for 80 gallon ... plus they use ambient room temperature to reheat water (they consume power from your HVAC) Since the water heater is running more, causing HVAC to run more, and there is a tax on your electric prices, then add those cost to the $30.24, and $41 is probably about right Resource: Best when set to efficiency mode water heater |
| Gene answers with more detail A high-priced plumber might think $.08 per day is ok in the era of global warming, and that $41 per year is better than having a timer that reduces cost to $20.50 per year. The pump is only part of the cost of pulling hot water out of tank ... the tank runs more//// how much more? I don't know for sure. Esimate might be $10 or more. Your pump has FLA 44 watts ... so it consumes 44 watts when running, with slight surge more amps when starting up each morning... If you run it 24-7 then that's like leaving a 40 watt light bulb running all the time. I say turn it off and save energy. For reasons of economics and planetary responsibility, a recirc pump should be on a timer or switch in my opinion.  Remember
too, that most small residential recirculating pumps come with a
built-in crossover
valve thermostat that cycles pump on-off based on temperature
... and nobody thinks about pump lifespan with that system ...
and
pumps last for decades.I suggest you plug the pump into a Kwh meter ... to arrive at real answer. Buy: Energy Kwh meters You can also add another meter to water heater ... ... might be revealing and help solve what is best decision. Resource: How-to-install-electric-meter-on-240Volt-water-heater |
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120Volt meter for refrigerator,
pump,
microwave, TV, computer etc Test any 120 volt appliance Resources: Kill a watt Instruction .pdf Buy: Energy meters |
 Meter on water heaterThe Kwh meter is din rail mounted inside enclosure. Records billable power usage in Kwh. Multiply by rate you pay per Kwh to get monthly/ yearly cost of running electric water heater. Buy Bomain power meter 240 volt pass through Kwh meter 2T2502GA beige plastic enclosure 2T2500GA grey plastic enclosure Resource: Enclosures How to install electric meter on 240Volt water heater |
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 | Hardwired, 120 and 240 volt Hour meters Example shows 240 volt Hour meter installed on pressure valve for 1 HP 2-person residential water well with light usage. After 12 days, the hour meter reading showed 3.5 hours 1 Hp pump, 735 Watts (start up amperage not included) 735 watts X 3.5 hours = 2572 wh = 2.572 Kwh per 12 days Extrapolate for 1 year = 78 kwh per year @ 12 cents = $9.39 to operate pump for 1 year Buy: UWZ48E-240V AC hour meter at Amazon Intermatic bezel to support UWZ hour meter Intermatic ac hour meters at Amazon Resource: Square D pressure switch .pdf |
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Install meter on Gas water heater Record exact usage of BTU of gas appliances. 3/4" pipe thread connections. Use yellow teflon tape or gas-approved sealant. No excess tape or sealant inside pipe. Use black pipe or yellow flex line. Keep pipes clean during assembly. Install in vertical position with inlets pointing up as illustrated. 12.2" Tall x 8.5" Wide x 11.2" Deep No power source required Digital readout in cubic feet Remote Readable option (pulse output) using EKM-25IDS Buy: EKM gas meter Approved gas pipe sealant Yellow gas line  Resources Gas meter spec sheet with optional remote monitor specs |
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Resource: Compare box timers DPDT timers Intermatic Control water heater w/ Z-wave |
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Resource: Intermatic control centers: and parts Digital control centers and parts Tork control centers and parts Subpanels All control centers and parts Enclosures |
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Resource: Programmable timers Countdown timers Off delay timers On delay timers Timer modules Din rail Twin & one-shot Woods Intermatic Tork |
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