If I pump water continuously through a small 6 gallon hot water tank recirculating from a 60-70 gallon water tank will this work to heat the water over 36-48 hours?

I'm not concerned about if it's the most efficient method, it's just a portable solution but I'm not sure if the water will heat if pumped through continuously.

  • 1
    So, neither tank has a heating element? You're relying on the energy input from the pump to heat the water?
    – DJohnM
    Commented Jan 19, 2019 at 23:09
  • @DJohnM one is a small hot water tank so yes one does have a heating element. Edited to add clarification. Commented Jan 20, 2019 at 23:20

2 Answers 2


Let's say we're dealing with 70 gallons of water, and we're adding heat just by running a circulation pump to stir the water around. And, let's say the pump is a standard hydronic circulation pump of 100 watts (they actually seem to be a bit less). And, let's say that all of that 100W goes into moving the water about, which then decays into 100W of heat through friction.

100W is the same as 340 BTU/hour, where one BTU heats a pound of water by 1 degree F (love them Imperial units!). 70 gallons is 580 pounds of water, so we'll be raising the temperature of our water by 1.7°F per hour. If we start with 60°F water, and want 120°F water, then we'll want to end up with 60°F of temperature rise, which will take about 36 hours. But, we guessed high a few times in our calculations, so I'll estimate it would take closer to 100 hours to get up to temperature.

But, that's with no heat loss whatsoever. To avoid losing significant amounts of your precious heat, you'll have to insulate it all so that when the water is at 120°F the temperature falls less than a degree per hour. That isn't easy for a single tank; add in pipes and it becomes really hard.

After all that, here is my answer to your question: no.

  • The standard burner on a 40-gal gas water heater is 40,000 BTU/h equivalent to about 12,000 W which would require a current of 50 A at 240 V. This is also equivalent to about 16 HP. Commented Jan 20, 2019 at 10:11
  • I'll take the short answer and thanks for thinking it through. Happy ending though, as we ditched the idea after this and other feedback and getting a 220v line put in to the area to run an on-demand tankless heater. Commented Jan 20, 2019 at 23:23

I would say no. Any heat made by the pump is going to be more than dissipated through pipe loss.

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