An Indiana homeowner shared real-life calculations on Reddit's r/Homeowners forum after asking how much it costs to run a dedicated, insulated hot-water recirculation loop with a tankless water heater.

Here's what to know

In the Reddit thread, the original poster described their Rinnai RXP199iN tankless water heater, which was paired with a 90-foot dedicated recirculation loop. 

OP wrote, "Got into a debate with a friend regarding [the] actual cost associated with a recirculation loop for my tankless water heater… I've seen statements around the web that the cost is between $35 - $65 / month for a recirc loop running 24/7. That seemed high."

The homeowner then estimated their yearly cost at $127, primarily using a pipe heat-loss calculator. Using 624 Btu per hour of loss, gas priced at $0.024 per cubic foot, and 1,030 Btu per cubic foot, they calculated 5,466,240 BTU over the course of a year.

By that math, the operating cost came out to a little over $10 a month, far below the $35 to $65 monthly figures the homeowner said they had seen elsewhere online.

They also flagged several limitations in the estimate: it assumed 100% water-heater efficiency even though the real figure is closer to 97%, counted pump costs as zero, and left out any water savings from shorter waits at the tap.

Even so, the OP's takeaway was simple: "For the joy of having nearly instant hot water a little over $10 / month is money very well spent."

When another commenter asked about pump electricity, the homeowner said it wasn't included. OP added that internet searches suggested their unit uses 78 to 98 watts while running and that, at $0.1375 per kilowatt-hour, "24 hours is about $.35 or a little over $10 / month." But that added expense may be overstated if the recirculation pump doesn't run continuously.

Another commenter said their own system, even when set to "24 hours," only turns on "every 30 minutes or so for like five seconds."

Commenters also noted that the loop's heat loss is not necessarily a total loss year-round. In winter, it can help warm the house, while in the cooling season, it can add extra work for the air conditioner.

What can be done?

For homeowners planning a remodel or repipe, installation details can make a major difference. In this case, the homeowner said the fully insulated loop and 1-inch PEX helped keep resistance down and avoid much pressure loss even with multiple fixtures running. 

For homeowners considering an upgrade, Cala makes customizable smart heat pump water heaters that can help them lower energy bills by heating water only when needed. 

That emphasis on timing mirrors one of the biggest takeaways from the Reddit discussion, where pump runtime was a major variable in total cost. Instead of treating hot-water demand as constant, Cala's smart heat pump water heaters are built around household usage patterns and customization. Buyers comparing convenience, efficiency, and operating cost may want to weigh recirculation controls against options from Cala.

Where can I learn more?

These stories explore the same trade-offs around recirculation loops, tankless heaters, and long waits for hot water. 

• A homeowner learned the right recirculating setup can cut daily waits and water waste.

• Another homeowner found tankless recirculation would save little if the shower still lagged.

• In Georgia, a homeowner found the recirculation pump never shut off, causing repeated error codes.

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