Hot water recirculation Systems

February 10, 2009 at 9:29 pm

I was walking around our favorite big box discount store the other day to see what new and great ways are out there to save money – and perhaps save the environment. Big section of shelf devoted to a product that promised instant hot water at every faucet in the house AND to save 11,000 gallons of water a year in the process. Wow! This I need to evaluate further. If it’s true and the savings pan-out, DH is in for another plumbing project.

Turns out we are going on a road trip in our hybrid car instead. Let me take you through the math on this device…

First, just what is this thing and how does it work? If my review were more positive, I’d link you directly to the manufacturer’s site and let them explain. BUT I don’t think they will appreciate my “advertising” so let me muddle through an explanation.

The problem that we’re trying to solve here is one of reducing water use. Most people don’t like to jump into the shower, turn the water on and start their shower before the water gets hot. SO, what we do, is turn the water on and let it run until the cool water in the hot water pipes is flushed out with water from the hot water heater before we get in the shower.

This device recirculates the water in those hot water lines by placing a sensor and a valve at the farthest end of your hot water line that senses when the water in the hot water line has dropped below 95° F, opens the valve and “trickles” the water out of the hot water line into the cold water line until the water in the hot water pipes gets back up to 95° F or so.

Got it? Make sense? If not, google “hot water recirculating systems” and look at some of the web sites, they have nifty diagrams. On to a review of the economics (and by extension the “ecologics”)

First, let’s take the manufacturer’s claim on face value. Let’s assume we really will save 11,000 gallons of water/year. Our current cost per gallon for water is just under $0.05 USD per gallon (includes supply and sewer charges). At that cost, 11,000 gallons/year is worth about $51.70. The shelf price of the unit at the big box discount store was $180. Even ignoring the additional electrical costs for operating the unit and some potential installation costs for adding this somewhat complicated system, it would take 3.7 years to pay for the unit. The warranty on the unit is only ONE YEAR.

But, you say, I’m saving the environment 11,000 gallons of fresh water every year! It’s worth the cost. However, one needs to consider the cost to the environment of manufacturing the unit in the first place. The rule of thumb I’m using is to look at the warranted period on the unit, and expect that the device should at least pay for itself within that warranty period. If not, then it probably is also not recovering the ecological cost within the time period either.

The story gets worse and worse as I continued to consider this particular device. I tried to confirm the 11,000 gallon of fresh water per year. It turns out that the manufacturer has assumed about 3.1 gallons of water is wasted each time you want hot water from any faucet in the house. They have also assumed that you will flush this much water TEN times every day.

Hmmmm, let me think about that.

3.1 gallons is reasonable for the end run in OUR house. DH did a test this morning, it took 1.5 minutes to get hot water with our low flow (1.5 gpm) showerhead. That’s more like 2.25 gals, but I’ll give them the 3.1 for this exercise.

10 systems flushes a day. Let me think about that. We take two showers (on average…) per day. We wash our hands several times a day in the bathroom – good for us! 🙂, but I don’t wait for hot water, instead I use … SOAP! to make sure my hands are clean. (As an aside, hot water doesn’t kill many germs, to be hot enough to do that, you will scald yourself…) So those, don’t count. We DO run the kitchen sink out to get hot water to do the dishes. OK, I’ll accept three times a day there. Where else? Laundry? no, the washing machine doesn’t flush the line, nor does the dishwasher. I’m thinking… really hard… nope, not coming up with other reasons to flush the hot water. The kitchen is less than half the distance, BUT I’ll give them that as well.

So, now, I’m at saving only 5,500 gallons of fresh water per year. And my savings has dropped to $25.85 per year. Now we’re looking at 7.5 years to payback. REALLY not making any sense… This is 6.5 years beyond the warranty. And I haven’t even looked at the additional electric costs of running the pump, and reheating the water that’s being recirculated back into the water heater.

Nope, that dog show in Charlotte is looking like a great trip for the weekend! No plumbing projects in our house.

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