Posted by: Li Ling Young | February 24, 2013

Demand Recirculator (not)

Nik has had it with how long it takes the hot water to reach our en suite bathroom.  Since I practice my always-on-the-cold-side approach to water use, I’m not much bothered by it.  But poor Nik has to shave, and in general has better hygiene than I do, so we’re going to do something about that wait.

A little pump under the farthest sink can deliver hot water to the sink fast without wasting water.

A little pump under the farthest sink can deliver hot water to the sink fast without wasting water.

Knowing how much I like to shop, Nik authorized me to buy a demand recirculator for our bathroom.  I’ve been recommending these to people for years.  Finally I get one of my own!  The product is called the Hot Water D’Mand Kontrol System and here is how it works:

  1. A small pump is installed under sink
  2. The pump connects the hot pipe to the cold pipe
  3. A button next to the sink activates the pump
  4. The pump pulls water out of hot pipe and sends it to the water heater via the cold pipe
  5. A temperature switch in the pump turns it off when the hot water arrives
  6. Hot water arrives at the sink within a couple of seconds of pushing the button; no water is wasted down the drain

The main benefit of these things is that no water runs down the drain while you’re waiting for the hot water to arrive, and of course the hot water arrives at the tap much faster than it otherwise would.  Saving water is great for the planet and we people, and there are big energy implications to wasting water, so save whenever possible.  But in terms of household energy, a demand recirculator has a very small impact.

Since it was a Sunday I started my shopping at the manufacturer’s website.  The Hot Water D’mand System is made by Advanced Control Technologies.  There I discovered that the D’mand comes in several sizes.  You want to get the smallest one that will do the job because an oversized pump will use more energy than you need.  The one that’s right for us is about $520.  Ack!  That’s a lot more than I was expecting.  As much as I wanted one of these things in my house I had to question what I’d be getting for $520 (plus we’d have to install an outlet under the sink for the pump).

I had Nik measure the amount of water in the pipe between the water heater and the sink.  He ran the water into a pitcher until it ran warm and measured half a gallon.  That’s pretty low, which means the sink isn’t really that far from the tank.  The wait arises from the low flow fixture at the sink.  Since it only flows at about .5 gallons a minute, it takes a long time for the pipe to empty of the cold water.  (All is relative; I’ve heard of waits much longer than that.)

With a potential savings of only half-a-gallon per hot water draw, and probably no more than a couple of serious hot water draws a day at that sink, that $520 loomed large.  Looking only at the cost of the saved water, it would take 135 years to save enough water to cover the cost of the D’Mand system.  That’s an extremely simplified way of looking at it and ignores the time value of money, water cost inflation, water heater fuel savings and pump energy, not to mention the whole point of getting this thing: convenience.  Nonetheless, it does put into perspective the cost.  I decided to check out other ways to make Nik happy in the bathroom.

I was looking for a residential version of a system I know of that goes on a continuously-circulating hot water system.  This device is similar to electric heat tape pipe wrap, but with controls that make sure it only adds energy to the water when necessary.  Nothing like that exists for residential systems, plus my idea would require a flow meter to turn the heat tape on and that would add a whole lot of complication and cost.  In the end I had to concede that the cheapest/simplest way to get instant hot water at the tap is to put a very small electric on demand water heater under the sink.  The one I like best is the Stiebel Eltron Mini 2.  It’s rated at .32 gallons per minute, but can handle higher flows if you’re willing to have water cooler than 120 deg F.  It costs $169.

The obvious drawback to this approach is that the little water heater will use electricity to heat the water (whereas the D’Mand only uses electricity to pump water around).  Will this little water heater add much to our water heater bill?  Remembering that the most heating it would have to do is .5 gallons at a shot, the cost to use this approach is 1.24 cents per draw, or $9.05 a year.  It also saves wasted water, so taking out the $3.84 for water savings, that’s just $5.21 a year.  All told, I think this a better approach.

Naturally, it’s more complicated than that.  I don’t know whether this water heater can be used in line with another water heater.  If not, we’d have to put a temperature gauge on the hot water pipe that would turn the Mini off if the water in the pipe is already hot.  That adds more cost.  Also, putting the water heater under the sink means we’d still have to wait for hot water (and waste water down the drain) at the shower.  To take care of both we’d need a a bigger Mini and we’d have to put it farther from the sink, which would entail a stranded-water inefficiency.  More to find out, more decisions to be made.  Hope to have an interesting update in a little while.


Responses

  1. We emailed tech support at Stiebel Eltron and they said we’d need a water heater with a thermostat in it so it can turn itself off when the water coming through is already hot. The one they recommended takes 240 volts, a 40 amp breaker and 8 gauge wire. That’s like putting a 4 lane highway in for a driveway.
    Back to the drawing board.


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