Hot water recirculation

Grundfos says their "comfort system" uses "less energy than a 25 watt light bulb," but it looks like that isn't the whole story. A foot of 3/4" 140 F pipe in 70 F air loses 30 Btu/h, about 10 watts, so when it is running, their recirc pump might add about 1 kW to the power consumed by a house with 100' of hot water pipe, or more, since Grundfos uses a pump and a timer at the water heater and doesn't sense temperature at the distant fixture. It looks like it pumps hot water continuously into the cold water pipe at the fixture, thus making a lot of the cold water piping hot and forcing a person wanting cold water to waste hot water out of the cold water pipe until it turns hot again.
Taco seems to have a better solution. Their $300 006DM-PK plumbing kit has a pump with a check valve under the sink, and temperature sensors. I'd omit the preheat pushbutton and use it with a motion detector.
Nick
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The Grundfos "comfort system" has no dedicated return pipe.

Chapter 44 of the ASHRAE Applications Handbook says 1/2" fiberglass insulation lowers the pipe heat loss from 30 to 17.7 Btu/h per foot.

Yes.
Taco wastes less energy than Grundfos, for cold-line systems.

That would cost more and waste more energy, without a motion detector.
Nick
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Personally, I think what you want to do is a silly waste of energy and money. You've got more important things to worry about. Having said that, if you have to have this, I'd add a tankless unit in the bathroom.
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They cost a lot and keep a small amount of water hot all the time. You might compare the capital and energy costs of the tankless and Taco solutions, with a motion detector that moves hot water for 30 seconds, using actual numbers.
Nick
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You aren't, and the pump would only run long enough to warm the pipe to the distant fixture, which might be less than 30 seconds. I suggest you compare that energy to the standby energy used by a typical tankless, over a day, using actual numbers. This might require an actual phone call to an actual tankless manufacturer. It's my impression that most tankless heaters keep a small volume of water hot 100% of the time, so the standby loss is not zero, and it may be more than the energy lost by pumping a half-gallon of hot water 6X(?) per day.
Then, you might compare the non-recurring costs.
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

Well, you are wrong about that Nick, or Bosch is guilty of saying what the native americans used to call, "that which is not so".
Take the "Interactive tour" and you'll see that they confirm there's NO standby energy use.
http://www.globecomsoftware.com/vendors/cec/cec_tankless/index.html
They use FLOW to trigger the heating, so, except for the little bit of heated water left in the coils when you close the faucet, there's minimal "standby losses".
Here's another one for you if you are still a disbeliever.

That probably would tip it in favor of the cheapo "jam it down the cold pipe" designs from an overall cost perspective, but that wasn't my original point. I was saying that I think those cheap systems are a stupid way to solve what isn't a very big problem anyway. And, that if you want to do it "right" then a separate return pipe with insulation or a tankless heater is the proper engineering solution.
Nice meeting you,
Jeff

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I don't have to or want to, but if I did, I'd want a lower-energy solution.

Maybe the latter. I see a number of Bosch units have standing pilots.

That's what they say...

There's one loss. The Bosch distributor CEC also mentioned a "5 to 10 second delay" between the start of water flow and actual hot water in the HX water ignition system. The closest thing I found to a zero-standby-loss instant heater was the $200 Powerstream RP12T... 240 V at 50 amps to heat 1.5 gpm "in a cold climate." But that seemed to have other problems, eg poor temp control with varying pressure, eg in a house with a well vs city water.

In my opinion, that's "not proven," and likely untrue.

Likewise.
Nick (ex K3VZW, BSEE '68, MSEE '87)
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snipped-for-privacy@ece.villanova.edu wrote:

Well Nick, since you encouraged me to "do the numbers", I just did.
We'll use your 6 times a day "waiting for hot water" figure.
Our city water (combined with a sewage charge that's more than the water charge!) costs us very close to $5.00 per hundred cubic feet. I know that's high, but we live in one of the 40 cities and towns that is still paying for the cleanup of Boston Harbor, claimed to have been polluted by over a hundred years of those 40 municipalities depositing their sewage into it.
I just used a gallon milk jug to check what it took to "get hot water" in the bathroom in our home furthest from the water heater. The jug was nearly filled when the water got "as hot as she gets" and it took 25 seconds to get there. Normally, I'd find the water was warm enough to start using in about half that time, but let's play it conservative. BTW, no one had been there to run any water for at least six hours before my great experiment.
That computes Captain, because that bathroom is maybe 50 linear feet from the heater, and assuming maybe 30 feet of 3/4" pipe and 20 feet of 1/2" pipe worst case, there's about 0.8 gallons of water in the hot water piping to sweep out, and the pipe itself needs to get heated in the process.
So, if we "waste" that one gallon of water down the drain six times a day, 365 days a year, the cost to do that, even at my outrageous water rates, will be less than $15 per year. If you live where water costs less than $5.00 per hundred cubic feet the cost will obviously be proportionally lower.
That's gonna take a long time to pay off the purchase and installation costs of an undersink unit. Say Nick, how many vanity cabinets have you seen with a power outlets inside 'em? Have you priced what an electrician would charge to install one there for you?
All the other factors cancel out, save for the cost of power to run the circulating pump and the cost of the water you "waste" waiting to get a glass of "cool" water to drink because warm water has just been pushed into the cold line. Of course, there's also some additional savings from decreased waiting times for hot water at other nearby faucets IF the motion detectored pump had recently been activated.
The pump power cost, if it really only draws 25 watts, is negligible. I figured it out to about half a kilowatt hour a year running for 30 seconds 6 times a day. That's nuttin' to worry about.
The "cold water waste" is a function of your thirst and taste of course, but it's gotta take something away the water you save by not wasting that water from the hot water faucet.
I suppose that if you're an impatient type and put a price on your discretionary time then the total waiting time for the hot water to warm up enough to use, which I make to be about 10 hours a year, would easily swamp all the other cost calculations. But, I try hard not to think about stuff like that while I'm waiting for traffic lights to change and supermarket checkout lines to move.
I think I've about "saucered and blowed" this one, Nick. Do what you want about it, Thank G-d it's a free country.
Cheers,
Jeff
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Good.
OK.
At about 2 gpm? Not much...

OK.
...O.8x6x365 = 1752 gal or 219 ft^3/year, about $11. Heating it takes 1752x8(120-60) = 841K Btu or 247kWh/year, about $25 at 10 cents/kWh.

IMO, the question is" "If a body desired instant hot water, which is cheaper, a) Grundfos, b) Taco, or c) Bosch?
Bosch may not be a good solution, with standing pilots or 5-10 sec delays. Taco is a better solution than Grundfos, with temp sensing and a motion detector vs a timer.

The Grundfos pump on a timer with no temp sensing would run a lot more than that. Running for a mere 2 hours per day, eg 8-9 AM and 9-10 PM, it might use 50 Wh of electricity plus about 2 kWh for pipe heat losses, totaling 2.05 kWh/day or 749 kWh/year, ie $75 at 10 cents/kWh. The Taco system might run for 3 minutes per day, filling half the pipe (only the hot pipe) at a cost of $36/year, about the same as the manual method, ignoring trips to the loo or the kitchen without hot water use.
Nick
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The Taco system might use about the same energy and less water than the manual (run water until it gets hot) method. A cleverer system might pump cold water back into the hot water pipe at the sink, after water usage has stopped, until the cold water reaches the water heater output, leaving the hot pipe filled with cold water between usage bursts, allowing house air to heat cold water in the hot water pipe, vs allowing stagnant hot water to lose heat in the hot water pipe...
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

I looked at these recirc pumps before I had replaced a dip tube that incidentally made a hot water problem moot.
I looked at a "chillipepper" brand. One thing it did was get turned on via X10 (perhaps with a timer).
If you run the hot down the hot and the warm/hot back up the cold for 30-60 seconds, when you turn on the tap, you get warm to hot water.
This makes my sleepy face happy.
Now, there's warm water backed into the cool a little bit with this.
But frankly, I'm washing my face, dealing with the AM. By the time I get to the bathroom, the hot water should be actually hot, so the device is now off. If I hit the cold water to get warm, as the cold pipe empties of lukewarm water to become cold (lets say 15 seconds of use, it's filled with cold).
Bottom line, the device turns on just enough to get hot water to the tap with about 30 seconds of advance warning.
If you do stuff and use NO cold water, then yeah, you leave some warm water in the cold pipe. Most of the time, people mix.
They also advertise that they draw the water fast enough to turn on a tankless waterheater.
So you're "6 times a day" means that you run this sucker 3-5 minutes a day at a few pump watts.
No special return pipe (which would be a pain in my existing bathrooms).
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I have been interested in building a new plumbing system with hot water recirculation. I just found this install guide from Grundfos that has some good information (PDF file)
http://www.us.grundfos.com/web/Download.nsf/Pages/D6774D5BEBD2117B8825652200650E2C /$File/Uprec-gd.pdf
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The way that they have it drawn it will work without the pump. Heat rises and cold falls so all you need to do is bring a return from the last hot water connection to the water heater drain with an extension, put a flap type check valve installed at a very slight angle so the flap hangs open on the return line. Then when you turn the hot water on the flap closes and you don't pull cooler water into the loop.
I did this in my ranch type house and it worked great, but used energy as you have a constant loss. But its a trade off for convenience.
Rich

http://www.us.grundfos.com/web/Download.nsf/Pages/D6774D5BEBD2117B8825652200650E2C /$File/Uprec-gd.pdf
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Bosch Aquastar has many units, you can get an old pilot model if you wish to waste energy or one of the newer models most people buy. 2 of them require no outside electricity supply, one has battery ignition , which I have, and one has a mini hydro generator. I dont see any more delay than with a tank. Recirculators waste energy.
Electric tankless heaters for most of the US cost double to operate than gas and easily require you to upgrade your main as a good Bosch unit takes apx 120 amp itself to operate.
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No more delay? :-)

The Taco system is close to the manual method, with less water use.
Nick
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My Bosch pilotless has a 4 second delay from faucet turn on till full fire. Not a delay worth considering. My 2 d cells are 2.7 yrs old , maybe new batteries would get it down, but still not worth mentioning at 4 seconds, and zero standby loss, it is pilotless.
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Correction, I just went back and took off my Waterpick restrictive tap filter and got a 3 second delay.
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