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.
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
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.
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.
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.
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 Wisnia (W1BSV + Brass Rat '57 EE)
"As long as there are final exams, there will be prayer in public
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.
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
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
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.
...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.
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...
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).
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)
Jedd Haas - Artist
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.
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.
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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