Home Ownership (misc.consumers.house)

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Phil Sherrod
 
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Default Power cost of idle electric water heater


On 31-Mar-2004, Jonathan Ball wrote:

Larger tanks have LESS increase in surface than the
increase in volume.


It is true that the volume increases faster than the surface area. So what?

Once you have a tank large enough to supply your hot water needs, any increase
in volume beyond that point will require an increase in surface area and more
heat loss. For maximum efficiency, you should use the largest single tank that
will supply your hot water needs.
  #44   Report Post  
 
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Default Power cost of idle electric water heater

In misc.industry.utilities.electric wrote:
| Jonathan Ball wrote:
|
|...Take a 1-foot cube, 1 x 1 x 1. The volume is 1 cubic
|foot, and the surface area is 6 square feet. An n x n
|x n cube of 2 cubic feet will have n = (approx) 1.26.
|The surface area is 6 x 1.25^2 = 9.52 sqare feet. The
|volume has doubled, but the surface area has gone up by
|LESS than twice.
|
| A lovely explanation, but larger tanks lose more heat.
|
|Not relative to volume.
|
| Who cares about volume?

Those who care about how it is relevant.


|The RATE of heat loss is based on the ratio of the
|surface area to the volume, and because the larger
|vessel has a SMALLER ratio of surface area to volume,
|it will lose heat at a slower rate.
|
| I'm afraid you are wrong, my good man. The rate of heat loss
| (vs temperature change) is directly proportional to the amount
| of surface. This is known as "Newton's law of cooling."

But you are totally ignoring the system. Pick a volume of water.
Now pick between having one large tank, or many smaller tanks.
Guess which one losses less heat.

It really is a matter of being proportional to volume.

--
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| Phil Howard KA9WGN |
http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
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  #45   Report Post  
 
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Default Power cost of idle electric water heater

In misc.industry.utilities.electric Phil Sherrod wrote:
|
| On 31-Mar-2004, Jonathan Ball wrote:
|
| Larger tanks have LESS increase in surface than the
| increase in volume.
|
| It is true that the volume increases faster than the surface area. So what?
|
| Once you have a tank large enough to supply your hot water needs, any increase
| in volume beyond that point will require an increase in surface area and more
| heat loss. For maximum efficiency, you should use the largest single tank that
| will supply your hot water needs.

In terms of cutting energy costs, a larger volume has benefits. And it is
practical to increase the volume. For example, a full days supply.

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
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  #46   Report Post  
Mark Sauder
 
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Default Power cost of idle electric water heater

Lou wrote:

"Mark Sauder" wrote in message
...
For instance my utility co. offers time of day rates. When I was
participating
the electric rate was about half the normal rate from 7pm to 7am and
all day weekends and holidays, and about twice the normal rate the rest
of the time.


That sounds somewhat askew - what you've just said is that the electric rate
was _never_ the normal rate. You seem to be telling us that if, for
example, the normal rate was ten cents per kilowatt-hour, participants in
this time of day pricing scheme could buy electricity for either five cents
or for twenty cents a kilowatt-hour, but never for ten cents.

Is that right?


Yes, that is correct. I just checked my utility co. web site
HTTP://www.xcelenergy.com and here are their rates.

Normal residential service - 7.092 cents per kWh
Time of day rates peak - 13.633 cents per kWh
Time of day rates off peak - 3.163 cents per kWh

The time of day service is an optional service, if you do not sign up
for it you will be charged the normal residential rates. If you can
move enough of your electric usuage to the off peak time you can save
money.

This is what my electric co. offers, other companies may have different
plans.

Mark
  #47   Report Post  
default
 
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Default Power cost of idle electric water heater

But more tanks lose even more heat for the same amount of water.
Fewer larger tanks beats more smaller tanks.


Utimately, your goal is hot water to USE, not hot water to store...
the only reason you have a tank at all is so you'll have enough
hot water when you want it. So you want a tank that JUST
big enough to meet your top demand, anything bigger than that
isn't helping you any, and IS increasing your heat-loss.

If you have a 30 gallon tank which is adequate to meet
your water demand, replacing it with a 60 gallon tank of identical
construction will cause your costs to go up.

--Goedjn

  #48   Report Post  
John Gilmer
 
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Default Power cost of idle electric water heater


"Phil Sherrod" wrote in message
news

On 29-Mar-2004, Bill Vajk wrote:

Your numbers are nice and thanks for posting the info,
however it seems to me 114 is a little on the low end,
with 120-125 being more common and I run my domestic
hot water at 140F.


I ASSume you are providing hot water for a complete bathroom and that your
guest would be taking a bath or shower.

I would suggest that you just place a switch at the water heater. When you
open the room, you turn on the power to the heater. With 4 kW coming in,
you can get water for hand washing within a few minutes.

You don't really need a two pole switch but you can get heavy duty 2 pole 20
amp switches for less than $20 and it might come in handy if you service the
heater (To set the temperature on most water heats you have to uncover BOTH
thermostats. It's a good idea to have the power completely off when you do
this.

If you only power the heater when you have a guest you just don't have to
worry about standby losses and you reduce that chance of something BAD
happening when no one would be around to turn off the water.


I agree. If this was my primary water heater, I would probably bump the
temperature up to 125. I may change it later.


Well, if you only used when you have a guest, the higher temperature setting
would let your guest take a longer shower without running out of hot water.


  #49   Report Post  
david fraleigh
 
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Default Power cost of idle electric water heater

Someone commented on the high cost of the instant hot water heaters...
Well about five years ago I made one out of parts that can be pretty
easily bought for around fifty dollars and it is still going strong...
Essentially it is made out of 1 inch galvanized pipe and T-fittings..
The T fittings allow you to screw in the screw-in type heating
elements into the heater... On mine I put in a flow switch (that I
got from a surplus place but that could be bought from a plumbing
supplier) I wired that to a 240 volt contactor large enough to handle
the flow of electricity... (I got mine out of a junked large air
compressor)... and I screwed in two heating elements in series into
the one inch pipe... All sort of hard to explain... but you get the
picture I hope.. It doesn't take up much room and remarkably it is
still working well years later.. I had commercial bought ones before
and got tired of replacing expensive little parts in them (like their
heating elements that would burn up if some air got in the line) or
their electronics which seemed to be damaged by power surges... etc..
This thing I have now just won't quit and when it does I can just get
the parts easily and cheaply... I don't have the luxury of a really
steaming shower (especially in the winter when the incoming water is
colder) but my power bills for a 3 person household are under
$25/month so I can live with it..
  #50   Report Post  
 
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Default Power cost of idle electric water heater

david fraleigh wrote:

Someone commented on the high cost of the instant hot water heaters...
Well about five years ago I made one out of parts that can be pretty
easily bought for around fifty dollars and it is still going strong...
Essentially it is made out of 1 inch galvanized pipe and T-fittings..
The T fittings allow you to screw in the screw-in type heating
elements into the heater...


Sounds interesting. I'm building something like this into a removable
clamp-ring lid of a 55 gallon drum, along with 300' of 1" plastic pipe,
to make a greywater heat exchanger with a little post-heating from
about 100 to 110 F from a $7 1500 W heating element.

On mine I put in a flow switch (that I got from a surplus place but
that could be bought from a plumbing supplier) I wired that to
a 240 volt contactor large enough to handle the flow of electricity...


You might use a $10 motion detector instead of the flow switch, and
add a $12 "single element water heater thermostat" (which is actually
2 in series, for safety.)

and I screwed in two heating elements in series into the one inch pipe...


Why two?

Nick

I'm using a new $35 55 gallon lined steel drum with a strong removable lid
(because the drum might end up under 2' of greywater head, with the inlet
and outlet above the lid) and bolt ring and a 3/4" bung and a 2" bung with
a 3/4" threaded knockout, with 100 psi/73.4 F pipe from PT Industries at
(800) 44 ENDOT. Their PBJ10041010001 1"x300'100psi NSF-certified pipe is
actually tested to 500 psi. The price is $59.99 from any True Value hardware
store. Lowes sells the rest of the hardware needed, all of which is
installed through the lid, so the drum itself has no holes:

sales total
# qty price description

25775 1 $5.73 24' of 1.25" sump pump hose (for greywater I/O)
105473 1 1.28 2 SS 1.75" hose clamps (for greywater hose)
54129 2 3.24 1.25" female adapter (greywater barb inlet and outlet)
23859 2 2.36 1.25x1.5" reducing male adapter (bulkhead fittings)
75912 1 0.51 2 1.25" conduit locknuts (bulkhead fittings)
28299 1 1.53 2 1.25" reducing conduit washers (")
22716 1 1.36 1.5" PVC street elbow (horizontal greywater inlet)
23830 1 2.98 10' 1.5" PVC pipe (for 3' greywater outlet dip tube)

The parts above are greywater plumbing ($18.99.)

23766 4 1.28 3/4" CPVC male adapter (for 1" pipe barbs)
23766 2 0.64 3/4" CPVC male adapter (fresh water I/O)
42000 2 3.84 3/4" FIP to 3/4" male hose adapter
23813 1 1.39 10' 3/4" CPVC pipe (for 1"x3/4" fresh water outlet)
23760 2 0.96 3/4" CPVC T (fresh water I/O)
22643 2 0.86 3/4" CPVC street elbow (fresh water I/O)
4 - 1" 3/4" CPVC pipes (fresh water I/0)
1 - 3' 3/4" CPVC pipe (fresh water inlet)
22667 2 2.56 2 SS 1.125" hose clamps (fresh water I/O)
219980 1 4.87 10.1 oz DAP silicone ultra caulk (bulkhead fittings)
150887 1 3.94 4 oz primer and 4 oz PVC cement

Parts above are fresh water plumbing. Subtotal $39.33.

26371 1 6.83 1500 W electric water heater element
22230 1 2.31 1" galvanized T ("nut" for heating element)
61294 1 11.76 single element thermostat with safety
136343 1 0.56 5 10-24x3/4" machine screws (mount thermostat with 3)
33368 1 0.37 5 #10 SS flat washers (mount thermostat with 3)
198806 1 1.38 10 #0 rubber faucet washers (mount thermostat with 3)
8763 1 0.67 5 10-24 SS nuts (mount thermostat with 3)

The above would make a standalone water heater, if needed. Grand total: $63.21.

For 4 10 min showers per day and 20 minutes of dishwashing at 1.25 gpm we
might heat 75 gallons of 55 F water to 110 with 8x75(110-55) = 33K Btu with
about 10 kWh worth about $1/day at 10 cents/kWh. If the "heat capacity flow
rate" Cmin = Cmax = 75gx8/24h = 25 Btu/h-F and the pipe coil has A = 300Pi/12
= 78.5 ft^2 of surface with U = 10 Btu/h-F-ft^2 (for an HDPE pipe wall with
slow-moving warm dirty water outside and 8x300Pi(1/2/12)^2 = 13 gallons of
fresh water inside), the "Number of heat Transfer Units" for this counterflow
heat exchanger NTU = AU/Cmin = 78.5ft^2x10Btu/h-F-ft^2/25Btu/h-F = 31.4, so
the "efficiency" E = NTU/(NTU+1) = 97% for hot water usage in bursts of less
than 13 gallons. This works best with equal greywater and cold water flows,
with either a 110 F water heater setting (preferable), or the heat exchanger
output feeding the cold water shower inlet as well as the water heater.

The Hazen-Williams equation says L' of d" smooth pipe with G gpm flow has a
0.0004227LG^1.852d^-4.871 psi loss. At 1.25 gpm, the pressure drop for 2 150'
coils of 1" pipe might be 0.0004227x150x(1.25/2)^1.852x1^-4.871 = 0.03 psi.

If greywater leaves a shower drain and enters the heat exchanger at 100 F and
fresh water enters at 50 F, the fresh water should leave at 50+0.97(100-50)
= 98.5 F. Warming it further to 110 F would take 8x75(110-98.5) = 6.9K Btu/day
or 2 kWh worth about 20 cents/at 10 cents/kWh, for a yearly savings of about
($1-0.20)365 = $292, or more, with a tighter shower enclosure and higher drain
temperature. The 1500 W heater might operate 2kWh/1.5kW = 1.3 hours per day.
We might wrap the drum with 3.5" of fiberglass and a 4'x8' piece of foil-
foamboard with 7 4' kerfs (knife cuts partially through the board) to make
an octagon and aluminum foil tape to cover the kerfs and hold it closed.



  #51   Report Post  
 
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Default Power cost of idle electric water heater

In misc.industry.utilities.electric default wrote:

| But more tanks lose even more heat for the same amount of water.
| Fewer larger tanks beats more smaller tanks.
|
| Utimately, your goal is hot water to USE, not hot water to store...
| the only reason you have a tank at all is so you'll have enough
| hot water when you want it. So you want a tank that JUST
| big enough to meet your top demand, anything bigger than that
| isn't helping you any, and IS increasing your heat-loss.

But you do want to have enough to store, to use, across periods of
rate/cost cycling, so you can optimize the time frames in which you
heat the water to minimize costs.

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------
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