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re: gas or electric water heater ?
10 apr 2004
lawrence wade wrote:
> i saw "sci.engr..." in the group's name; i assumed i'd be able to
>get advice off a bunch of mechanical engineers who specialized in hvac
i think this group is mostly workers. a few mes design appliances. most
are off designing systems for large buildings, rockets, cars, and so on.
>> btw does anyone know which is cheaper to use/maintain in the long run ? a
>> gas or electric water heater?
>...1 kwh of electricity yields 3,413 btu of heat. one cubic foot of
>natural gas gives 1,008 to 1,034 btu of heat - average it to 1,021 btu.
good.
>...we'd be done, except for one thing. electric and gas hot water
>heaters will both cool down from exposure to air. if they're the same
>physical size and have the same amount of insulation, we can ignore
>that loss, because it'll be the same for both of them - and you just
>want to know which is cheaper.
he also wanted to know which was cheaper to maintain, which is a good
question for this group, with their years of experience. i'd guess there
isn't much difference in maintenance costs.
there could be a large difference in installation costs, especially for
a house with no other gas appliances. there are also safety and health
differences. a few years ago, i saw a whole warehouse full of charred gas
appliances (mainly dryers) that had caused house fires. a friend of mine
won't have any gas appliances in his house, because he says the pipes
always have tiny leaks, even after multiple passes with soap solutions.
he cites a 1950s aaas article showing that people who live in houses with
gas appliances have slightly higher cataract rates.
>the gas hot water heater loses energy through its chimney
>and surroundings - not all the energy goes into the water. the
>manufacturer will then give you an efficiency rating, typically as a
>percent - that's the percent of gas which heats the water, the rest
>being lost. say you choose one that's 90% efficient - then you'll need
>10% more gas to couple those 10,000btu of heat to the water, or 1.1
>times the amount of gas. 90% is probably a good ballpark, but i
>haven't looked it up for any particular brand. you do that.
as i recall, it's closer to 60%. i suppose this includes the pilot light,
but not the effect of the chimney, another hole in the house that allows/
requires more air infiltration, which raises the heating bill. tankless
gas heaters are more efficient, close to 90%, with a smaller amount of hot
surface and less standby heat loss. but they can't use off-peak electricity,
which costs about 6 cents vs 15 cents per kwh where i live...
i've suggested a greywater heat exchanger with a small electric heating
element: a $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 100 psi/73.4 f pipe from pt
industries (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.)
75450 9 1 0.29 2 3/4" conduit locknuts (fresh water i/o)
23766 2 0.64 3/4" cpvc male adapter (fresh water i/o)
141830 1 0.42 2 3/4" reducing conduit washers (fresh water i/o)
23813 1 1.39 10' 3/4" cpvc pipe (for 1" 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)
23574 4 3.88 3/4" cpvc fip adapters (")
54142 4 3.28 3/4"x1" male adapter barb (fresh water i/o)
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 $42.08.
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: $65.96.
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.
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 up to 110 f would take 8x75(110-98.5) = 6.9k btu/day,
ie 2 kwh worth about 20 cents/at 10 cents/kwh, for a yearly savings of about
($1-0.20)365 = $292. 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.
nick
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