92-93-94 even 98% water heaters are common even 96% boilers, even a
94% tankless. AO Smith Cyclone tank, Takagi tankless and a Canadian
firm makes a 98% commercial hw boiler, 5 years ago I installed at my
apt a 92% 1900000 btu AO Smith Cyclone. these are all condensing
On Wed, 19 Mar 2008 04:32:02 -0700 (PDT), ransley wrote:
I think the reason for this is somewhat misleading.
If I understand this correctly, almost all the heat energy put into an
electric heater gets put into the water. Basically, the water cools the
heater coils down by taking the heat off the heater coil.
In the case of a gas water heater, the water cools down the flame by taking
heat off the flame (figuratively speaking) but a LOT of heat goes up the
They baffle the flue to slow down the rising air but they have to let the
hot air out. If they cooled the hot air to room temperature, it wouldn't
rise and get out of the house and that would be a bad thing from the
standpoint of carbon monoxide poisoning.
So, I think the fact that all none of the heat energy that went into the
electric coils goes up any flue - it's all absorbed by the water - is what
makes the electric water heater 98% efficiency.
But, as someone stated, I suspect the power generation is about 70%
efficiency, so, the true efficiency of electric water heating must be
vastly lower than 98% taking distribution into account.
But, how can we account for that true efficiency?
Electric are all 100% efficent, all energy consumed is used to heat
water, and energy factor should be near 100 as well with great
insulation. Almost all gas water heaters burners are about 80-83%
efficent, but an additional 17-20% goes up the chimney 24 hrs a day,
Energy Factor ratings account for loss up the center uninsulated flue
part of the tank and reflect overall efficency, which for most gas
tank is 50-60 with one I saw of 70. Condensing gas water heaters,
Boilers, furnaces, are different, have a second exchanger that lowers
flue temp to near room temp and are forced out the flue by a fan. A
condensing 93% water heater wont loose 20% in flue loss since the fan
stopped some of the heat loss, but even the best condensing tank water
heater of 93% may only be 83% Energy Factor [I guess]. Condensing tank
water heaters are really commercial units costing thousands. AO Smith
has them, I own one a 175000 btu unit, a Cyclone. For most, electrics
are and always will be more expensive to run unless you have a cheaper
Hydro Dam nearby, since for most oil- gas products generate
electricity. Someone stated 70% for electric, that is not true to you
for what you consume and pay, he was talking about transmission line
loss, for you electric tank is 100% efficient, but here electricity is
still 30% more than NG. If nobody in your neighboorhood has an
electric furnace then you can bet Ng is still cheaper per Btu. Now in
the last 6 months all petroleum products are going up fast, but
electric will follow in the long run.
so, the true efficiency of electric water heating must be
Average power generation isn't anything like 70% efficient. Typical
efficiency for a coal fired station is 30-40% with only the latest
generation achieving 60%+. Gas fired around 47% and nuclear around 38%. Then
another 5-6% is lost in transmission. The average depends on what mix your
country has but I can't see it being much above 40-50% overall by the time
it reaches your house.
This web site compares the cost of different fuel sources in the UK. It's
the only site I've seen that takes into account boiler efficiency. The key
figure is the middle one "Pence per kWh after boiler efficiency". The actual
boiler efficiency is in brackets...
Note that electric heating is indeed 100% efficient but the cost of that
electricity makes it expensive to run.
Heat Pumps have efficiencies of over 100% and in the case of a ground source
heat pump (GSHP) around 350%. This more than compensates for the loss of
efficiency producing the electricity needed to power. Overall a GSHP is the
cheapest system to run (ignoring capital costs). It would be interesting to
know if anyone makes a small scale gas or oil powered GSHP and how the
efficiency of those compare.
In theory it would _just_ be possible to use the heat from a GSHP to power a
Stirling engine to power the GSHP. This would not violate COE because there
is a heat source (the sun) providing power into the system. However most of
the heat produced by the GSHP would go into the stirling engine with very
little left over to heat your house. Stirling engines that big would also be
rather big physically. Overall such a system would be too big and expensive
to be practical - but it would be free to run.
you can get too would up over efficency ratings, nothing is 100% even
electric loses a little to the room.
and one must be aware that cost to buy can exceed savings on whatever
your trying to be more efficent with
Basically what we're talking about with the efficiency of water heaters is
the percentage of the energy that's put into the system that actually gets
applied to the task of heating water. And, of course, a lot depends on where
you define the boundary of the system.
on a tankless feeding a regular tank, it should cost no more to
operate than a regular hot water tank.
the tankless initially heats the water to whatever it can, then sends
the water to a regular tank that does its normal job.
endless hot water regular tank conveniences and the only extra cost is
the line between the tankless and regular tank, ideally it should be
short and well insulated.
true the tank will have normal tank losses.
today i have to stop at home depot and while i am there price some hot
water tanks. just to verify some of these issues:)
Maybe not, but is sure costs a lot more in terms of buying and
installing 2 water heaters, one of which is tankless and more
expensive. With this approach, you incur the higher cost of tankless
and by having the second regular tank, you still have the standby
losses, which defeat most of the advantage of the tankless that
justify it's expense. I fail to see the point. Plenty of folks
have a gas tankless for their whole house needs and are happy with it.
There is no point to this approach, its backwards and will loose you
all the savings you just paid for. If the tankless and tank are 82%
efficent you are heating with one 82% burner and keeping it warm with
another 82% burner. You are heating with the tankless and allowing it
to cool in the tank, at about a 20% reduction in efficency rating.
Most of what you just paid for in increased efficency goes up the
center of the tank and out the chimney. The tank if hooked up should
before the tankless and only hold water unheated to allow it to warm
up by the surounding air to temper it, it works for me. Even better is
to strip of the insulation on the tank, your basement will always be
warmer then the incomming water main.
A much better way to do this is to get an electric tank water heater,
remove the heating elements, and wire the thermostat to run a pump on
a loop to the tankless heater. Incoming cold and outgoing hot are
from the tank iteslf.
This way, the standby losses are that of an electric tank, which is
less than a gas tank due to the lack of a flue down the middle. An
advantage over tankless only is that the delivered hot water pressure
is higher, because the pressure drop from a tank is noticeably less
than from a tankless.
When I get around to installing solar hot water, this is probably the
way I'm going to go; the solar can be on another loop from the tank.
On Tue, 18 Mar 2008 07:30:39 -0700 (PDT), ransley wrote:
I should have mentioned that I searched for that PDF during my GAS water
heater replacement. http://tinyurl.com/38eh4d
That PDF only contained residential GAS water heater specifications (very
many hundreds or even a thousand or more).
It did not have any residential ELECTRIC water heater efficiency ratings
(some of which approach 98% due to the fact no heat goes up the flue; it's
all absorbed by the water).
If you're gonna spend lots of money on a water tank you might as well
get an oil fired demand water heater. Gives you unlimited hot water and
no cost to maintain a tank of hot water. If you want to put a tempering
tank in your hot attic save even more.
my tankless add on was only for use when familiy is visiting, the
remainder of the time my 50 gallon high btu tank is fine.
now 7 people pile in here, and it can become a problem espically when
incoming water temperature is 40 degrees
Its alot of money to put in a tankless and not get the savings year
around, first you need to get the supply tested with all other gas
apliances running to be sure no upgrade is neded. Do 2 people shower
now at the same time, I dont think you will benefit having a tankless
before a tank and it will actualy cost more to run since both units
burners are probably near in efficency, I put my tankless after my
tank with bypass valves incase my old tank leaks, but i havnt used it
since installing the tankless, the cheap Bosch.
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