Maybe you've spoke of this subject B-4.
But I see Lowes big box store has a variety of instant hot water
heaters. The one like Rinnai <sp>...
Are these water heaters worth the price to switch out from a water tank?
There are various models and prices...
Have you had any experience with any at all? Are they efficient? Is
Rinnai the only one to trust?
Seems to me that a great deal of money can be saved using these heat on
units...am I wrong?
Please comment on what you know.
And I thank you for any input...
Pat in Denver
Make up your mind Harry.
1 - First you said the essential difference was the standby losses.
2 - Then you said how much you save depends on your usage patterns.
3 - And then you said losses from tank heaters are almost constant
regardless of how much you use.
For the record, I agree with 1 and 3.
For me, I look at my gas bill in summer, when all gas is used for is
the water heater and my outdoor gas grill. It's about $17 or so a
month and that is with a std efficiency water heater. That includes
those standby losses and usage. I could surely do better with a high
efficiency tank type. From that, I've concluded if I need a new
one, I'd go with a tank type higher efficiency one.
One simple test to settle the standby loss issue would be to read the
gas meter before going away for a few days to a week. Before doing
that, draw enough water to fire it up. Upon returning, see how much
gas it used.
They were leaders in jet aircraft. Remember the DeHavilland Comet?
It was involved in the first fatal crash of a passenger jet airliner. Soon
afterward one dropped into the Indian Ocean . . ."The crash was attributed
to structural failure of the airframe with witnesses observing the wingless
Comet on fire plunging into the Indian Ocean."
On 10 January 1954, 20 minutes after taking off from Ciampino, Comet G-ALYP
("Yoke Peter"), BOAC Flight 781, broke up in flight and crashed into the
Mediterranean off the Italian island of Elba, with the loss of all 35 on
And who could forget the White Star Line's most famous ship, the Titanic?
Silly boy. The English owners wouldn't want to pay British steelworker or
real estate rates. They exploited the Irish in the finest tradition of an
"The first company bearing the name White Star Line was founded in
Liverpool, England by John Pilkington and Henry Wilson, and focused on the
U.K. - Australia trade, which had increased following the discovery of gold
there . . . the company's bank, the Royal Bank of Liverpool, failed in
October 1867. White Star was left with an outstanding debt of £527,000, and
was forced into bankruptcy. On 18 January 1868, Thomas Ismay, a director of
the National Line, purchased the house flag, trade name, and goodwill of the
bankrupt company for £1,000, with the intention of operating large ships on
the North Atlantic service. Ismay established the company's headquarters at
Albion House, Liverpool."
That $17 probably includes overhead.
I asked my gas company what my bill would be if I used no gas at all; their
answer: about $17.00. That amount helps pay for physically reading the
meter, mailing out a bill, recording the payment, etc. I can't begrudge them
the admistrative fee.
So, living in a converted duplex, I connected the gas lines from the two
sides of the building and cancelled the service to one side.
Saved $17 bucks a month.
1) Natural gas prices have been very low the past few years, and there's
no indication that's going to change in the next few years. Anything
you spend to reduce natural gas usage will have a proportionately small
return on investment given low gas prices.
2) Conventional hot-water tanks are pretty efficient from a
standing-loss standpoint, and what little heat they do radiate can be
reduced by a relatively cheap external insulation blanket. On the other
hand, the radiant heat loss from the tank is captured inside your house,
the advantage of which is proportional to your northern geographic
location (or as a function of altitude).
3) heat loss from a conventional tank flue is minimal if you have a
power-vented system (when the fan isin't turning, it's acting like a
baffle preventing air flow through the flue). I suppose a
power-operated shutter could be added to completely close the flue and
prevent heat loss when the burner is not on.
4) efficiency of heat transfer is inversely proportional to the heat
gradient. The burner of an on-demand heater needs to put out 10's of
thousands, even 100+ thousand BTU in order to heat incoming water during
the water's short residency time inside the heater for the water to
reach conventional hot-water temperature (typically 140 to 160 f). The
more north you are, the colder your incoming water supply will be, and
the more capacity (in BTU) the burners will need to be to bring the
water up to the desired temperature. Exhaust heat loss from these units
is significant while they are operating, and during their off-cycle as
they cool down they can't dump much heat energy into the water because
there isin't much water stored in the unit.
Conversely, the burner of a conventional water tank is capable of much
less BTU heating, and the heat from the burner has more time to come
into contact with the internal tank surface and transfer it's heat into
the water. The exhaust gas temperature in the flue of a conventional
heater can be so cool as to require a small electric blower to properly
exhaust the gas out the flue. This is an indication that most of the
combustion heat is being transfered into the water and not being
exhausted out the chimney.
In other words, perhaps 50% of the combustion heat of an on-demand
heater is actually being transfered to the incoming cold water and the
other 50% is being lost in the exhaust, while 80% of the combustion heat
is absorbed by the water in a conventional tank. The difference is that
an on-demand heater is on perhaps 30 to 90 minutes per day, while a
conventional tank might be on for 4 hours a day. But remember that when
a conventional tank is on, it's burners are using a much smaller amount
of gas compared to the on-demand heater.
5) the efficient use of an on-demand heater is challenged by short
hot-water usage events. In most houses, the hot water lines are
minimally insulated and thus the water in them quickly drops to room
temperature. Anyone turning on a hot-water tap in an upstairs bathroom
will notice it take 10 to 30 seconds to actually get hot water. It
doesn't matter what type of heater you have (assuming the heater is in
the basement). A short hot-water use event (say, washing your hands)
will end up dumping a lot of waste heat out the exhaust when an
on-demand heater is signalled to turn on and then soon after turned off
to heat the water for that short-use event.
6) because of the very high heating capability (BTU capacity) of
on-demand heaters, the extreme thermal cycling of their internal
components will age the unit much faster than a conventional water
heater, and they do or will require more maintainence and repair vs a
conventional water heater (they have control devices, electronics, etc,
that are not present in conventional heaters, and as we all know -
electronics and HVAC equipment really don't tend to co-exist very well
for the long term).
7) on-demand heaters have electrical or electronic controls that require
a source of AC current. Thus they will not function during a power
failure. Anyone living in a northern climate that is subject to
sporadic winter power failures will not appreciate the lack of hot water
during extended outages.
No home owner that has a working conventional gas water heater will ever
live long enough to recoup the savings from replacing his existing
working heater with an on-demand unit - and it's not a given that there
will actually be any measureable savings in gas use.
What has been observed is that the behavior of occupants change in terms
of how they use hot water when a conventional heater is replaced with an
on-demand heater, and that change usually results in less hot water use
(shorter showers, changes in shower heads, installation of low-flush
toilets, etc, insulating hot-water supply lines inside the house) so
it's not always clear where the savings come from and why.
Replacing an old / leaking conventional water heater is very easy for
most novice home owners / handymen, and at a cost of only a few hundred
dollars, the cost/reward ratio is still heavily in favor of replacing a
old conventional water heater with a new conventional unit.
You will get more bang-for-the-buck by
1) putting an insulating blanket around your existing or new
conventional water heater
2) insulating as much of the hot water supply lines inside your house as
you can reach
3) use a low-flow shower head
On-demand water heaters are basically a crock of shit designed to give
plumbing and HVAC companies a very lucrative new revenue stream.
Well, nobody ever `splained it to me that way before.
Thank you... I read your whole soliloquy,
and I do understand what you wrote.
Especially that changing over to an on-demand water heater will never
return what you spent to install it...EVER.
You may get an argument from some in this group... but I thank you for
making me understand the big picture...
And thanks to all who took the time
to respond as well....
Pat in Denver
You need to go back to school, what you state is nuts. No gas boiler
is over 100% efficient, no gas boiler is 100% efficient, the best is
around 98%. There is wasted heat out the chimney and there is the
loss. If you burn an unvented flame inside like a gas stove, that is
There you are wrong, a Btu is a Btu and this is 6th grade stuff, you
cant get back more btus then you put in. Its a fundamental part of
energy. If you could get more out than you put in your reasoning
would lead to perpetual motion of other energy forms. Look at our AFUE
ratings for furnaces, we correctly rate that condensing furnaces and
boilers start at about 92% and go to 98%, its tested and proven by
science you dont get more than you put in burning gas for condensing
units. You still waste energy with condensing units, it goes out the
chimney. I have a condensing furnace, boiler and condensing AO Smith
water tank, None of mine are rated over 96% and nothing sold is rated
over 100%, even 99% is likely a lie as heat is wasted out the
chimney . If your point was correct running a stove would be over 100%
efficient, but it takes energy to condense water.
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