Hot Water on Demand

CheannbalJune 9, 2004

I'm thinking of having my 40 gal. gas-fired hot-water tank removed and replacing it with a hot water on demand system. They're selling them at Home Depot. Anyone with any experience? It's just my husband and I in the house now so we don't have to worry about teenager's showering habits. Any thoughts?


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On demand heaters save ONLY on storage losses . . . . and they have some "quirks" you should know about before you buy.

They do NOT use less energy to HEAT the water . .. ONLY less to store it. With an efficient, well insulated water heater the savings will be minimal.

They have certain flow rates at certain temp rise of the water . . . . at very slow rates they may not heat correctly; and at the high flow rates some will NOT be able to keep up with the flow . . . such as filling a tub where there are NO flow restrictors. Multiple hot water faucets on at the same time may tax it as well. You may need to run a larger gas line to feed it. . .. though they heat ONLY when water is required; the instantaneous rate at which they heat the water is quite high. For electric ones; the line you must run is quite large.

Another issue to me is serviceability . . . . will the mfr still be in business a few years from now when you need a part ? Will it be easy / fast to get as you will be without hot water until then . . . . .

They are not cheap compared to conventional water heaters; what little you may save will take MANY years if ever; to recoop the additional expense up front.

I'm not intending to knock them; but know of their quirks etc before you buy one. To me; other than space restrictions in a particular situation; I would not use one . .. . .


    Bookmark   June 9, 2004 at 7:51PM
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Thank you very much, Bob, that's just the sort of info I was looking for. I won't be getting one. I appreciate the time you took to reply.


    Bookmark   June 10, 2004 at 5:52PM
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Please read both letters below.
One is for one against.
Just to balance out the discussion!
Just thought I'd let you know that when our tank water heater began to leak (due to calcium build up, as they do here after about 10 years) a couple of years ago, we replaced it (in our attic) with a tankless water heater. I have been very impressed with it overall. The fire risk must be quite small, I would think, and the savings are substantial. We would have paid $500 for a tank water heater to be installed and paid $700 for a tankless plus about $100 to have a larger opening for the larger exhaust pipe put in our roof. At $100/year savings, the energy savings will have paid the difference by next year and paid for itself in savings in another 4. I'm not sure how easy it is to get a desuperheater installed or to find one, but the tankless heaters are at Home Depot and probably Lowes as well. They also are warranteed for 15 years (if professionally installed) and are expected to last much longer. Once they finally live out their expected life, the parts can be recycled (rather than taking up huge space in a landfill). Anyway, just thought I'd let you know that I'm satisfied with mine and would definately recommend it.

Best wishes,

>Subject: [EnergyRater] Tankless Water Heaters: Dispelling the Myths
>Date: Wed, 09 Jun 2004 11:23:14 -0500
>Although numerous websites can be found that SUGGEST that tankless water
>heaters deliver hot water for less energy, this is the only site I've found
>with HARD numbers!
>I've heard it said that the standby losses of standard water heaters (with
>tanks) is so small that there is negligible savings. HOWEVER: Apparently
>this author/scientist disagrees and EEBA thought him a worthy presenter
>last year. --->
>Rob Hammon, ConSol
>" ...
>Water heating 20% to 25% of our energy use
>ÂStandby losses are 10% to 20% of water heating costs
>ÂTankless water heaters are 30% more efficient than a standard tank-type
>water heater
>ÂTankless have had problems in the pastÂ
>6. How Efficient?
>Tankless 8 gal/min Energy Factor 0.82
>13. Cost - Savings
>ÂTank electric or gas $350 - $400
>ÂTankless electric or gas $650 - $1200
>Â$600 price differential
>Â$80 - $100 annual savings
>Â6 Â 7.5 year simple payback
>However, despite these numbers I'm not sure I'll be recommending them because:
>1. Most installations of water heaters are inside of the conditioned spaces
>of homes or their garages or attics.. These practices should be avoided
>since such installations are fire hazards in the last two cases and CO and
>CO2 hazards as well, if installed in conditioned spaces. To minimize the
>CO and CO2 poisoning effect it is advisable to provide more ventilation in
>their vicinity, this solves a health problem while increasing the
>infiltration of the home. A tankless water heater will need more
>ventilation because it uses more source combustion air faster than a
>standard gas unit.
>A few of my clients have changed their gas water heaters in their garages
>to electric because of the fire hazard!
>To avoid these problems I recommend sealed-combustion water heaters or
>water heaters installed outside of the home, garage and attic
>altogether. When that recommendation is not taken, I always recommend a CO
>meter in the same envelope as the water heater.
>2. The second reason I'm not apt to recommend tankless water heaters is the
>prospect of using alternative heat sources besides gas to heat water even
>more efficiently:
> A. The first choice here is the desuperheater.
> B. Second choice is solar water heaters.
> C. For the small group of my clients who want ground-source
>heat-pumps for cooling and heating of their homes, I recommend
>water-to-water heat-pump water heaters. This actually has a negative $500
>capital cost since it can avoid 1 ton of over-sizing of the ground
>loop. When standard efficiency units of this type are installed, you can
>expect to get a COP greater than 3. When compared to gas this creates an
>effective gas efficiency of slightly greater than 1.0.

    Bookmark   June 11, 2004 at 9:02AM
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Uh-oh! Okay, I think I'll continue my research. Thank you!


    Bookmark   June 11, 2004 at 4:54PM
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Electric on-demand water heaters use a lot of electricity when in use; a lot more than a storage tank heater. I've seen models as high as 100 amps (at 240 volts).

Some electric utilities do have restrictions on the amoung of sudden current drawn. That limit is above the common home tank water heater wattage of 4500. Faster heaters could be required to have staged element turn-on to minimize the effect on the quality of electric power. If a big water heater suddenly starts drawing 100 amps, you'll notice the lights dimming. If you go with electric, check what current will be needed for the heat-rise flow rate you need. if you go with gas, check the cubic-feet rate it needs.

While on-demand heaters can save energy over the loss levels from tank storage heaters, this small savings is probably undone by the fact that you will often need hot water when electric demand is high. More and more electric companies are wanting to go with variable time-of-day rates, and in these cases, any usage during the peak periods would be at a loss compared to a storage tank large enough to ride you through most of the peak period.

I used to think they were cool. Then I started to think about how I will be using energy economically (including peak-shaving from, if not out-right switching to, alternative sources). Storage of heat from any sources (solar in the day, electric at night with off-peak rates, and gas during other times) would be a good way to manage energy if you can afford all the sources (for example, you might omit the gas and just go with solar supplementing electric). Solar heating saves you the most, but it is only effective with storage (gather it when the sun is beating down on you).

    Bookmark   June 11, 2004 at 8:49PM
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For me the determining factor is fuel availability.

The studies that I do for energy efficiency shows
that gas is the best fuel type for this system.
One electricitian told me that for electric 3/30 amp
breakers had to be installed for an electric
tankless unit . 90 amps of elec,
regardless of one gallon or 100 gallons.
Maybe that was simplifying matters, but in an
understandable way for most people.

For many techs the standard installation is a
known factor, when you ask for something a bit
out of the norm, therein lies the problem.
Not that it can't be done, but just that it is not
done commonly.
If the tech has not installed this type of system.
complete reading of directions is necessary,
not just a 'looks like this would go here' kind
of thing. Doesn't sound like a lot to ask, but
sometimes asking 'didn't you read the instructions?'
can be offensive.

This is just my experience with these installs.
The gas units are efficient (energy factor .80 and
higher) but knowledge of install is also is also a factor.

Here is a link to a unit that I see installed often
( and correctly!!)
although it doesn't look like a working link, it is!
Best of luck with your decision!

    Bookmark   June 12, 2004 at 12:55PM
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I had no idea there would be so many opinions on these on-demand heaters. Mine would be fueled by natural gas so I don't have to worrying about the lights dimming.

1n 1983, I backpacked around S-E Asia and saw my first on-demand water heater in Bangkok, Thailand. Each room in the 'hotel' we stayed in had a unit. I thought it was absolutely ingenius and I tested it by running the hottest, longest bath in history. It was impressive.

Thanks to all for the info.


    Bookmark   June 13, 2004 at 5:26PM
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As long as your usage is within the tankless water heater capacity, you will be just fine and have all the hot water you want and save money in the process.

    Bookmark   June 15, 2004 at 8:33PM
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I installed a SETS system a few months ago, and am on a well. Although I have had to educate my children about the differences, they no longer run out of hot water while trying to get ready for school. For the daughter who prefers a bath, she has had to learn to draw the water more slowly, but then the other can take a shower right behind her and have hot water! This was not possible with a HWH. Although I think gas is more efficient, right now it is not the cheaper way to go. My electric bills are lower, but I have not studied it in depth to be able to quote an accurate percentage. My parents, who are in FL had a Seisco installed in their new house and have been very pleased. I am glad I made the change!

    Bookmark   June 27, 2004 at 2:31PM
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I read this thread because I had the same question. Everyone has info on the money you save. Does anyone have info on the water you save. Right now I have to wait a few minutes for hot water to reach my upstairs baths. Aside from the absolute waste of water, the water where we live is very expensive. Does anyone know what the possibility of savings (Of water)a tankless would give?

    Bookmark   July 22, 2004 at 10:07PM
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Assuming you put the on-demand heater in the same location as the existing tank heater, the savings on water usage would be zero.

    Bookmark   July 23, 2004 at 9:33AM
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re: " Does anyone have info on the water you save."

Water can be saved by installing a recirculating system.

Here is one example.


    Bookmark   July 25, 2004 at 12:04PM
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Like the original poster, I encountered on-demand hot water in Asia (Japan - I lived there for 2 years). I thought it was friggin brilliant! In a country where they have gas spigots like we have electrical outlets, it was energised by gas, and it was located on the outside of my house (It didn't freeze like it does here-- the snow removal system on the streets was warm water!). I was pinching yen like a Miser, but I NEVER ran out of hot water, even when I took an onsen (looong hot bath) or a shower, or washing clothes and didn't notice a huge jump in gas bill from summer to winter (no central heating).
I came back about 7 years ago and wondered why such a great idea didn't come over I see -->the Asians live, typically, in very small homes 500-1000 sq ft, but I now live in a 3 storey 2700sq ft home, and such a water heater in the basement might not make as much sense.
Thanks for the info!

    Bookmark   July 25, 2004 at 5:23PM
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Anyone who uses a tankless water heater to avoid running out of hot water could use more energy with one rather than less. With the endless supply of hot water, people might waste more of it.

Also, "losses" from a tank-type water heater often aren't really losses. During the winter, the heat loss from the water heater helps to heat your house, assuming it's in a part of your house that's heated. (And if it's in the garage, it may help to keep the garage a little warmer so pipes don't freeze -- since you must have some there if the water heater is there.)

Any "hard numbers" about water heating costs are based on a whole lot of assumptions, none of which will be true in any specific area, so they're probably not such hard numbers after all. In Ontario, energy is generally cheaper than in the U.S. That makes the tankless heaters even less advantageous there than here. Also, every household has a different pattern of hot water usage, and this will have a big effect on how much energy you'll use in a tank-type versus a tankless water heater.

For most people, probably the best way to save energy on water heating is to keep the existing water heater and reduce the temperature as much as possible. This minimizes standby heat loss from the water because the temperature difference between inside and outside the tank is less.

    Bookmark   July 27, 2004 at 11:40PM
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For any plus comment, there is a negetive comment. In europe, where the use is common, one issue that has been forgotten is the water savings. My Hotwater heater is in the garage. It takes about 3-5 minutes for the hot water to get hot in my bathroom during the 3 months of winter. during the majority of the year the water is hot within 1 minute. If the water is 9 gallons per minute, then 5 minutes is 45 gallons of wasted water. This would be for every shower. If I need hot water to wash my hands, I go to the kitchen. It is the closest distance to the heater.

When talking about savings, one needs to understand that electric saves energy over gas, because the gas has a natural heatsink thru the vent opening down the center of the heater. Electric does not have that vent. Electric is more expensive to operate, but does not need gas piped to the bathroom. Return water systems waste water when they are not used. The water circulates all of the time. This causes more energy to be spent to maintain overall tempreture. When reading savings, one needs to be careful. I have calculated the savings in adding an instant heater to the bathroom. For the 2 showers per day the system would be nice, but would never save money. I would expect to install the unit myself and save the installation costs, and I would also expect to buy the unit wholesale.

I see a better option in using a solar or geothermal pre heater for the water. This could be in conjunction with gas or electric. The conversion would probably cost about $600 for my needs. Until the last 2 years, my gas bill was $16 for 9 months a year with a jump to $100 for 3 months. The recent world events has jacked up the bill to $45 for 9 months with $300+ for 3 months. When I started bill averaging the cost is $65 per month. If the components lasted for 3 years I would break even. A circulating system would probably cost about $350, and the payback would be about 20 years. The quipment would not last this long, so I would never recover my investment.

    Bookmark   July 30, 2004 at 2:01AM
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Bob, I'd think another option you might want to consider is two smaller tank-type water heaters rather than one. Three to five MINUTES to get hot water is unreal. I've never seen anything like that; you must have a very, very large home. My water heater is pretty far away from one of the bathrooms, but the most I've ever seen is maybe 30 seconds to get hot water.

    Bookmark   July 30, 2004 at 1:57PM
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The one thing everyone's avoided mentioning thus far is that the energy cost for the standby "losses" is actually greater in the summer. My HWH is in the laundry room, which is inside the conditioned envelope. Living up north, this might not be so bad, but down here in FL, this means I'm paying to heat the water, paying to keep it hot, and paying to remove that heat with my AC. And there's some substantial loss going on. My laundry room ends up getting to 85F if I keep the AC vents closed in that room.

    Bookmark   August 10, 2004 at 2:13AM
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forget the tankless water heater; go with the traditional one and get a recirculating system (about $200-$300) from AutoCirc, Grundfos or Metlund. You will definitely save lots of water and money. Trust me, I have a recirculating system (metlund) and have saved tens of thousands of gallons in the 7 years I've owned it. 5-10 seconds and instant hot water! Get one!

    Bookmark   January 8, 2005 at 1:05AM
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anyone heard wheather the noritz is better then the bosch?

    Bookmark   March 13, 2005 at 6:23PM
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The Noritz has a larger capacity. I don't know if it is better than the Takagi Mobius which is its comparator. I had a TK series Takagi and loved it. I am going to install two Noritzs in my home.

My gas bill in a previous home went down from $100/month to $20/month without a tank. My current gas bill runs anywhere from $200- $250 and $100 of that is just my water heater. I know this because I had a problem with the heat for over 2 weeks and used electric heat.

Good luck!

    Bookmark   March 21, 2005 at 9:08PM
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I am considering using tankless to replace both domestic hw and hopeless steam boiler.

The major reason is that, in a NYC rowhouse, it will let me recover the boiler room that is required by code. It will also save the cost of relining a 50' chimney, which should have been done when we converted to gas.

The boiler room will eventually hold a wet bar, an extra fridge, extra freezer, 2 wine fridges and so forth.

Does anyone know if it is too cold in NYC to hang a tankless heater on the outside of the building?

    Bookmark   April 10, 2005 at 4:09AM
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Unfortunatly, there is no way you can replace your steam boiler with a tankless water heater. Water heaters make hot water, steam boilers make steam. If you could make it produce steam by some 'modifications' it would be subjected to temperatures and pressures it was never designed for. The likely result would be it blows up.

Of course, you can buy smaller domestic boilers which direct vent through the wall. And replacing the normal water heater with a tankless sounds like a good idea, although you can only have them outside in climates that don't freeze.

    Bookmark   April 11, 2005 at 9:10AM
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In our NY home we have had a Takagi for 4 years on Natural Gas. It is the best. We have saved about $35 per month on gas since installing it and about $5 per month on water. My water bill is like $30 per quarter. I highly recommend it!

We are moving to Florida and plan on installing an electric tankless system after we close.

    Bookmark   May 11, 2005 at 10:21PM
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Interesting thread. There is yet another option. The stainless steel tank which heats water to a much higher than usual temperature, then mixes it with cold water when there is demand. This gives a smaller tank the effective capacity of a tank about 50% larger. At the same time reduces the risk of growing germs and mould.

This is the link to a site that explains it in detail.


    Bookmark   July 14, 2005 at 11:43AM
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One of the concerns that has been bought up several times in this thread is that tankless heaters use a lot of energy. This is quite true, but also misleading. While regular tank heaters use a moderate amount of energy and require a fairly small electrical connection, the elements in them are running for a long time, many hours a day, and not just when the hot water is being used, they also switch on frequently to maintain the temperature of the stored water, which in itself consumes a fair bit of energy considering the thermal mass of all that water, and that it's constantly loosing energy through the tank walls.

Tankless heaters use lots of energy and have suitably big electrical connections, but only run for very short amounts of time. There are genuine energy savings to be made with them, it would be mistaken to assume they actually use more just because they have a large Kw rating and need a huge fuse. A little often does add up to much more than a lot infrequently.

Also, with a few exceptions, tankless heaters do heat the water more efficiently. The ones I've seen average about 80% or more, which does compare well against a lot of the tank heaters available.

    Bookmark   July 15, 2005 at 6:22PM
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I bought a tankless just because I wanted to never run out of hot water. It seemed that at my old house I couldn't even finish taking a shower before I ran out of water (it was a rental that I was sharing, so I couldn't upgrade anything then, and not everyone could take a shower in the morning). It is misleading to call them on-demand though, that to me implies a recirculating pump.

    Bookmark   July 21, 2005 at 2:49PM
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