Request for info on demand water heaters

Just finished fixing a demand heater. First time ever working on one; I rebuilt the "water valve", the part that senses water flow and turns on the gas. It was pretty easy, even though the guy I did it for had contacted several plumbers (I'm not a plumber, I just play one on TV) who all told him, basically, "that's too complicated, it makes my head hurt, can't do it". I mean, this was really not rocket science: he had a rebuild kit, consisting of a diaphragm, a piston assembly and a pushrod assembly, with very clear instructions.
Anyhoo, I have another friend who is considering installing one of these in his house, and I'm doing some research for him to see if it's worthwhile or not. He has some misgivings about them, the first being that they're "complicated" (which is the plumbers' complaint, which turns out to be unfounded).
But another concern is: are they even worth the trouble? I remember back in the 80's, when they first came on the market here in the US, that they were touted as using far less gas than a regular (tank-type) heater. But now I'm skeptical as to whether this is really true. The argument in their favor goes that since you're not heating a big old tank (20, 30, 40 gallons) of water, that it takes less energy overall to heat water as it's being used. But after working on one and seeing it up close, I have to wonder if this is really true. When one of these things fires up, there's a fairly enormous flame started; they use a much greater volume of gas when they're on than a tank heater (which also means that venting them is more critical). And I was surprised at the relatively small size of the heat exchanger; there were only a few loops of tubing with fins attached.
So I'm wondering if anyone here knows how efficient demand heaters are compared to tank heaters. While I would like to hear peoples' experiences with them, I'm actually more interested in finding sites with hard data on their performance.
I also heard from someone else (a local home inspector) that there are only a couple of makes that are reliable, and that you should stay away from the other ones. Even this one (a French model), which seems well-made and reliable, requires servicing every 5 years or so at the least, more often if your water is hard.
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The problem with tank heaters is that they are ALWAYS hot and much of the fuel is used to replace heat loss from the tank to the room it is in rather than to heat water for domestic use. The theory of the demand heater is "no heat loss when no hot water is being drawn."
And another of my favorite rants, people who want to avoid waiting for the cold water in the line between the tank and the faucet to run out before the hot water comes out so they install a circulating system. They do avoid waiting, but at the cost of increasing the amount of heat loss while no hot water is being drawn.
David Nebenzahl wrote:

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John_B spake thus (reply moved to bottom):

Yes, I know that argument, but I'm not sure it holds water (to make a bad pun); after all, if the tank is well insulated, there should be little heat loss. Anybody have any DATA on this?
Besides, I can tell you that in this particular installation, there *is* heat loss, as the run between heater and the nearest appliance is uninsulated copper pipe. So at least some of that heat is going to warm up the living space.
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I'm not quite convinced of the validity of this. The thought of my water heater running all night to keep that tank of water hot, even when I am not awake to use it, is the whole reason why I was interested in on-demand water heaters.
When I began to investigate the situation, though, I realized that my tank water heater is not firing up as often as I imagined it was. Current tank-style water heaters are VERY well insulated and require very infrequent "warm ups". [This assumes that your water heater is not located in an unheated garage (in a cold climate) or something. Mine is in my heated basement.]
I compare this to the virtual 'gas inferno' that the on-demand units use to do their instant heating, and it becomes unclear about which one is actually more efficient.
Like the OP, I would like to see some hard data comparisons between the two types of water heaters, from a reasonably unbiased source, like Consumer Reports.
=========================================Tank Water Heaters Pro: Hot water is always available when needed Con: Heats water, even when it is not needed
On-Demand Water Heaters Pro: Does not heat water when it is not needed Con: May take some time to get the water hot
On-demand heaters use a big blast of flame to get the water hot, as opposed to the on-going smaller flame of the tank water heater.
WHICH ONE IS MORE EFFICIENT ON AN ONGOING-BASIS? =========================================
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really need a FAQ on this....
power failure probably no hot water...... electricity needed for operation
initial cost far more than tank type
because of higher BTU probably need upgraded gas line...... maybe even meter..
energy wise its just the standy losses, in the winter these losses help heat your home
because instaneous costs so much more than tank type the payoff period exceeds the expected life of the heater.
there are high efficency 90+ stanless steel tank type heaters. but no one guarantees them past 10 years,
on low flow like faucet partial;ly open probably no hot water since flow is below trip on level....
in a water failure the tank is a excellent source of emergency drinking water
nice idea, costs too much, saves little, service personell limited and costly.
tank type heaters are dependable realtively long lived devices.
heck a 20 year old new homeowner might buy one every 10 years. if he lives to 80 thats just 6 tanks.
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Paul spake thus:

That is the question.
By the way, let me amend your list of pros and cons a little bit. After this weekend's repair job, I can testify that demand heaters do indeed heat water *on demand* as their name implies; it takes very little time to get hot water out of the tap.
Another consideration because of the higher gas volume:
On-demand water heaters: Con: may require more extensive/expensive venting than conventional heaters
And another one: Con: Requires periodic maintenance (including possible parts replacement)
(as opposed to a tank heater which only needs to be drained periodically)
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I will add these to a FAQ.
want to add in areas where winter temperatures are low......
incoming water temp drops, which may mean cool showers in cold weather...
if someone wants a dont run out of hot water a tank type 50 or 75 gallon 75 Thousand BTU will probably do the job, its twice the heat of a regulat tank, I know i can shower forever and not run out, after hurting my back long showers helped so much.....
jogher BTU cost more but i wouldnt have it any other way:)
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Since the efficiency of a heat exchanger drops with the firing rate, a large well-insulated tank with a small fire could be more efficient. And a valve that closes the flue when there is no fire.
Nick
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They save a gas tankless cut my bill apx 50%, The proven overall rating is the Energy Factor, tankless ng go from 82-90 or so, gas tanks your normal type are apx rated 65% total efficency. This is not the burner rating whoich is 80-84% but a overall rating overlooked by most. The more in you family the less you save since its in constant use with tankless, but they work and only use the energy needed, tanks are always cooling especialy up the flue.
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On Tue, 3 Oct 2006 08:22:17 -0500, snipped-for-privacy@webtv.net (m Ransley) wrote:

It is just too dependent upon the way that you use the water heater. If you call for hot water at a flow rate higher than the tankless unit can handle, you get cooler water. That doesn't work for many people. The tank unit is only limited by the size of the tank. It can deliver high flow rates with no problem.
From a usability point of view, the tank-style seems to have advantages.
The key question about the gas usage during an average family's use of both styles of water heaters can't be answered without hard data from a reasonably unbiased source.
I have submitted a request to Consumer Reports. They came out with a report on dehumidifiers just 3 months after I requested that one. That's pretty good response time, or they coincidentally had that report on their honey-do list.
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