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Posted by John_B on October 2, 2006, 11:11 am
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:
> 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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