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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