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.