Most states’ building codes (maybe all states?) for the last several years have mandated thermostatic control devices or whatever they’re called to prevent just such things occurring.
Huh? How does it “go” from hot to cold? Do you lose power/gas supply in the building or something? I mean, there’s no holding tank to empty out or anything. It should be running at the same rate no matter what, so long as it is on.
The unit I’m familiar with (actually, both of them - we replaced the first one which had died of old age) had a delay of a a few seconds between detecting pressure and the heat exchange firing up. Presumably, this was to allow any trapped air through first in a situation like that.
But that too had its downside. Say you wanted a sinkful of scalding hot water to soak pans or something - you needed to run the tap first until the water came up to temperature, put the plug in, and then leave the tap running. If you turned the tap off partway through and then back on again, you’d have cold water in the line for that delay period. This might sound trivial, but it’s one thing I used to bear in mind, because it did make a difference.
As for noise, neither of those devices - both gas-fuelled - made any noticeable noise other than a “FLUMP” sound when the heat exchange fired up. And this was actually a benefit for me, having two young children. How? I could magically tell if they’d used hot water to wash their hands after using the lavatory upstairs. They really did think I had Supernatural Powers.
Are you also heating with propane? If that’s the case, kill two birds with one stone and get a ground source heat pump (geothermal, geoexchange, other buzzwords). They’re pricey, but since propane is even pricier than natural gas, your payback will be that much quicker. Make sure you get a desuperheater for your hot water, and you get free hot water most of the time the system is actively heating or cooling.
I don’t know anything about how it’s hooked up or how it’s *supposed *to run, I just know that 7 times out of 10 when someone wants to shower, it takes about 10-15 minutes to warm up (we’ve learned to turn the shower on and then brush teeth, use the toilet, whatever, to kill some time) and then when you’re in the shower, 3 times out of 10, you’ll lose all hot water abruptly during the shower. when that happens, none of the other taps have hot water either for another 15 minutes, or sometimes more than an hour. It’s incredibly inconsistent.
I have no idea what’s going on in the other 5 units in the building when that happens, of course.
I installed a LP Aquastar when I built my house. I love it. It cost around $500, and it was simple to install. Just hang it off the wall, attach plumbing + gas line + turn it on. No electricity needed. No hot water tank to heat the house in the summer. Endless instant hot water. (Note: I wouldn’t reccomend installing it yourself unless you knopw what you’re doing.)
The only downside I’ve found is that it’s easiest to control with separate hot + cold water knobs, as a single knob controlling both changes the flow rate of the hot water, changing it’s temperature. So if you adjust it up slightly, the hot water flow increases and the heater increases temperature to compensate. This results in an additional increase in temperature when the water that was in the lines between the heater and the shower runs out + the hotter water gets to the shower. You also cannot get a trickle of really hot water without turning on another faucet, but I’ve never needed it.
If it takes a long time to get hot water, your water heater is either defective or simply a long way from the faucet, and you’re running the room temperature water out of the lines. If your water isn’t getting hot enough, either the heater is too small for the volume of water you’re heating, or it’s set too low. I have no idea how you could ever run out of hot water with an on-demand water heater unless you run out of gas or the power goes out.
I have no experience with the electric ones, but I would think they would be expensive to operate. Still probably cheaper than a tank.
Given the disadvantages you mention, I wonder if the cost difference becomes negligible (or at least insignificant). The only ineffeciency in the storage tank design is the heat that’s lost as a result of the water cooling when you’re not using it. Modern units have so much insulation that it’s possible that if you have a lot of hot water demand, your biggest energy use is for replacement water rather than due to water cooling. In my geo system I opted to keep my existing gas heater as the backup supply. Because its insulation is nowhere near as good as a modern, electric unit, it’s actually cheaper for me (now, when the geo heats the water) to make sure I use the hot water daily, rather than to give it a chance to cool off and let the gas burner kick on.
Five units being served by one on demand heater? That might be the problem right there. I think the large majority of dissatisfied customers are the unfortunate victims of a poorly designed system. On-demand water heaters should, in my opinion, be sized only for one or two loads and placed as close as possible to the taps. If it takes 30 seconds to wash your hand but you have to wait 2 minutes for the hot water to reach the sink, that’s two and a half minutes of heating time of which you’re only using a fifth (if that - most people would probably give up waiting and wash under the cold water and shut the tap off just as the hot water arrives).
This is also the reason people get frustrated trying to adjust the temperature. It can take a little while for the temp change to get from the heater to the tap. Using a mixing valve rather than seperate hot and cold valves helps remedy this partially.
Also, since these heaters should feed only low-flow taps, it can take a little while to get the hot water from the heater to the tap. When I did my shower renovation I made sure the heater was as close as possible to the shower mixing valve. I’d estimate there is not more than six feet of pipe between the heater output and the shower head, and even at that close range it can take 15-20 seconds for the hot water to make its way to the shower head. The output of the water heater is hot instantly (I can feel the pipe get hot the instant the shower is turned on). It just takes time for the heated water to get to the shower head.
I’d also recommend using ½" cpvc to keep the volume of water in the pipe low, and to reduce heat loss that you’d get from a copper feed.