Small electric hot water systems...

Greetings All...

Some time back I started a thread that sought the names of manufacturers of Instantaneous Electric Hot Water Systems. It was apparent by the end of the thread that the single-phase versions were just not going to have the grunt needed to heat up sufficient quantities of cold water for a decent shower. It now looks like a central solar-heated tank or two might turn up at the block of units in question, but I am wanting to avoid the water wastage that will result from clearing the long pipe run between the central solar-heated tank and the Flat every time I want some hot water. I am therefore wanting a small water heater of some sort that will provide a "buffer", so to speak, until hot water from the solar tank can make it up to the flat.

At the moment, hot water is provided in the flat by a mains-pressure 50L underbench storage system in the Kitchen. The Plumbers and plumbing-savvy amongst you will realise that these things are generally too low in power to instantaneously heat up water to any great degree, but the fact that they aren’t hooked up to a Zellweger - and can thus start reheating whenever necessary rather than only during off-peak - means that the 50L "buffer" of already-hot water inside the unit, combined with the low but nonetheless immediate reheating, is usually enough to keep two or three people out of trouble.

I’ve seen some even smaller storage units from Dux, Rheem, and Edson, that all work along the same principle, and would therefore be even more ideal for hooking onto the other end of a pipe from a central solar-heated tank, if the purpose was simply to provide a small reserve of already-hot water until the pipe coming up from the solar tank could heat up. The only problem is that they all need a Safety Tray, and they all require at least two Relief Valves and an overflow. It doesn’t take much in the way of leaky Relief Valves to end up with damp and smelly underbench conditions...

I noticed, however, when perusing the PDF brochure for the Stiebel Eltron SN "Sink Heater", that "Open Vented" heaters do not require Relief Valves, and that heaters in general up to 10L in size do not require Safety Trays. Perfect, I thought! Being single-phase, the circuit for the existing 50L storage system would do the job, and whilst low-powered, the "buffer" from the 10L storage should "hold the fort" until solar-heated water could arrive en-masse from the remote solar storage tank. And with no Relief Valves, underbench dampness shouldn’t be a problem either. Heck, I’d mount it on the wall above the sink anyway...

I then rang Stiebel Eltron and told the chap there that I wanted to put one of their SN’s in line as the first port of call for hot water coming into the Flat, and therefore having to supply all hot water outlets in the Flat, but with backup being not far behind from the proposed remote solar storage tank. He replied that it would be suitable for use with the sink, but not for the shower, and added that this had something to do with being "Open Vented" in design. The way I see it, cold water goes into the thing, and pushes hot water out. I wouldn't have thought that the unit could be unpressurised, because you can get underbench versions from which the hot water has to flow upwards to the taps (which, however, are of an apparently special design with three lines attached. Maybe there's some siphoning going on, and the thing is indeed unpressurised)...

My questions are thus as follows:

1) Can anyone confirm that you couldn’t have one of these Open-Vented Sink-Heater style units inline as the first port of call for hot water coming into the flat, and with the task of simply providing some instant hot water buffer until the "Cavalry" (being hot water from the solar tank) could arrive? (ie. Why wouldn’t the thing be able to send some hot water down the line to the shower? Is it indeed unpressurised?...)

2) Does anyone know of a small (perhaps wall-mountable) mains pressure electric storage heater that doesn’t need a Safety Tray or Relief Valves due to it’s small size?

Sorry for the long-windedness, but it helps to get you inside my mind...
Many Thanks,
Batpig.