under sink water heaters

Surely as a water heater, it won't connect to your hot water plumbing (apart from a tap)?

Assume it requires mains pressure cold, can you feed it that?

Not an answer to your question, and perhaps an obvious point, but fill it with water and connect up the power /before/ you plumb it in. When I moved in there was a 10l water heater (Ariston?) in the garage that tripped the RCD as soon as it was switched on. I assume there was an internal leak somewhere as there was no sign of water under it. I got rid of it and fitted a small instant-heat Triton for hand-washing. This sort of thing:

It is a very long run from the hot water tap upstairs so about 15 litres has to run through before it is hot so I was thinking of putting it between the hot water supply and the existing hot tap. As the hot water tank is small it would save reheating the whole tank after a bath has been drawn.

The manual says lowest pressure is 0.5bar which is about 15ft of water and we have that.

I see the problem now; mains pressure ones have a non return valve and pressure relief to a drain.The non return valve would make the heater sealed and unsafe if used in a vented system.

I think you might be reading a bit too much into that...

The heater will likely be designed so that it can operate as an unvented cylinder - i.e. designed to be able to withstand (close to) mains pressure and run directly from a mains cold feed without needing any of the additional clobber required for a vented system.

They are designed to give a quick supply of hot water in locations where piping hot to it is not possible, or the dead leg would be so long as to make hot delivery very slow and wasteful.

You probably *could* feed one from a cold supply from a cistern rather than the mains if you wanted (assuming adequate pressure etc), but would be adding complexity where none is needed.

Some of the small unvented cylinders specify what kind of check valves can be installed before them since they rely on being able to push water back into the main to provide expansion space rather than having their own expansion vessel (a slightly dodgy practice IMHO!)

Have a look at:

ajh snipped-for-privacy@loampitsfarm.co.uk> wrote in news: snipped-for-privacy@mid.individual.net:

Did you really mean that you intend to feed it from the _outlet_ of the hot water cylinder? That would mean that any use of hot water from the under sink heater would result in draw off of hot (wasted) and intake of cold into the cylinder, requiring re-heating.

If you mean using the cold tank feed then no problem or mains feed. I don't see any issue with there being an inlet non-return valve as the situation is no different from a mains operated one. Essential to follow their instructions, some allow exit pipework to accept expansion or they will require the addition of a small potable water expansion vessel.

Fine.

How does the temperature regulation work on these? If it's like an electric shower, the heating power is fixed and the flow is adjustable. But it's designed to raise water at say 15C to 40C, whereas if you put 60C water into this you might get 85C which would be dangerous. I presume there's an overtemp cutout but I wonder how it will work when supplied with hot water? Would it say lockout and refuse to heat until power is removed?

Theo

In message <0RECN.10280431$ snipped-for-privacy@fx16.ams, fred snipped-for-privacy@for.mail writes

I had a similar problem at the farmhouse. Long pipe run from a ventilated hot tank leading to excessive run off before hot water arrival and subsequent energy loss during re-heat. At the time, 1995, Screwfix were marketing an Italian under sink heater. Because the tank hot water was from the softened supply and the cold for drinking not, it seemed best to use hot for the local heater.

As AJH found, there are rules about lengths of feed pipe necessary for thermal expansion and an overpressure relief with tundish to waste.

Water regulations are fussy about possible backfeeds contaminating potable water and using unsoftened water would cause lime scaling here. This seemed the best solution and worked well for 25 years (new owners).

The small unvented cylinders work pretty much like a traditional hot water cylinder with an immersion heater. They pre-heat and store hot water. As it is used, that is replaced by cold, and the thermostatic heater will re-heat the cylinder.

(there are also "under sink" inline heaters that work more like a combi with no stored hot water, and have a very limited flow rate. The unvented cylinders offer full flow - but can only do so for a limited time before the water cools)

Tim Lamb snipped-for-privacy@marfordfarm.demon.co.uk> wrote in news:F$ snipped-for-privacy@marfordfarm.demon.co.uk:

A good pragmatic approach, the only neg being, as I said is, that you're effectively heating the small amount of water used at the sink twice. Once in the under sink heater and once by filling the pipe between the main HW cyl and the under sink unit with hot water, left to go cold unless you use more than the amount held in the pipework.

If that's not an issue then it makes decent compromise sense.

Thanks for that John, if the check valve is external then I have no worries as I can install the heater and miss out the check valve.

Yes there will be some waste in that the under sink heater will draw some hot water from the DHW tank but the 10 litres will be immediately hot and the kitchen sink is only used for washing up.

It will be heated by solar PV in the summer and that mitigates against using the vented ones as nearly all of those state that they must be permanently live and not operated by timer switches as this causes a reset..

The only other thing is that the online instruction for this one states it should not be used in a hard water area, which puts it out of court because our water is hard, which I don't understand as it comes from the Thames treatment plant.

It's what I have done for our kitchen sink water heater. It does waste a bit of hot water but nowhere near as much as gets wasted by running the water for ages to get hot water from the immersion heater which is a long (pipe) way away from the kitchen.

It's great having instant hot water at the kitchen sink.