Why are expansion vessels so expensive, more than the heater they're needed for?

[Chris Green]

That's a point, removes the back feed issues as well, the hot water isn't potable.

[Chris Green]

To expand on this, can anyone see any reason at all why I can't put an unvented water heater in the *hot* feed to the sink? It would solve virtually all problems in one go (at least the rules on how you plumb it problems):-

The hot feed is low pressure, we have a conventional old-fashioned header tank fed hot water cylinder

No back feed issues as it's non-potable water anyway

Minimum length of pipe to take-off is easy to fulfil (even though it's irrelevant)

It sounds a funny way to do things but it would actually work quite well, if we happened to want a *lot* of hot water then we'd eventually get hot feed from the cylinder. There will be a bit of heat loss from the pipes when we draw off small amounts but I don't see that as being a big issue. What the 'local' heater will do for us is give immediate hot water and stop the waste of water.

[Tim Lamb]

The other possible upside is that the hot water supply is likely to be from a water softener which may reduce scaling in the undersink heater.

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[dennis]

Unless there is a minimum flow rate specified I would think it would be OK.

You wouldn't want to leave the heater off for long though as you would want to kill any bacteria that may have got there from the header tank that wasn't killed in the hot water tank. 60C should kill most things so see what the stat can be set to on the heater. 60C is quite hot for a tap if there are kids or forgetful adults about.

[Chris Green]

I've not seen any minumum flow rates or pressures specified.

It'll be on all the time and we will have the water quite hot (most have a maximum of 70 or 75), it's never for handwashing or such, just kitchen type things.

[Fredxx]

That is what I have done. It works well and I have instant hot water at a temperature of my choice.

[Roger Hayter]

It will give you the convenience of immediate hot water, but I don't think it will reduce any waste. When you draw off hot water your new heater will fill up with cold water from the pipe, and the pipe will still be filled with hot water from the main tank, which will then cool down unused.

[Roger Mills]

It probably won't save any energy, but it will prevent you throwing

*water* away, waiting for the tap to run hot. Could be significant if the water is metered - and more environmentally friendly even if it's not.

[tabbypurr]

and will save money if the DHW if heated by gas or immersion on E7

NT

[Chris Green]

It will save a huge amount of water. We had to draw many litres of water before we got hot through, it was a good sink full if not more so maybe 20 or 30 litres.

So, if we use, say, 5 litres of hot water to do some washing up we will heat that 5 litres of water twice. It's still *way* less than the 20 or 30 we'd have needed previously and saves a lot of water too.

So, yes, it's not as good as it could be maybe but it's a whole lot better than it was and also much more convenient.

[Chris Green]

Exactly! ... and (as I explain in my response above) I think it will save quite a lot of water heating energy.

[Tim Lamb]

Also.. mains water may be 10 deg. C whereas your hot pipe is not going to cool below room temperature:-)

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