Unvented unersink water heater fed by hot water?

[G]

Unvented water heaters typically say 'cold feed only' - but is there any issues with feeding them with hot water?

I want to use a small one as a bit of a 'buffer' so I can have instant hot water while the boiler fires up.
They use the preceding pipe work as their expansion vessel for small ones - being fed by hot water I believe should make this less not more of an issue?

[Hahadumball]

*puts on plumbers hat*

absolutely fine to do, most cold taps have better pressure + it hardly seems worth putting hot water into a heater to heat up, you need to make sure your hot feed is at least around 1.5 bar of pressure on top of that...

downside.. you'll be creating more insoluble carbonates from the hot feed.. thus limescale build up in the pipe + filter.. more maintenance.
____________________
Fin: no matter how much I look at It I can't understand what was going through my head, all I remember is going about 80 and redlining it to stop it seizing.
360 Deg... Five 1/4 turns. :- Teflon-Mike 18 Jan 2015

[G]

Cheers.

The electric heater won't hold enough hot water for a decent sink's load and won't heat up quickly enough generally - also I'd prefer to use gas where possible - so the electric is just there as a buffer zone while the gas fires up.
Hoping the initial shot of cold water while the gas is firing up will mix well enough to give warm enough water for the initial bit.

Yes, presumed a bit worse for limescale - but shouldn't be a big issue.

[Ebbs73]

I take it your putting this on a combi ?


If so have a look at this

https://combisave.com

I have one and it works really well , my combi's in the loft and takes an age for the hot water , it's a 38 kw model too .
I measured it before and after and I save 5 ltrs per washing up bowl .

I think I paid £45 .

It slows the hot water flow from the outlet of the boiler , the boiler heats up quicker , once it hits the temp you set it opens full bore .

May help

Chris

[G]

Cheers - in this case just an unvented gas boiler - ie combi without the heating bit, so effectively the same.

Even going for the under-sink boiler, worth considering to minimise the period the tank is mixing cold into hot.

I'd also wondered about having a temp sensor that would activate a bypass when the incoming water hits temp - but that'd lose some of the value in trying to use gas to heat the water.