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Unvented unersink water heater fed by hot water?

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G
The Voice of Reason



Joined: 02 Feb 2002
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PostPosted: 10:03 - 14 Apr 2018    Post subject: Unvented unersink water heater fed by hot water? Reply with quote

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?
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Hahadumball
World Chat Champion



Joined: 07 Oct 2015
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PostPosted: 22:09 - 14 Apr 2018    Post subject: Reply with quote

*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.
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G
The Voice of Reason



Joined: 02 Feb 2002
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PostPosted: 12:30 - 15 Apr 2018    Post subject: Reply with quote

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.
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Ebbs73
Nova Slayer



Joined: 19 Jul 2015
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PostPosted: 17:39 - 15 Apr 2018    Post subject: Reply with quote

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
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G
The Voice of Reason



Joined: 02 Feb 2002
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PostPosted: 00:20 - 16 Apr 2018    Post subject: Reply with quote

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.
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