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Forum Index : Other Stuff : Feeding gas HWS with elec HWS

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renewableMark

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Joined: 09/12/2017
Location: Australia
Posts: 1678
Posted: 10:43pm 27 Apr 2019
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Hi all,
my system often goes to float early in the day and the 10kw of panels go to waste.
The Mad GTI controller has a function to % load a HWS (elec) with any spare power it is producing.
So my plan was to feed my current gas HWS with an additional electric system which will be powered by my extra elec HWS yet to buy.
Here are the pics of my gas unit.




So the pic at the bottom of the tank has a pressure restrictor and a one way valve.
Can I just feed that pressure restrictor with already hot water from the elec system?
I assume it will need to be set slightly cooler on the elec so it doesn't mess with the heat restrictor on the gas unit.

That seems too simple, so I'm guessing it will be more complicated than that.
Cheers Caveman Mark
Off grid eastern Melb
 
Solar Mike
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Joined: 08/02/2015
Location: New Zealand
Posts: 1138
Posted: 12:26am 28 Apr 2019
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Hey Mark, what you are talking about here is a preheat tank powered from excess output from your PV panels; this then feeds preheated water into your main HWC, gas in your case.

The Mains water pressure reducer and 1-way valve would be moved to the incoming cold water supply to your new tank, the heated outlet of this tank then feeds the cold water inlet of your existing gas HWC. The existing anti-scald valve (blue adjuster) would have its cold inlet moved to the inlet of your new tank, so it gets cold water not warmed water.

You would have to make sure both tanks can operate at what ever pressure the pressure reducer is set to.

As your gas HWC doesnt seem to have any sophisticated controls, it should work fine feeding it with preheated water at some random temperature, but you should read its manual and make sure it can accept heated water, some have a max limit as they have plastic components (these would mainly be the instant gas type).

Here in NZ, it is not allowed to use an uncontrolled heat source to heat a HWC tank unless it is open vented, because if the tank boiled there would be an explosion. So providing your new tank works off AC power and fitted with an approved thermostat and safety cutout switch, shouldn't be a problem. If you were going to power the element with DC then that will be a problem as the existing thermostat etc will arc and self destruct first time it opens.

An open vented preheat tank with an internal heat exchange coil running at the mains pressure is another option as a thermostat etc isn't required.

You might want to talk to a friendly plumber first...

Cheers
Mike
 
Warpspeed
Guru

Joined: 09/08/2007
Location: Australia
Posts: 4406
Posted: 12:57am 28 Apr 2019
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I did something very similar a very long time ago with a pumped solar thermal system.
Solar heat was stored in a large pre-heater tank which then fed the main gas storage water heater. Even in mid winter the system warmed the water going into the main hot water heater, but not by very much. In summer it was difficult to prevent the system from boiling.

Anyhow its definitely a practical workable idea.

That top blue knob thing is a thermostatic mixing valve. The top entry is the hot water, and the bottom entry MUST be fed with stone cold water. As Mike says it is to prevent scalding, and is a statuary requirement for situations where hot water can contact skin directly.

What you need to do is fit your pre heater tank into where the cold water enters the gas storage tank at the bottom, leaving the vertical cold water feed to the thermostatic mixing valve as it now is.
Cheers,  Tony.
 
renewableMark

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Joined: 09/12/2017
Location: Australia
Posts: 1678
Posted: 02:23am 28 Apr 2019
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Thanks guys, wow, so it is simple after all!

The excess output from the GTi controller is designed to run a ssr to activate the existing 240v wiring of a std elec HWS.

Great, I'll put this project on the list.
Since all that power goes to waste I may as well use it somewhere.
Cheers Caveman Mark
Off grid eastern Melb
 
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