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Author Topic: Using Aga/Rayburn "back boiler" in reverse?  (Read 5166 times)
r-morris
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« on: June 02, 2012, 02:21:24 PM »

Hi,

This is a question for Aga/Rayburn experts out there...

Supposing you already have a thermal store with ample heat from other sources (biomass/solar/etc). Instead of burning solid fuel (relatively inefficiently) or having separate gas burning going on in an Aga or Rayburn, would it be viable to use the "back boiler" in reverse, as a heat exchange from a circuit going through the thermal store? If the heat exchange occurs at a fast enough rate to heat the stove, I can see this having various advantages:

1. Access to heat from multiple sources, burning solid fuel in a boiler is presumably more efficient than in an Aga/Rayburn?
2. Stove could be heated on a timer-controlled circuit - less unnecessary heat (depending on building layout and use of course)
3. Stove doesn't need a flue
4. No need to bring dirty fuel storage and boiler loading/cleaning out activities into the kitchen

As an aside, how viable is it to retrofit one of these into an Aga:

http://www.ebay.co.uk/itm/Aga-135-Cast-Iron-Deluxe-Boiler-Vitreous-Lined-/360459498717?pt=UK_Home_Garden_Kitchen_Ovens_Hobs_Cookers&hash=item53ed0f74dd


Robert
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martin
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« Reply #1 on: June 02, 2012, 02:30:19 PM »

In a word "no" - until the appropriate valve was installed, our Rayburn back boiler had thermosyphoned heat  from our solar heated hot tank circulating through it, the result being that all the heat we'd gained from the sun shot up the chimney, with little warming effect on the Rayburn itself! Roll Eyes
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r-morris
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« Reply #2 on: June 02, 2012, 02:50:46 PM »

In a word "no" - until the appropriate valve was installed, our Rayburn back boiler had thermosyphoned heat  from our solar heated hot tank circulating through it, the result being that all the heat we'd gained from the sun shot up the chimney, with little warming effect on the Rayburn itself! Roll Eyes

OK, but what if the Rayburn wasn't connected to a chimney, and there was an appropriately controlled (ie pump + motorised valve + TDC) separate circuit from the thermal store?

Robert
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martin
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« Reply #3 on: June 02, 2012, 02:56:15 PM »

I think the losses would be colossal (it'd act like a thumping great radiator) - I'll stick with my original "no" Grin
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r-morris
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« Reply #4 on: June 02, 2012, 03:17:28 PM »

I think the losses would be colossal (it'd act like a thumping great radiator)

How does this differ from what an Aga or Rayburn is normally doing? ;-)

But in all seriousness, I don't understand (maybe because I don't know enough about how these things work) why heat from an alternative source within the stove would work differently from heat from the stove's own internal heat source (ie wood/coal fire or oil/gas burner). The question really was whether the "heat exchanger" within the Rayburn (or Aga with one of those units in my original post retro-fitted) would be efficient enough to keep up with the heat demand from the ovens/hotplate - and heatlosses into the room.

Robert
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martin
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« Reply #5 on: June 02, 2012, 03:54:08 PM »

" would be efficient enough to keep up with the heat demand from the ovens/hotplate - and heatlosses into the room" - no! ralph
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r-morris
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« Reply #6 on: June 02, 2012, 03:56:42 PM »

Actually after writing my previous post I realised I'd overlooked a fundamental flaw with the idea: the hot water in the store will only be 85-90C maximum, and higher temperatures will be required for cooking; so even if the heat exchanger was good enough, once the temperature inside the cooker exceeds the hot water temp, the heat exchange would then work the other way! So water is a fundamentally unsuitable thermal storage medium for cooking which requires 200C or so.

Robert
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martin
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« Reply #7 on: June 02, 2012, 04:01:41 PM »

 extrahappy penny dropped! genuflect
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renewablejohn
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« Reply #8 on: June 02, 2012, 04:50:40 PM »

Nothing at all wrong with your theory however you will need to use thermal oil as your heat transfer media which will do your cooking upto 230C but you will need a steam evaporator as your heat exchanger between hot thermal oil and hot water otherwise you could end up with flash steam and a powerful bomb.
Have a look at commercial thermal oil ovens used in high quality patisseries to give you some ideas.
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Justme
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« Reply #9 on: June 02, 2012, 05:35:37 PM »

We use ours as a manual heat dump for when we go away. We have the standard controlled dump to the rads but if we are going away we leave the Raybun's single valve (to stop thermosyphon) turned on so that the excess heat can escape via the Rayburn.

With this in operation the TS can not get above 50c no matter how sunny it is or how little water is used. That way even if the power goes off or the controller or other parts of the dump system fail we are still protected.
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