How best to heat water using excess heat from oven

[brucefehring]

Wondering if anyone has had experience in running piping through the insulating layer during construction of a new oven to heat water for a nearby hot tub after baking? Concerns are creation of steam (bad), best type of piping to use, size of piping, etc. I am considering a constantly rising coil of 1/2 inch flexible copper piping around and around the dome's insulating layer which will utilize normal house water pressure to keep water in constant supply and movement while oven is heating and cooling.

Your feedback, please...

[Ken524]

Your water heater will pull heat out of the oven (we spend a tremendous amount of time and effort to avoid this very thing). The insulating layer is there to keep the heat in the bricks. Putting a heat exchanger between the insulating layer and bricks defeats the purpose of the insulation.

Heating the oven AND water = more wood.

If you are using the heat exchanger after you are done cooking pizza, the oven will cool down rapidly making retained heat cooking impossible. I can cook for 3 days with one firing.

The other thing you'll have to figure out is what happens to the water trapped in the heat exchanger as the oven heats up. If the water isn't flowing, it will become superheated under extremely high pressure. The bricks in the dome hit over 1000F. Those temps combined with the pressure should provide a pretty exciting show. You'll need some sort of relief valve.

Sounds like an interesting idea, but I suspect it will take a LOT of careful engineering (and wood) to heat your hot tub.

[Nic The Landscaper]

I would consider using the exhaust (smoke) from the the oven to heat the water. Maybe you could bend the flue back over the top of the dome and create a series of chambers or heat exchangers that utilize only the heat that is already escaping from the oven?

[brucefehring]

OK, thanks, fellas. I am ALMOST convinced that a heat exchanger built into the chimney is the way to go. However, not quite...

We intend to have a thermal blanket and 4" of vermiculite insulation over the top of the dome. Because we intend to fill up the entire square space around and above the dome with insulation, there will be as much as 12" of insulation from the closest point of the dome to furthest reaches of insulated space.

How about a guesstimate, given a temperature of 800F degrees at the dome firebrick, of how hot the vermiculite insulation would be at 4" from the dome? at 6"? at 9"? at 12"?

Many thanks, Bruce

[Ken524]

Bruce,

That's VERY similar to how my dome is insulated. 2"-4" blanket with 4" of vermiculite/cement on top of that.

Before I built my enclosure, I fired the oven a few times. The vermiculite was ambient temp while cooking pizza. Several hours later it was warm, but not hot to the touch. The insulation likely works even better now that the oven has been fully cured and there is no more moisture in the bricks, blanket or vermiculite.

[david s]

I have been mulling over an idea of building a spa over a WFO with a 1" layer of vermicrete between the dome and the spa so the heat is released a little more gently, hence less problems with resulting expansion, cracking and excessive heat conduction. I thought building a thin ferrocement structure over the vermicreted dome and then following this form so the spa has a dome rising in the centre. Light a fire in the chamber and come back a couple of hours later to a nice warm tub.

[brucefehring]

David- Thanks for the input. I pondered a similar idea. The concerns that came to mind were 1) making sure that the weight of water plus body (-ies) plus tub were properly supported by the substructure and 2) that smoke might render the tub unusable at times when would was actually burning.

I'm hoping that others out there with any experience in trying to create a warm outdoor soak with some of the generated heat will share their experience...

Bruce

[brucefehring]

Hi Ken. Thanks for that feedback. When you say that the insulation now likely works better now that the whole thing has dried out, are you saying that you think even less heat is transferred to the outside of the insulative layer, and thus it would not even be warm to the touch?

Thanks, Bruce

[dmun]

are you saying that you think even less heat is transferred to the outside of the insulative layer, and thus it would not even be warm to the touch?

Bingo. A properly insulated oven will not be hot except near the vent and opening. Of course we all know that energy is neither created nor destroyed, and the heat has to go somewhere, but it is dispersed so slowly and diffusely that heating water with it seems to be a non-starter.

They make nice little immersion units for wood fired heating of hot tubs. Seems like a more practical idea to me.

[david s]

This is avery rough idea of what I had in mind.