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My house has a boiler in the attic that provides hot water for taps and for central heating. On the opposite side of the attic is a chimney pipe for the fireplace on the ground floor. I was thinking of running the boiler's intake along that chimney pipe, coiling around it for instance, so that when a fire is burning the heat that leaves the chimney would heat up the cold water that the boiler takes in, thereby reducing the amount of gas the boiler requires to heat the water. This is similar to the idea of a heat exchanger than runs waste water from the shower along the intake water line to re-use the heat that was flushed down the drain.

However, the fireplace isn't always burning, and the chimney is basically an open pipe to the cold outside. I'm wondering if I'm not actually going to cool the water that goes into the boiler most of the time, thereby defeating the purpose. Then again, the chimney is inside, so possibly it's always at more or less room temperature, which is by definition warmer than the water from the water mains.

Is there anyone with experience doing the same? Any calculations I could look at?

Edit: I'm reading up on this some more and realized that cooling the air in the chimney could have an adverse effect on the chimney's draft and even cause condensation within the chimney... not good. Still, I'm annoyed that so much heat is just flying out of the house rather than stored. There must be some way to utilize it.

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    This is probably very specific to the situation in your home. How about, as a start, rigging up something to measure the chimney temperature and the water temperature over a longer period? That would at least tell you what the temperature difference is. Plot that difference against time and plot your warm water requirements in there as well, and you will see if these overlap sufficiently to even begin thinking about implementing this. Sounds like a nice science project. You don't have school-going kids by any chance ;-)? – Jan Doggen May 10 '17 at 14:31
  • Alas, I don't. But I'm a school-going kid at heart. I like your idea, I'm going to see if I get two raspberry pi's or something to collect the temperatures in both the chimney and the water intake over a couple of months or a year or so and see what the data tells me. Thanks! – Bas May 11 '17 at 9:39
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    The temperature of the water intake will be quite related to the outside temperature. But so is the chimney temperature, especially when the fireplace is not in use. To me it doesn't sound like you would be cooling the water this way, ever... (The chimney being inside your home and not being perfectly insulated, there will be heat loss to the chimney if the fireplace is not in use. I don't think your chimney will be lower than the water temperature. In extreme conditions, you'll want to use the chimney anyway, right?) – Earthliŋ May 16 '17 at 16:37
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I'm glad you found that the extra information about the problems of cooling down your chimney pipe. That is the first point I was going to make in this answer (and I'll make it anyway). You do not want to cool your chimney exhaust gases. Chimneys rely on a temperature difference between the exhaust gasses and the surrounding environment to create the chimney effect which pulls those gases out of your living space. There is some amount of "waste" in the heat of these gases but some portion of the heat escaping is doing a job for you: it's keeping your fire from smoking back and choking you out of the house. If there is a lot of extra heat (that is, flue gases at a temperature in excess of that required to generate the necessary chimney effect) you could steal some of this for pre-heating but you will have no particularly reliable way to know how much you can steal nor to regulate how much you're stealing on a dynamic basis as the conditions in your fireplace change (eg when your fire starts, the flue gases are colder, there is less heat for you to steal before you cause a chimney stall and smoke yourself out).

Then there's the matter of creosote buildup as a result of condensing flue gases and the resulting risk of a chimney fire.

So: don't do this.

What you can do is explore alternate forms of wood heat. A fireplace is indeed a very low efficiency heating device. I recommend learning about rocket mass heaters and then trying to determine if one can be made to work in your living space. The three basic ideas of a rocket mass heater are:

  • Combust all of the fuel. A conventional fireplace or woodstove wastes a significant portion of the fuel, often intentionally. Combustables are left in the flue gases due to insufficient temperature or oxygen to burn them.
  • Use the increased chimney effect from higher combustion temperature to drive a more substantial chimney, allowing for greater heat exchange.
  • Incorporate a significant thermal mass to soak up the resulting increased heat production for slow radiation over a number of hours or days.

A rocket mass heater burns hot and fast and then radiates slowly to provide heat while it's not burning. By only ever burning at top efficiency, much less energy is wasted.

Erica & Ernie Wisner have substantial additional information, both on their website, other online forums, and in a number of published books.

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