Diplomat Chimney Fireplace Flue Heat Exchanger/Hot Air Exchanger, Exhaust Gas Cooler Black, XL Diameter 130 mm, 5 Pipes with Damper
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I do however have easy access to some raw materials, so what I was thinking of making a heat exchanger coil from copper pipe and putting it inside a metal box which flue gas would go through before venting to the atmosphere. Make sure the air pressure is relieved when rewelding so the pressure doesn't blow out the melted steel (another obvious idea that didn't occur to me at first). The purpose of a heat reclaimer is to capture the heat from the exhaust air escaping up your wood stove's flue and put it into the room. try to make it indestructible (HTP high heat coatings come to mind) and expensive, or just accept that it's eventually going to be a casualty so make it cheap and replaceable. Daily cleaning involves using the Soot Scraper knob to scrape the soot off the heat exchanger tubes several times during the day, In addition to scraping the tubes, burn paper or cardboard every other day to assisting with cleaning the tubes.
The 6" Magic Heat Reclaimer is a single piece unit that is quickly and easily installed in-line with new or existing single wall stove pipe of any air-tight stove, furnace, or freestanding fireplace.ETA: Regarding materials, yeah, I would probably use steel for the heat exchanger, and if it's going to be a a "firetube" design, I wouldn't expect it to last long. Never over-fire your appliance because it can damage your chimney and pose a fire risk, and don't operate the stove at too low a temperature as this can lead to excessive soot and creosote build up.
I have a nice CAD drawing of it but the CAD program keeps crashing today, so I'll post a drawing later. Many boiler stoves available are still only rated for use on open vented systems, however a lot of heating systems are sealed or pressurised so you cannot simply join the pipes up.In regards to the latter point - getting the exhaust ports through the heat exchanger down to a single pipe - the question is how to do that. If we went to a stove - wouldn't that alone be extracting a ton more heat from what goes up the flue than an "uncontrolled" fireplace? Racking out burning coals when the water circulation has stopped is not fun, I have done it, lucky the Aga was on a quarry tiles floor so any burning coals spilt would not cause a fire, but once you add a boiler, then water must flow.