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Sweep's Library: Stovepipe Heat Exchangers

Q: Hi, I bought a wood stove, chimney, and installation from you folks this year. So far I'm quite happy with the results. My new system works well - good draft, etc. The chimney install went smoothly. - Thanks! What are your thoughts on these heat extracting boxes with fan and tubes that attach to the stove pipe right above the stove? There's this guy on a wood heat forum on the WWW who raves about them. - Jim Pitman

Sweepy Hi Jim,

Thanks for the inquiry! Whenever someone raves about the increased heat output that can be accomplished using one of those stovepipe heat exchangers, what they're really saying is, "I have a poorly designed woodstove that loses too much heat up the chimney, and I need this device to get some of it back."

We call those devices stack robbers, and they date back to the pre-airtight days when Franklin stoves and cone-shaped fireplaces were the most prevalent hearth products in people's homes. Those early stoves and fireplaces weren't designed using today's sophisticated thermal engineering, and therefore lost most of the heat from the fire up the chimney, often resulting in flue temperatures of 800+ degrees. In an effort to reclaim some of this lost heat, a few companies ( Torrid-Aire and Magic Heat come to mind ) began to market stack robbers, which partially blocked the stovepipe, slowing the exhaust passage while extracting heat from the super-hot exhaust gases using heat exchange tubes with a blower.

Times have changed. Today's EPA approved airtights are designed to maximize heat output into the room and minimize heat loss up the chimney. In fact, some models have to be "de-tuned" a bit during the design-testing process to release more heat up the chimney and ensure that flue gas temperatures remain hot enough to maintain adequate updraft.

Aside from the fact that a strong updraft is needed to pull the wood exhaust through today's secondary burn chambers, the goal is to maintain flue gas temperatures of at least 325 degrees, as below this temperature liquid creosote begins to condense in the chimney. We've seen many cases where people have used stack robbers with airtight wood stoves and caused such heavy creosote condensation that frequent flue blockage and/or chimney fires resulted. I have personally witnessed a couple of extreme cases where liquid creosote was actually running out of the stack robber and splashing onto the hot stove! Since excessive creosote formation often leads to dangerous chimney fires, code authorities have outlawed the use of stack robbers with controlled combustion wood stoves.

Here's an easy way to demonstrate that a stack robber would not be a good idea for your high-efficiency airtight. Get a magnetic flue gas thermometer and stick it onto your stovepipe about 24" above the stove. Now, turn the draft control down to adjust your fire to the all-night burn setting, and monitor the flue gas temperature periodically throughout the burn, keeping in mind that you need at least 325  degrees for safe and efficient operation. I'm betting that you'll find you have little or no wasted heat for a stack robber to rob.

Next time someone on your wood heat forum raves about the increased heating efficiency created by their stack robber, do them a favor and suggest they could get a lot more heating efficiency without the heavy creosote formation if they'd upgrade to a better-designed wood stove.

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