Digging deeper into Draft

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stockdoct

New Member
Oct 19, 2008
194
ilinois
I've learned tons from this Board, and I sure appreciate the exchange of ideas.

I'm thinking about chimney "draft", and how it affects the efficiency of wood burning. As I understand it, draft is proportionate to the difference in temps between the outside air and the temperature of your stove ..... hence, a "down draft" may occur when your stove is cooling and the outside weather is warm.

I have a Lopi Freedom insert with a blower, but I'm not planning on running it 24/7 ; it will be a wonderful addition to my natural gas furnace, but I will still use the furnace for about 25-50% of my heat. My problem, is I don't have an unlimited supply of wood and it makes sense to me to burn what I have in the way that maximizes efficiency i.e. get as much total heat as possible out of my wood pile.

It makes sense to me that excessive draft is my enemy. The fireplace's blower has a limit to how much it can extract from the fireplace, and although I'm sure I'll get more BTU heat from burning my fireplace at 600 degrees than 300 degrees, I DOUBT I'll get twice as many BTUs at the higher burn temperature. The extra heat goes up the chimney in "draft". The force of the draft, being proportionate to the difference in temperatures will also be stronger if I burn at 600 than at 300 degrees, so the air moves faster and I lose even more BTUs of heat because of powerful draft.

I have to assume the stated "efficiency" of my stove (70.1%) is under optimal conditions, and the hotter the stove gets, the less optimal the conditions become due to the stronger draft and the limit of heat extraction by the blower. Does it make sense that to maximize the BTUs obtained from a pile of wood, I should burn the wood at lower temperatures, but not so low to build up creosote or cause a downdraft? By burning your woodpile at a cruising temp of 400 degrees, your wood pile wood pile will last longer and deliver more total heat than if I burn at 500?
 
There is a sweet spot between too hot that all your heat goes up the chimney and too cool that you don't get good combustion efficiency. There are too many varibles to determine this point without testing. My Napoleon 1400 says max efficiency occurs at stove top temps between 500F-600F, check your manual to see if it has a similar number. Either way it doesn't seem that 100F is going to make a major difference.
 
It's proportionate but not necessarily linear. And it's very much dynamic in the early stages of your fire. So if you're not running 24/7, you're going to be working hard on managing that warmup / transitional curve just getting to cruise consistently. That said, I doubt it should be anything at all as hard for you as it is for me with soapstone feeding a 35 ft tall 6" liner and soggy oak... I go very suddenly from "can't burn sh*t" to "why are the flames all licking up past the baffle plate?" in about 5 minutes. problem is that those 5 minutes don't happen til 75 minutes after I light the match...
 
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