Overall Efficiency - Small Splits vs. Large...

In the overall analysis, there are a number of variables that come into play, depending on one's scope of consideration, such as energy expended to obtain enough smaller splits for a worthwhile comparison.....etc, etc.


I'd say that the smaller splits from the same wood split at the same time as the larger splits would be drier and have more surface area, producing more heat and shorter burn times, while giving off more gas.


The smaller splits should edge out the larger ones regarding burn efficiency, but whether or not the gain is ultimately worth it is a matter of personal choice depending upon overall efficiency consideration.



TS

Don't know why there would be any penalty at all. It is good to mix.

Since you asked for any and all input I'll chime in with a best guess.....

1. The first consideration is releasing the energy from the wood. As long as there is no visible smoke you are extracting all/most of the energy from the wood and converting it to heat. So a "proper" burn is critical to efficiency regardless of the size of the split. It's easier to get a smokefree fire with smaller splits but provided you have a proper burn it is also accomplished with large splits.

2. Since either small or large splits can be burned to extract all of the energy, the efficiency is therefore resolved not by the amount of heat energy obtained from the fire but by whether it gets into the house or is lost up the flue.

So my intuition would be that a slow burn with large splits would be better because the slower airflow would allow the flames to have longer contact time with the stove, as opposed to being lost into the flue via convection.

I'd also guess that the efficiency difference would be minimal, so I wouldn't worry too much one way or the other.

Smaller splits will get up to secondary lite off temp quicker, but you will lose out on burn time and have to feed more often. Most stove manuals will state that small hotter fires are most efficient.

If you take the 'small split' idea all the way to its conclusion, you wind up with pellets burning in a pellet stove... and I believe they are considered generically more efficient than solid-wood burners.

I don't have a conclusion to offer from this, just noting it as a point to consider...


Eddy

From an efficiency standpoint, what is the prevailing thinking on how hot the flue needs to be in a high efficiency stove?

It seems like an awful amount of precious heat simply goes up the chimney. Even with complete combustion (no smoke), there's nothing saying the optimal amount of heat actually makes it out of a stove and into the living space.

I have a BIS Nova fireplace (Built In Stove). While I'm impressed with how cool the outside of the enclosure stays (literally cold on top), it still seems like the lower 6 feet of the 6" insulated chimney gets quite hot (almost too hot to touch with a hot fire). That seems to me to be a lot of BTU's going up the stack.

Seems like optimal flue temp would need to consider:

- Adequate draft to support the needed air draw and smoke evacuation (reminded each time I start it)

- Keep the smoke hot enough to not condense on the chimney sidewalls. In the case of an EPA stove, seems like this condensation issue is less important because most condens-ables have been burned in the secondary combustion. Is this a fair assumption?

Are there other considerations for flue temperature?

How might split size effect the heat loss up the stack? Seems like the larger, slower burning, lower temperature large splits might be more efficient from that standpoint. I suppose that also depends on the radiation and convection properties of the stove design.

I have a thermocouple meter...I might try inserting it into the stack at the top of the fireplace and see what that temp is. Seems too hot to me given the above considerations. What temp should I expect?

Good point about the pellets. I've used pellets as fire starters in my stove for years. It is the 'rocket stove mass heater' in miniature. It is fun to watch them take off.....they gassify so readily, the efficiency displayed is amazing.



TS