Standard instructions for wood inserts seem consistent in recommending higher fan settings with higher burn rates. This makes no sense to me, and I'd be interested in others' opinions or concrete knowledge from theory or practice. What follows is a semi-technical expression of what I've seen and inferred; happy to expand if useful.
Seems to me that the only reason to use a fan is to extract more heat from the insert. In my experience (with only a few different models), inserts deliver heat poorly with anything but a very hot fire: flames licking the glass, or a broad exposed area of glowing coals. This is consistent with the mid-infrared cutoff of relevant glasses - which only pass much energy in the visible and near-IR of a hot fire. They only radiate effectively if the window is itself quite hot, since emission goes like the fourth power of frequency. With my insert, the only time I can get much heat out without a fan is with a very fast fire; with anything less, most of the heat output is from blown air.
Another observation: the heat pathway between firebox and blower manifold makes it a high-impedance source. That is to say, increasing the load - by increasing fan speed - does not cause a temperature drop proportionate to flow rate. That, in turn, means that more heat is extracted with higher flow rate under any fire conditions.
I have also heard and read that the fan speed should be kept low until the firebox is hot, to avoid slowing the buildup of a hot fire. I'm dubious about this as well. In my current unit, there is no close connection between the active fire and the extraction manifold. Blower air is probably warmed only moderately by conduction through firebrick, and mainly (I'm guessing) by the relatively small area of bare iron above the firebox and separated from it by a solid insulating baffle. (I would have made this contact area much larger, but presumably the designers knew what they were doing.) So the only way extracting more heat would affect the fire is by lowering exhaust temperature slightly, which shouldn't matter with a proper draft. In any event, opening the secondary air gate allows a huge updraft that would overcome any such effect.
My insert, despite salesmen's promises, can only delver about three hours of strong heat on a full charge of dry wood. The way I usually use it during waking hours is with a steady feed of ca. 3-4 splits per hour. This maintains good flames and coals with the air gate completely closed. In this condition, heat output is directly related to fan speed, so I keep the fan near max to get the most heat with tolerable noise.
So - I don't get why the maker's advice is to match fan speed to air-gate setting. Can someone explain?
Thanks-
Theo
Seems to me that the only reason to use a fan is to extract more heat from the insert. In my experience (with only a few different models), inserts deliver heat poorly with anything but a very hot fire: flames licking the glass, or a broad exposed area of glowing coals. This is consistent with the mid-infrared cutoff of relevant glasses - which only pass much energy in the visible and near-IR of a hot fire. They only radiate effectively if the window is itself quite hot, since emission goes like the fourth power of frequency. With my insert, the only time I can get much heat out without a fan is with a very fast fire; with anything less, most of the heat output is from blown air.
Another observation: the heat pathway between firebox and blower manifold makes it a high-impedance source. That is to say, increasing the load - by increasing fan speed - does not cause a temperature drop proportionate to flow rate. That, in turn, means that more heat is extracted with higher flow rate under any fire conditions.
I have also heard and read that the fan speed should be kept low until the firebox is hot, to avoid slowing the buildup of a hot fire. I'm dubious about this as well. In my current unit, there is no close connection between the active fire and the extraction manifold. Blower air is probably warmed only moderately by conduction through firebrick, and mainly (I'm guessing) by the relatively small area of bare iron above the firebox and separated from it by a solid insulating baffle. (I would have made this contact area much larger, but presumably the designers knew what they were doing.) So the only way extracting more heat would affect the fire is by lowering exhaust temperature slightly, which shouldn't matter with a proper draft. In any event, opening the secondary air gate allows a huge updraft that would overcome any such effect.
My insert, despite salesmen's promises, can only delver about three hours of strong heat on a full charge of dry wood. The way I usually use it during waking hours is with a steady feed of ca. 3-4 splits per hour. This maintains good flames and coals with the air gate completely closed. In this condition, heat output is directly related to fan speed, so I keep the fan near max to get the most heat with tolerable noise.
So - I don't get why the maker's advice is to match fan speed to air-gate setting. Can someone explain?
Thanks-
Theo
