Several years ago I followed the consensus opinion of North American stove builders and started to build stoves using over air as primary combustion air. This was obviously an improvement over under air, but did have one noticeable disadvantage. The embers would take much longer to burn when blasted from above by over air than when having under air forced through them via the grate.
(High Performance Heating)
The fall in a stoves efficiency as thermal charge leaves via the smoke path during the ember stage is considerable and well documented by Alex Batsulin. (Determination of Heat Loss)
The object towards the end of combustion should be to finish the ember stage as quickly as possible so that the smoke path can be closed, and loss of the stove's thermal charge, via an open smoke path, stopped. The ember pile may look as though it is hot, but the stove will be losing more heat than it is gaining.
It is for this reason that adding an extra log to the ember pile and opening up the smoke path at the end of combustion, will probably be detrimental to efficiency. Light and heat are radiated directly from the fire box, giving the psychological impression of warmth, but the mass of the stove probably looses more charge than it captures during the logs combustion.
Fanatics have been seen to shovel the embers from a stove and then run across the room, screaming “look out! hot embers!”, to throw them outside, before running back inside to quickly stamp on any pieces that fallen on the floor. This though, in most circumstances, is unacceptable.
In temperate zones where only small fires of 10 kg are needed, great attention is paid to the selection of wood and the stacking of the fire.
Hard wood is split relatively thin and burnt mixed with softwood, to avoid large lingering coals.
The damper can be closed in increments during the ember stages to allow just enough draw to evacuate the gasses and reduce loss of charge from the mass. Doing this though will considerably prolong the ember stage, especially in a stove using over air or having no grate.
In recent over air fired stoves an under air option has been added, where at the removal of a small cast iron door on the stoves facing, under air is allowed to flow beneath the grate. This means that the embers burn much more rapidly permitting the damper to be closed a lot sooner.
The cost of installing an under air option in a heater with basement ash drop, is negligible in both time and materials. The same installation in a stove with an ash removal box directly beneath the fire box is slightly more complex.
The disadvantages of the under air option are:
- That the area directly below the grate needs to be kept free from build up of ash, i.e. cleaned out regularly, for it to work.
- That the under air option may be used inappropriately by a free wheeling client.
- Also with this set up having to remove a door to let air into the under air channel, and place the door somewhere until it is replace 15 to 20 minutes later, is not ideal. A draught slide would be more convenient.
The below images illustrate three slightly different projects which employed the supplementary under air option.
Core Built on a slab with no basement below.
All images have an HD option. Click on image.
The layout of the first of the 4 courses of the cores base. The opening for the ash box must be precise to prevent over air running through it and becoming under air.
Four courses of clay brick layed in type N mortar. The opening for the ash box is just behind and to the bottom of the primary air channel and the under air intake at the right running from the corner of the core to the void of the ash drop. The opening of the ash drop has been reduced and is narrower than the width of the ash box. This helps prevent ash falling over the sides of the ash box and is closer to the dimensions of the grate, that will rest on the two courses of refractory brick above.
The opening for the ash box is bridged by a piece of quarter inch thick flat bar.
The first course of refractory brick bridges the under air intake channel.
The ash box must fit tightly into the opening. There must be a small amount of play to allow for the slight expansion of the ash tray. This, though, is not enough to allow a consequential infiltration of air.
The first courses of this ‘see through' core are in refractory brick. There is no basement in this example, the stove resting upon a slab.
The under air channel opening beneath the grate.
The opening for the under air channel should be as far away from the primary air channel as possible in order that the under air channel door is not located uncomfortably close to the much larger ash box door on the facing.
The final course of the hearth before the fire box walls.
This core is built on an elevated concrete block foundation which acts as an ash dump, allowing ashes to fall directly from the grate into the void within the foundation. There is no ash box. As with the two above examples primary over air is taken in through the draught slide in a cast iron ash box door attached to the facing. The small opening in the foundation slab, below the primary air channel, leads to the void of a block in the last course of the foundation. This allows the option of opening the block and attaching an outside air duct.
Note: The three elongated voids in the last course of the core, are of no relevance to the under air channel.
The function of the sloped walls of the fire box is to coral the embers onto the grate where they will burn faster together than they would dispersed across a flat hearth.
The faced core seen in the previous two images. The small half size flue access door to the top right of the ash box door is taken off to allow under air and replaced to close the under air channel. When the under air channel is open the primary air intake in the ash box door and secondary air intakes in the fire box door, should be closed. This will increase the velocity through the under air channel. The embers will burn much more quickly with under air coming through the grate than with over air.
It is advisable to insert a wire pan scrub into the under air channel just behind the door, to reduce the chance of embers being blown out by a back draught.