Over air conversion


This masonry heater and cookstove were built in 1991. The masonry heater is set up to use under air, i.e. all the primary combustion air is drawn up through a grate in the fire box floor. The draft slides in the fire box door are designed to take in secondary air, i.e. air that will bypass the primary combustion and feed the secondary combustion of the smoke.

The masonry heater has a bypass damper from the upper chamber to the chimney, its control handle being just below the damper handle on the chimney.

There have been 3 owners of the house since the stove was built. I had not been back to look at the stove until the current owner contacted me this autumn saying he needed advice.

On visiting the house I was told that the masonry heater had been used as a slow combustion stove for several years before the house was bought by the current client. He was having short hot fires, but had no real idea how to use the stove.

About 15 years ago the guillotine damper had started to bind. After being left closed to rust for a season the guillotine plate would only pull 20%, where it had jammed. Consequently the stove had been fired, by necessity, for several years with its bypass damper completely open. After the fire the bypass would be closed, essentially being used as a shut off damper.

As the smoke stream was no longer being drawn down the side channels, and up the chimney via the rear manifold, it was entering the chimney directly from the upper chamber. The chimney flue, below the bypass, and the side channels, which had become inactive, had filled with compressed fly ash. The objective of the intervention was:
  1. To free the shut off damper, and make it operable.
  2. Removal the compressed fly ash from the manifolds and chimney flue.
  3. Conversion of the stoves primary air intake, from under air to Over air.
  4. Instruction for the client on how to use the masonry heater, during one fire lit from the top.

The over air conversion was straight forward due to the type of fire box door installed. This Upo door has a large draft slide in the bottom of each of the two doors. The position of the draft slides, and the amount of air they let enter, means that all the primary air can be taken through the door, and deflected on to the wood load. Other door models have small secondary air draft slides, or one slide on the top and one on the bottom of the door, making this set up impractical.


A strip of 22 gauge stainless steel plate is cut and positioned at the front of the fire box as shown.


The steel plate is designed to fit tight against the back of the doors when they are closed, and force the air coming through the draft slide, forewards onto the fire. Without the deflector, the air would rise directly into the fire tube, gripping the glass of the door, and bypassing the fire. This is the intention of the secondary air draft slides. I.e. to provide air that is not burned up in the primary combustion, to nourish the secondary combustion. The deflector rests loose upon the fire box floor and can be repositioned before each fire if necessary.


The deflector is made from a flat strip of steel plate (seen from the bottom) which has a short leg folded on each end to position it at the right height just above the draft slide in the door. The folded flap in the centre is to allow the plate to fit flush with the doors when they are closed.

Generally it seems that there is rarely a lack of secondary air. It can be assumed that enough unburned primary air is still available as secondary air. Or that secondary combustion is much more closely associated with primary combustion for there to actually be a need for secondary air as such. Therefore it is not detrimental to the operation of the masonry heater, to divert air from the draft slides in the door, to be used as primary air. And, in this case, additional air can always be allowed through the grate via the draft slide in the ash box door.

Note: The chimney is lined with refractory concrete 8 x 12 inch flue tiles, which are in excellent condition even where the bypass damper intersects them.

See High Performance Masonry Heating.

Marcus Flynn


Articles by Marcus Flynn

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