Observations 18 Years Later #1

These images detail the condition of the fire box of a contra-flow heater after 18 years of continuous seasonal use.

The refractory brick used were generic low duty, layed in Duchaine HTC.
The heater is fed under air.

All images have an HD option. Click on image.

The heater has acted as the sole heat source for the home since 1992. Seen here in 2010

The inside of the fire box showing destruction of the surface of the brick on the rear and to a lesser extent on the side walls.

The same image taken with flash for comparison.

View towards the throat. Note that the destruction of the surfaces is confined to the areas in proximity to the wood load.

Detail of the sloped surface of the fire box ceiling in perfect condition.

View of the unshielded quarter inch thick 4 x4 inch angle bar lintel that bridges the fire box opening. The lintel has kept its form and not suffered any destruction despite the high temperatures it has received to cause it to whiten off. Also exposed are the bottom surfaces of the voussoirs.

The fire box floor is intact. The skew cut pieces that form the angled course each side, broke their bond during the first season, though have remained in place. The points broke off the corners of these pieces within the first two seasons. The grate is warped at the back as the under air tends to run through the front, pushing the ash build up, beneath the grate, towards the back. This ash below the back of the grate has repeatedly insulated the cast iron bars and caused them to overheat and warp.

The upper chamber, looking up towards the three capping slabs. The brick wall at left and the slabs are in perfect condition. The imbalance in the way the capping slabs have whitened off is due to the throat, (which runs top to bottom of this image), being of centered i.e. closer to the front wall at left. This is because the first two courses of brick over the angle iron lintel, of the fire box opening, were layed plum. This meant that the corbel started two courses higher on the front wall than on the rear, i.e. causing the throat to be off centered


Destruction being limited to the areas of the firebox in close proximity to the wood load , and the clinkering of the surface in these ares, suggests that the cause is chemical attack.

The excellent condition of the angle bar lintel even in this relatively short, under air, fire box, shows that this way of bridging the opening is viable over time, and that the lintel wont burn out.

The grate can be replaced, and new skew cut brick cut for the sloped floor of the fire box.

The rear wall would be more difficult to repair. It could be patched with refractory mortar. A process that would have probably to be repeated each season and provide only a limited solution. The brick could be removed and replaced with new brick though this would be a major task, and run the risk of disturbing previously sound areas of the masonry above. This type of intervention is possible though and has been documented in the article by Norbert Senf http://heatkit.com/research/callback-ruppell.htm

The fire box will be left as it is and the destruction monitored each season. It has been several years since the heater was last inspected. The rear wall of the fire box is not much more damaged than it was then, indicating that the stove could remain in this condition for some time, before becoming dangerous or structurally unsound, and the fire box having to be rebuilt.

Note: Today most professionals use a fire box wall construction technique comprising of two wythes of refractory brick layed as shiners, allowing the interior wythe to be easily replaced if it becomes damaged in time.

The heater shown here was built before this technique was widely adopted.

Marcus Flynn


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