The easy/conservative theoretical answer is “yes” the bench flue runs should be gasketed and the bench facing should support the benchtop slabs. But masonry heater builders in the real world know that there is such a high level of detail in masonry heater building that it is interesting to consider where it is possible to avoid extra work (and therefore extra cost for one’s self or a client).
There is no question that the hottest portions of the core need to be gasketed. The firebox and firebox’s throat need to be independent of the facing because they reach temperatures where the masonry heats up to expand significantly enough to cause issues in functional/aesthetic facings.
(As an aside, some very interesting single skin heaters such as the Gymse address this by floating the refractory firebox core inside of the single skin facing.)
The question here is whether flue gas temperatures have lowered significantly enough by the time they reach bench runs (after primary updraft and downdraft runs) to make gasketing the bench unnecessary and save time, money and expense.
I pose these questions to the community to see what opinions and personal experience is out there.
I happen to be the customer in question above. Thank you @Max for getting this conversation started.
This morning I came across a FAQ by Norbert Senf, a deeply experienced mason who has done a lot to advance the cause of MHs in North America.
Q:Is refractory mortar used between liners? What else do I need to know?
A: For heated benches, I use ordinary mortar between the liners. This is because it is hard to cut liners closer than about 1/4", sometimes 1/2". They often tend to be warped and out of square a bit, making them hard to mark very accurately. There is not much heat stress at the tail end of a contraflow heater, so using regular mortar is no problem. I also use ordinary mortar around any liner connections, and slush in mortar in those areas as well.
The mason then simply builds the bench as if it were a horizontal chimney, except for the benchtop. I tell them to slush all the connections solid with mortar. The benchtop is usually 2" stone, and it is mortared directly onto the flue liner. For long pieces of stone, you could create a slipjoint by laying a piece of fiberglass mat on the liner first.
The benchtop can get fairly hot and the joints between the benchtop stones have a tendency to hairline. Here is what I do: use a 1/4" joint, leave it open. Pump the joint full of type 1 silicone. Immediately, dust dry “mortar mix” powder onto the silicone and press it in slightly. This gives the appearance of a cement joint, but it is permanently flexible.
My heater only has one down draft before it arrives into the bench, so I’m not sure these sentiments would apply to my case - I’m planning to use refractory mortar and allow for a slip-joint as opposed to slushing it in solid, but it’s a nice point of reference for most folks with a more conventional heater design.
Dwyer, thank you so much for adding this link/reference. Indeed, @mheat (Norbert) has contributed a remarkable amount to masonry heater development in North America.
Norbert is right that it is hard to cut wonky flue liners precisely enough to meet the tight joint tolerances that refractory mortar was designed for.
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