Click on picture for larger image
Click on picture for larger image
Seven and Eight Foot Rumfords
You'll notice that on a couple of plans we sloped the fireback. We hate to do that after arguing elsewhere that "real Rumfords have straight backs", but we thought we had a problem. Our segmented throat tiles are designed for a 36" Rumford. We can make do in the larger Rumford designs by using the Vestal damper to help bridge the too big throat. In our six foot Rumford we move the whole throat assembly back 4". But we thought the seven and eight foot Rumfords were just too big. We wished we had a larger throat tile with a bigger radius curve for these big Rumfords but we don't. So we swallowed hard and slope the fireback to help close down the throat opening far enough for a Vestal damper to fit.
More recently we made a couple of seven and eight foot Rumfords with straight backs anyway because it was easier to make the panelized fireboxes if the firebacks were straight. And the fireplaces worked fine.
So we have presented several variations of seven and eight foot wide Rumfords on this page. Click on the image to make it larger. If you slope the fireback don't slope it much. You can vary the width and height of the firebox so long as the flue size is at least ten percent of the opening area. And we like to stay close to the Rumford formula that the depth and width of the fireback are about equal and at least one third of the fireplace width. Also be sure that the net throat opening is about one fifteenth the area of the opening. We can design the throat to be as small as one twentieth of the fireplace opening size with good streamlining but the Vestal dampers are a little abrupt and restrictive.
You can leave out the Vestal damper and allow the throat opening to be quite large and the fireplace will draw fine - it just won't be as efficient. Use a chimney top damper or, outside where you don't care about wasting outside air up the chimney, use no damper at all.
Really Big Rumfords
Somewhat surprizingly, the segmented throat tiles designed originally for a three foot wide Rumford work well for much larger Rumfords. The resulting throat is too large, of course, and the stramlining is not as good as it could be when we push the throat system 8" or 12" back or slope the fireback to reduce the throat opening. But it works pretty well.
I suppose there is a limit, however. Or maybe you just want to build a big Rumford to be as effient as posible, although a big efficient Rumford is sort of an oximoron.
For this big ten foot wide Rumford the masons built a custom form and poured the arched rounded Rumford throat in place using refracotry concrete.
And here is an idea from our freind, Norber Senf ...
Something worth considering is an old 1930's technique called sand casting. There is a large bank in Ottawa done this way, and you have to look very closely to figure out that it is not natural stone. Very realistic building "stones" were cast from concrete by making molds out of damp sand: you take a pile of sand and carve a negative, or pount a positive into it. You then mix a very soupy concrete mix and pour it in. Stir a bit to get bubbles to come to the top. The sand sucks up the extra water, and you end up with strong concrete, with a nice sand finish.
We have done this with castable refractory in the past to cast large custom Rumford chimney breasts, where it did not pay to build a mold for a one-off size. If I were to try a pizza oven, I would pound up a damp sand dome, and trowel a very wet castable mix over it. Maybe even figure out how to incorporate some chicken wire, etc. Could be very quick, cheap, and strong......................N
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