Rumford fireplaces are tall and shallow to reflect more heat, and they have streamlined throats to eliminate turbulence and carry away the smoke with little loss of heated room air
Back in England, Rumford applied his knowledge of heat to the improvement of fireplaces. He made them smaller and shallower with widely angled covings so they would radiate better. And he streamlined the throat, or in his words “rounded off the breast” so as to “remove those local hindrances which forcibly prevent the smoke from following its natural tendency to go up the chimney…”
The objective is to reflect or re-radiate as much heat out of the face of the fireplace as possible. It doesn’t hurt to whitewash the firebox or to insulate behind the firebrick. It does hurt to cover the opening with glass doors, which cut about 80% of the infrared radiation. Tall, wide shallow fireplaces [the Rumford design] tend to smoke. The other half of Rumford’s genius is his intuitive understanding of fluid dynamics – way ahead of his time. By rounding the breast to “remove those local hindrances which forcibly prevent the smoke from following its natural tendency to go up the chimney…” he essentially created a venturi, a nozzle, like an inverted carburetor, that shot the smoke and air up through the throat into the receiving smoke chamber.
Because the true Rumford design is based on the science of airflow, it does not smoke. If you have a Rumford-like fireplace that does tend to smoke, something is wrong: either it’s a pseudo-Rumford with a bad throat design, or the throat’s obstructed or has been damaged. True Rumford fireplaces simply do not smoke—that’s why they were so popular.
The fire in a Rumford fireplace is laid with a large horizontal log at the back and smaller logs placed vertically, leaning over it. View a Rumford Fireplace (PDF)»
The main thing that distinguishes a masonry heater is the ability to store a large amount of heat. This means that you can rapidly burn a large charge of wood without overheating your house. The heat is stored in the masonry thermal mass, and then slowly radiates into your house for the next 18 to 24 hours.
If you burn wood fairly rapidly, it is a clean fuel. If you try to burn it too slowly, the fire will change from flaming to smoldering combustion. The burning process is incomplete and produces tars. Atmospheric pollution increases dramatically. This is important if you are planning an energy-efficient house. The average energy demand of your house will be quite low. For most of the time, it may require only 1 to 2 kW of heat. For most conventional woodstoves, this is below their “critical burn rate”, or the point where they start to smolder.
Masonry heaters fill the bill perfectly. If you need even a very small amount of heat, such as between seasons when you simply want to take off the chill, you simply burn a smaller fuel charge–yet you still burn it quickly. The large surface is never too hot to touch. You have a premium radiant heating system with a comfort level that simply cannot be equaled by convection or forced air systems.
- A mass of at least 800 kg. (1760 lbs.).
- Tight fitting doors that are closed during the burn cycle.
- An overall average wall thickness not exceeding 250 mm (10 in.).
- Under normal operating conditions, the external surface of the masonry heater, except immediately surrounding the fuel loading door (s), does not exceed 110 C. (230 F.).
- The gas path through the internal heat exchange channels downstream of the firebox includes at least one 180 degree change in flow direction, usually downward, before entering the chimney.
- The length of the shortest single path from the firebox exit to the chimney entrance is at least twice the largest firebox dimension.
Soapstone is quarried like granite and marble. It is a steatite stone and its primary components are magnesite, dolomite, chlorite, and talc. It can range in age from 300 to 400 million years old depending on which part of the planet it is drawn from. As talc in soapstone is soft to the touch, it gives the smooth feeling of rubbing a piece of dry soap. Thus the name was derived – “Soap” stone. No. You can’t wash with it.
For thousands of years, soapstone has been used throughout the world for tools, carafes, vases, goblets, sculptures, fireplaces, etc. In early American history, soapstone was used primarily for building blocks, sculpting and urns. As villages and towns began building home structures, a popular choice for the do-it-all sink was soapstone. It could be easily cut to shape with non stone cutting tools. Four styles of common sinks from the 1800’s and early 1900’s were the Philadelphia, Chicago-Wright, Boston-Williams and the good old New England Double Bowl (our most popular). In early New England, Soapstone uses ranged from fireplace hearths to countertops, sinks, and oven fireplace stoves. In different parts of the world, soapstone is still used as a daily staple for mixing bowls, cook-tops, cookware, and oven baking decks. Currently in the USA and in different parts of the world, soapstone is used for the largest variety of items ever yet – including balusters, stair treads, window sills and island tops. It’s fast becoming a very popular choice by designers and architects and it’s one of a kind texture and look make soapstone one of the most aesthetically pleasing stones to be used for the job.
True Soapstone is inert. Alkalis and acids won’t affect it as they will a granite, marble, or slate. For over one hundred years, soapstone sinks and tiles have been used in science class rooms and labs along with work tables and counter tops. Its longevity to long term – high traffic use is amazing!
Because of its truly remarkable and natural heat retention characteristics, soapstone is widely used for masonry heater fireplaces, wood stoves, fireplace liners and pizza ovens. Soapstone heaters and fireplaces heat very quickly from burning coal, pellets or wood, the soapstone will then slowly radiate heat very evenly for hours on end. Even after the fire has long gone out!
Q: Since soapstone is soft to the touch, does this mean it’s absorbant?
A: Though soapstone is soft to the touch it is far from absorbent. If the surface picks up a stain or discolors, this is literally – just on the surface – and can be scrubbed or sanded off. Slates, marbles, most granites, limestones and travertines are absorbent. Soapstone is not.