Saturday 6 July 2013

Masonry Heaters - 1

Masonry heaters work by radiating the energy stored in their masonry mass. They are simply heat storage banks. A short hot fire heats the masonry mass which stores and radiates it back to the space slowly and evenly for many hours. This creates a gentle heater with almost imperceptible warmth.

Right from the first time the fire is lit the heating cycle is very even, only slightly cooler in the morning than in the previous evening. Quite unlike traditional wood heating systems which create a very hot space around them, cool considerably overnight and then super heat the area when reloaded in the morning. Radiant masonry heating produces all round warmth as the thermal mass is warmed and re-radiates the warmth.

The generally accepted advantages of having a masonry stove are:

- They have the highest efficiency that can be achieved by burning wood.
- They create almost no wood smoke emissions.
- High safety. Properly constructed they don't produce creosote.
- Require minimal supervision and maintenance.
- Are custom designed and can be faced with a variety of materials to suit house's concept.
- Are wood fired and therefore a sustainable choice.
- Minimal fuel consumption. They are fired only once or twice a day.
- Low heating costs and independence from any future energy crisis.
- Independence from electricity.
- Healthy air inside house.
- Excellent suitability for solar passive design houses.
- Long lasting heat as heater releases stored heat 12-24hrs after fire is out.
- Safe to touch.
- Even room temperature from floor to ceiling and between rooms.
- Radiant heat. Sunshine like warmth while leaving the air fresh.

Flue Gas Channels courtesy of Heinz Maresch

Design and construction

The basic concepts masonry heaters go back hundreds of years, particularly in Eastern Europe where a dearth of wood meant that every last drop of heat needed to be gleaned from what wood was available and led to a basic design developing for the "Kachelofen" or "Russian stove". Whilst these stoves were traditionally used across Eastern and Northern Europe they only started to gain a wider audience when the Russian immigrants to North America started using them to replace the high consumption wood stoves that were common in the more Northern States and Canada. A seminal paper on the design and construction of ceramic stoves was produced by Heinz Maresch in 1972. (1).

Diagram courtesy of Missouri Department of Natural Resources

Diagram courtesy of Missouri Department of Natural Resources   
From this paper it can be seen that the masonry stove is based on the use of baffles, horizontal or vertical, made of brick or stone that direct the hot flue in an "S" shaped system. This serpentine pattern slows down the air speed and increases the length of the flue. Each additional square foot of masonry surface absorbs some of the heat from the flue gas until an optimal temperature is reached. As the fire is allowed to burn out with each firing the hot bricks slowly transfer their heat through the wall and into the living space as radiant heat. This slow, steady low level heat is very comfortable and it is possible to design a masonry stove whose appearance most suits the environment and house's concept. An important factor in my case.

Many articles and books have been produced detailing the history, design principles and in a few cases potential designs for incorporation into new build and to a lesser extent retrofits into existing buildings. The main problem with the latter is the sheer mass of a masonry stove with the finished weight of a masonry heater in the range 6,000-12,000lbs (2700-5400kg) depending on the size and design and the need, therefore, for an adequate foundation.

(1) "Kachelgrundofen (Ceramic Stoves): A Guide for the Practitioner" by Heinz Maresch