Tuesday October 30, 2012

In the past two weeks I’ve written about wood stoves and pellet heating. This week I’ll focus on another way to burn wood cleanly and efficiently: using a masonry heater.

A masonry heater, also called a masonry stove or Russian fireplace, is a wood-fired heating system that is fired intermittently at very high temperature to heat up the large quantity of thermal mass, which then radiates heat into the home. The heater has a circuitous path through which the flue gasses flow. Here, the heat is transferred to the stone, brick or other masonry elements of the heater.

Key benefits of
masonry heaters

From an environmental standpoint, masonry heaters burn fuel very rapidly at a high temperature. This results in very complete combustion with little pollution generated. Except when first starting the fire, there should be no visible smoke.

From a performance and comfort standpoint, masonry heaters take a long time to heat up, but they then continue radiating heat for a very long period of time, typically 18 to 24 hours. The outer surface of a masonry stove never gets as hot as a cast-iron or steel wood stove, but it retains its heat much longer. The surface area provides a large radiant surface, contributing to comfort.

Operation of masonry heaters

Unlike a wood stove, where you typically start a fire and then keep it going for a long period of time by adding fuel, with a masonry heater you operate it in batches, and the fuel is typically entirely burned by the time the next fire is started. This means that you have to start a lot of fires -- which some people will find less convenient.

Because the firebox may not be very large in a masonry heater and because a fast-burning, intense fire is desired, the firewood is cut and split differently. Often the length of acceptable firewood is less than with a wood stove (sometimes as short as 12 inches), and the optimal diameter of split wood is smaller -- typically 3-5 inches.

Because the heat from a masonry heater won’t warm up a space quickly (it may take several hours for the outer surface to reach peak temperature and peak heat delivery), it isn’t as effective as a wood stove at quickly taking the chill off. You need to plan ahead. And if it’s going to be a sunny autumn day and you have a lot of south-facing windows, starting the masonry heater in the morning may result in a period of overheating later in the day when the solar gain peaks.

Some masonry heaters include bake ovens or warming areas built into the modules, offering a nice feature for those interested in wood-fired baking. Others include integral benches for seating.

Product options

Masonry heaters are often custom-built, and such units can satisfy a wide range of design needs and special requirements. Because they are large and heavy, provision must be made for such units -- such as a concrete slab or concrete bearing walls beneath the heater. The Masonry Heater Association of North America is an excellent resource on masonry heaters and includes a directory of masonry heater builders. For information visit www.mha-net.org.

There are also some manufacturers of modular masonry heaters that can be assembled relatively easily. The best-known manufacturer is the Finnish company Tulikivi (www.tulikivi.com/usa-ca). Tulikivi heaters are made from soapstone or ceramic and are available in a wide variety of styles, both with and without bake ovens. Some include integral bench seats.

If a house has the space for it, a masonry heater is often the best way to heat with wood. In new construction, particularly in rural areas, it’s definitely worth looking into.

Alex Wilson is the founder of BuildingGreen, Inc. and the Resilient Design Institute (www.resilientdesign.org), both based in Brattleboro. Send comments or suggestions for future columns to alex@buildinggreen.com.