Heating Your Home: Should I Install a Mass Heater?
Author’s note: the following article on home heating is the final one in an eight-part series. If you are thinking about installing a wood-burning mass heater, this article should help you.
Operating a Mass Heater
Mass heaters are a different approach to burning wood. If you don’t don’t understand this, you won’t just be disappointed — you’ll fill your house with smoke or, worse, poison your family in their sleep! Getting proper performance from a mass heater requires a little planning, and some involvement, on your part. The tradeoff for this extra effort is safety and tremendous energy efficiency.
Because the heater sucks so much energy out of the flue gasses before they depart, it’s up to you to establish sufficient draft (the force that pulls the smoke up the chimney) before lighting a fire in a cold heater. How do you establish draft? By purging a cold air “plug” that can form inside the chimney with a small fire (e.g., a single lit piece of newspaper) placed directly underneath the chimney via one of the ash cleanout doors. When you hear a dull “roar” from the newspaper, it means that hot air is moving up the chimney and it’s time to light the main fire. This step is not hard, but skip it at your peril!
You want to be around when the fire burns down to small embers. At that point, you should close the flue damper and the air intake door. If you don’t, the heater will continue to draw air into its vast mass, heat it, and send it up the flue. The next morning you find a cold stove and realize two thirds of your heat went up the flue. On the other hand, if you’re impatient and shut it down too soon, you risk filling your house with poisonous carbon monoxide gas, a normal by-product of combustion. Timing is important!
If you’re starting to feel a little intimidated, don’t be: anyone capable of boiling pasta can operate a mass heater. However, installing one requires more planning, and designing or building one yourself is still more involved.
Mass Heater Installation Checklist
Mass heaters are subjected to tremendous thermal stress. Consequently, they should be built with high-quality, enduring (and necessarily quite heavy) materials. Retrofitting them into existing buildings can be challenging, as they require substantial foundations. When retrofitting a heater into a suspended floor system, be prepared to open the floor for new footings. To be effective, mass heaters should be located centrally in large, open spaces. Securing or buying firewood can be challenging or cost-prohibitive in some locales. Storing and carrying firewood to the heater can be messy or difficult in retrofit installations. Ideally, you want the trip between the heater and the outdoor wood supply to be as short and easy to sweep as possible. The up-front cost of mass heaters is greater than for other heating systems. Finally, if your daily schedule is erratic, or you’re thinking about one of these for a weekend retreat, it might not be such a good idea. In this scenario, thermal mass is not your friend (it takes a while to get it warm).
Installing a Mass Heater
Most people are well advised to hire a mason to install a mass heater. However, for the reasonably motivated and skilled, it’s a realistic home project. Plans for simple heaters can be found on the Internet for free. (For example, see Missouri Stove.)
After designing and building several prototype heaters, I found a commercial kit I like better than anything else I could find or develop on my own. The engineered “core” of the system is sold as a kit that you build by stacking and mortaring the pieces together much like giant blocks. You wrap this core in a decorative masonry facade to add still more mass and integrate it with your decor.
Whether you’re building one yourself, or hiring someone to do it for you, here are some considerations when choosing a design or system. First, know that when you subject any material to a range of temperatures of almost 2000 degrees F, it will expand and contract significantly. To withstand the heat, the combustion path should be refractory brick or refractory concrete, and should be assembled with special refractory mortar, not the sacked stuff that’s readily available at home improvement centers. Airtight expansion joints should be part of the design. If a decorative facade is applied, it should be isolated from the refractory core to keep it from cracking. All transitions between different materials (e.g., refractory, masonry, metal) should use ceramic fiber gasket material to isolate and seal critical joints. Remember, clean combustion happens at really high temperatures. The details are vital if you expect the heater to be able to burn cleanly and last a long time.
Good designs simplify maintenance. Ideally, heat exchange channels should be primarily vertical (rather than horizontal) so as to concentrate fly ash, an inevitable product of combustion. Passages should be readily accessible for cleaning.
Wood Heat: Local, Carbon-Neutral, and Clean
If passive solar isn’t a possibility for you, the next most comfortable, healthy and renewableoption may be radiant heat. If you can find or buy firewood, you may have the option to heat with a locally produced, clean-burning, carbon neutral fuel. To do this, you will need a somewhat open floor plan, a slab foundation or exra piers, and, last but not least, a mass heater.
Take the plunge. When the power goes out and your neighbors are freezing their behinds off, you’ll be going about your business as usual — or maybe you’ll simply enjoy that roaring fire.
Previous Articles in this Series:
- Heating Your Home: Mass Heaters
- Heating Your Home: Why Woodstoves Aren’t the Answer
- Heating Your Home: Why Fireplaces Don’t Heat
- Heating Your Home: Thermal Mass
- Heating Your Home: Forced Air
- Heating Your Home: Heat 101
- Heating Your Home: Radiant Heat, Wood Heat
Related Articles:
- Wood Burning = Green Heat?
- Green Building Talk: Save Money on Your Heating and Cooling Bill with Geothermal
- Geothermal Energy and Ground Source Heat Pumps
Resources:
Photo credit: Temp-Cast Enviroheat LTD
