Thursday, May 20, 2010

The Masonry Heater - Part I

Ever since I started this blog, I've been excited about making this entry. Unless you're from a very cold climate (Russia, Finland, etc.), most people have never heard of a masonry heater. Indeed, I had never heard of a masonry heater until poking around on the web one day. It all started with our very strong desire to have a fireplace in our new house.  The warmth and ambiance is something we just adore, and it tends to bring the entire family together in a non-electronic, peaceful setting.  Unfortunately, we knew that traditional fireplaces weren't terribly efficient ways of heating a home. In fact, they typically increase your heating load.  Plus, a little research shows that wood burning fires are notorious for emitting particulate pollution, which is terrible for the environment. Alas, in The Consumer's Guide to Effective Environmental Choices: Practical Advice from the Union of Concerned Scientists, the authors strongly advocate against putting in a conventional fireplace in a new house.  Sadly, it appeared as though our dream of having a fireplace was toast.  Or was it?

One of the reasons that fireplaces are so terrible at providing heat for a home, even though they are lusciously warm up close, is because they tend to use far more air than is necessary to burn; up to 16 times more air than necessary, and as much as 300 cfm according to the US Department of Energy (DOE).  This air typically comes from the interior of the house, which must then be replaced.  Ultimately, this air is replaced by cold air from outside the house that comes in through leaks in the house's envelope, which actually cools the house down more than the fire is heating it up.  That's one of the reasons why constructing an airtight house is so important when it comes to energy efficiency.  In addition, only about 5% of the heat from the fire is actually emitted into the house.  The other 95% goes right up the chimney.

So perhaps the solution is simply to include an exterior air source for the fireplace?  This definitely improves the situation significantly.  Unfortunately, the large volume of cold air actually cools the fire and keeps it burning at a relatively low temperature (often under 1,100°F), which is precisely the condition that leads to high particulate pollution. Fortunately, high-efficiency fireplaces and fireplace inserts are now available that improve heating efficiency while lowering pollution.  The most common of these are catalytic or advanced combustion wood stoves.  These appliances provide a lot of heat when burning at "full throttle" and burn at a temperature above 1,100°F.  This is hot enough to burn combustible gases which reduces particulate pollution considerably.  In addition, wood stoves are sealed so that the amount of air entering the fire is controlled, hence less cold air is drawn into the house.  The result is that much more heat is available for the home.  According to the US DOE, advanced combustion wood stoves have advertised (combustion) efficiencies of 60%-72% while catalytic wood stoves are efficient up to 80%.  This represents an incredible improvement over a traditional fireplace.

When presented with this information, it was pretty clear that an efficient wood stove was the right decision for us.  Unfortunately, this just wouldn't have the same ambiance that we had grown to love with a fireplace.  So we started thinking about using a wood stove insert with a conventional fireplace hearth and chimney.  This is when we stumbled upon masonry heaters.

Masonry heaters, also known as "Russian," "Siberian," or "Finnish" fireplaces, have been around for hundreds of years.  According to the US DOE, they produce more heat and less pollution than any other wood- (or pellet-) burning appliance, with combustion efficiencies over 90%.  Masonry heaters consist of a firebox, a large mass of masonry, and a long twisting exhaust channel that runs through the masonry (to promote heat absorption).  The heat is absorbed in the masonry during the 2-3 hour burn phase and then radiated throughout the house for the next 12-18 hours. The following schematic diagram from Energy Smart Homes magazine (vol. 3) details the basic airflow pattern of a masonry heater.

Heater Airflow Diagram (Click image for more photos.)

Typically, masonry heaters burn small, hot fires (over 2,000°F in the firebox) that release very little pollution.  This temperature is so high that there is almost no creosote buildup (creosote is burned as fuel).  According to the Masonry Heater Association of North America (MHA), well constructed masonry heaters have combustion efficiencies between 96%-99% and an overall efficiencies of around 75%.  Such a 75% overall efficiency is much higher than it sounds because in the US (but not in Europe), overall efficiency is defined to include the energy used to vaporize the water contained in the wood.  This results in a maximum possible overall efficiency for any wood burning appliance of only about 83%.  Therefore, a 75% overall efficiency means that masonry heaters achieve over 90% of the theoretical maximum in terms of potential heat delivered to the home.  Very impressive indeed!

Having decided that a masonry heater is what we wanted, it's not like you can go down to Lowe's or Home Depot and purchase one. Masonry heaters are massive structures that are hand crafted by expert masons. Indeed, this is probably the biggest drawback of a masonry heater, the expense. Whereas a traditional fireplace might be constructed for around $8,000, a hand crafted masonry heater will cost at least double and could easily run even higher. If this is your primary heat source, such an investment makes sense. Since we will not use this as a primary heat source (although we will definitely benefit in lower heating costs), it was a difficult decision. When you factor in the cost of an efficient wood stove insert, a masonry heater was not going to be all that much more expensive. Still, it is a costly item and we would save a lot by not having any kind of fireplace or wood stove. In the end, we decided to go for the heater; this was one place we thought it was worth splurging a little.

Because masonry heaters are so specialized, we knew the chances of finding a heater mason in Carlisle, PA was essentially zero. Fortunately, the Masonry Heater Association of North America has a directory of certified heater masons across the country and this led us to Brian Klipfel of Fire Works Masonry in New Jersey. Brian is building us a Finnish contraflow heater with a wraparound heated bench. He has been working on it for about a week and has the basic heater core, bench, and backwall complete.  He'll return in a couple of weeks to finish up the chimney and install the exterior stone.  The construction of the heater has been quite impressive so far. I'm very glad we decided to splurge.

Phase I Complete (Click image for more photos.)


Unknown said...

Hey I stumbled on your blog while looking for masonry heater info. (I know it's an old post.) Thanks for sharing! I'm thinking about moving East from Oregon to New Jersey (where it's actually a lot sunnier), hopefully have a farm and make one of these heaters. Currently working on a masters in math. I'm going to check out the rest of this blog :)

Unknown said...

Thank you David/Virginia for all this great info. Based onthat I will build up by myself a masonry heater in my farm in Central Mexico, I think this Idea will motivate many local people to turn back to this great ancient devices. Fortunatelly the labor and materials are much cheaper than in the state, not to mention the skilled hand labor of local peole in masonry construction. When it get done I wll post how much I spent, but hardly it will be more than 2K-3K USD. Thanks George and Margaret

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