Stove efficiency numbers...propaganda?

There are a lot of numbers thrown around when it comes to efficiency. Everyone looooves numbers even when they are meaningless. So, does anyone know what the efficiency are based on? That is, how much wood is turned into heat vs how much wood is turned into HOME heat?

I have an old stove that puts a lot of heat up the chimney and should be replaced. There are so many directions to go: the forum favorite RMH; EPA certified re-burner or cat stove; retrofit with the IntensiFire insert.

I've seen RMH fans talk about 90% and from the look of the exhaust, I believe it. The EPA says the stoves they give their blessing to are 50% more efficient. If I assume my stove is 60% efficient, then an EPA stove would be 90% efficient. Probably not since a cursory search shows manufacturers state values around 75% for re-burning and 80% for cat stoves. And lastly the Intensifire insert claims 60% increase. So, 93% efficient? A condensing natural gas unit maybe.

Thoughts? Scientific or anecdotal evidence welcome.

Ben Logsdon wrote:There are a lot of numbers thrown around when it comes to efficiency. Everyone looooves numbers even when they are meaningless. So, does anyone know what the efficiency are based on? That is, how much wood is turned into heat vs how much wood is turned into HOME heat?

I have an old stove that puts a lot of heat up the chimney and should be replaced. There are so many directions to go: the forum favorite RMH; EPA certified re-burner or cat stove; retrofit with the IntensiFire insert.

I've seen RMH fans talk about 90% and from the look of the exhaust, I believe it. The EPA says the stoves they give their blessing to are 50% more efficient. If I assume my stove is 60% efficient, then an EPA stove would be 90% efficient. Probably not since a cursory search shows manufacturers state values around 75% for re-burning and 80% for cat stoves. And lastly the Intensifire insert claims 60% increase. So, 93% efficient? A condensing natural gas unit maybe.

Thoughts? Scientific or anecdotal evidence welcome.

First thought. As far as I know, legally, a wood heater that is considered 100% efficient is actually 84% efficient. That top 16% must be used to get the smoke up the flue. I think gas appliances are similar, or at least used to be till some of the newer model furnaces started using a fan to get the exhaust out of the house and could be more than 100% So all of those percent numbers are based on that. Some RMH setups do horizontal exhaust at lower temperatures, but all wood heaters installed with a permit must have a minimum exhaust temperature at the bottom of the chimney.

Second thought. There is a huge difference between how a stove is tested in the lab and how it is used in the home. When a stove is tested in the lab, a standard fuel crib is burned at the best burn rate it can do (fast and hot) and the exhaust temperature is measured as well as the temperature rise in the room. The amount of energy in the wood that heated the room can be calculated as a fraction or percentage (with the last 16% removed as we said). During this test the temperature in the test room will be much warmer than comfortable or maybe even livable, but that doesn't matter, the stove has in fact performed as advertised. Then the new owner installs the same stove at home with the same flue set up and burns the same kind of wood.... but, the new owner wants a nice 74F and has to burn the same wood with less air so there is comfort... then they want the last load at night to keep the house warm till morning. The only way to do this is to run the stove at 10% to 20% efficiency. So the stove is capable of 70% efficiency, but will never be used at that level because it doesn't feel good.

Third thought. The outside surface of an iron wood stove even running slow and dirty is over 400F, a masonry heater (generic term for high mass heater) should have a surface temperature that will not cause a burn from touching it... under 200F. So for the same level of insulation in the wall or window with 0F outside, the heat from the iron heater will radiate through the wall or window twice as fast as the high mass heater and so will use twice as much radiated heat to do the same job.

Fourth thought. Most homes are light build with high insulation. This means that the air inside is the most significant mass for holding heat. The stove heats the air and we feel warm, while the stove is burning we feel the radiation. Once it goes out, that radiation stops and the iron cools quickly and the user feels cold in less than an hour (typically). So the stove is run for as long as the house is occupied at a rate that keeps things cozy.... generally this means the wood is smoldering. The high mass heater burns very quickly and cleanly for 2 to 4 hours, but instead of heating the air up it uses the heat to heat it's mass (often several tons) and that mass radiates for 12 to 24 hours keeping the house warm for the whole time.

So those are some of my thoughts. People (manufactures) are telling the truth, but the efficiency the heater is being used at tells a different story... the heater is often not as effective as the efficiency would make it seem. This is why someone with a high mass heater uses a lot less wood (as much as 90% less) for the same heating season.

Masonry heaters generally cost much more than an iron box... 10k plus. The RMH started out as the poor mans masonry heater. Some people have found it to be a better compromise of responsiveness to mass storage. Because most of them have not been built with a permit they may even be more efficient because some people do not use a chimney, but rather just exhaust at shoulder height at a lower temperature that a permit would allow. The RMH is still very much a work in progress or a design under development. The original design still works fine, but there are many people working to make it better.

I hope that gives you some idea of where all the numbers come from and why they don't always work practically even when they are accurately measured.

As Len is saying, there are actually two aspects of efficiency. The first is what percentage of the BTU's which are in the wood are extracted by combustion. The second aspect is whether most of that heat is kept inside the building. In order to draft reliably, current thought is that the exhaust should still be 140°F at the exit. There is less condensation at this temperature.

The temperature leaving the heat riser in a Peter van den Berg burner is well over 1,000°F, so even if you stop extracting at 140°, there is still plenty of heat to store.

At Dragon Heaters, we test our designs with a Testo 330 which is a flue gas analyzer. One of the settings on the device is the moisture content of the wood being burned. The probe from the device is inserted into the stove pipe at the exit where the exhaust is heading out of the building. It measures the amount of Oxygen, CO, and CO2 to compute the approximate efficiency of the combustion process. The latest version From Testo also measures the particulates which the EPA claims to be worrying about.

So, you can go to our blog and read the test charts from various burns. We also use multiple thermocouples inside the build to read the temperature as the burn progresses.

manufacturers state values around 75% for re-burning and 80% for cat stoves

Another deduction you could make is that the "cat burners" show to be more efficient because the catalytic converter cleans up the exhaust so that it reads better on the Testo. Probably not because it is actually burning more efficiently.

The EPA says the stoves they give their blessing to are 50% more efficient.

more efficient than what? a standard open fireplace?

No official organization has tested a J-tube combustion system. Not the EPA nor any of the state equivalents. There are no protocols for how to run the test since it is not a "load it up and shut the door" sort of wood-burning device.

Same boat here, really. 30 year old cast iron stove, weighs 400#'s but dang will that sucker ever heat! It came with a catalytic converter, but we found that it needed to be cleaned every week, account it clogged with fine ash.... We don't use it.

Myself, I'd want to visit with a RMH owner or three before committing to the expense and the build. Just my 2 cents....

Best, TM

Come on down to Houston! It's a cheap flight from most places. You can see 3 different sizes in operation.

You don't say where you are from, maybe we have someone doing a build nearer you.

Do you see smoke coming out your chimney? Visible smoke has the volatiles (released by the wood when it burns) in it which represent something between 40% to 60% of the BTU's in the wood you are burning. It requires 1,200°F to burn all of the volatiles and most cast iron stoves don't get that hot.

If you look at the engine spec sheet or test report, a Ferrari has more horsepower than a Kenworth and it should pull that semi trailer just fine. Not the whole picture.

I have worked with several test labs--what labs do is remove variables so you can measure the one you want. They want single equation, one unknown math if at all possible.

The real world doesn't work that way, and that often gets lost in marketing.

So much info. So many directions. I think maybe I'll put some firebricks and an iron baffle in my old stove to make a hotter burn chamber and increase flame length. Right now I have a double walled stove pipe and elbow (about 2" long). Replacing that with single wall, since the burn is hotter, can extract a little more heat without worry about creosote. Next year I'm going to try a hybrid combo like this http://www.handprintpress.com/ovens/bring-in-the-mud/
A masonry mass on top of the stove! Luckily my stove is quite wider than that guy's so it won't look quite as "hat" like. But Could really pull more heat out of that exhaust.

As I recall in my early training, there are two types of efficiencies that are reflected in real life operation. The first is combustion efficiency. The design of the stove reflects combustion efficiency. Reburners in the stack are not part of the stove so they have their own efficiency criteria. Then there is heat transfer efficiency. The stove has its own efficiency criteria and the chimney, usually just the connector with or without a reburner, has its criteria as well. It was my understanding that stove manufacturers combine the two efficiencies in some kind of a weighted average...your guess is as good as mine on that one. From my personal experience in observing different installations over 26 years, I wouldn't recommend a catalytic unit of any kind...they burn out fairly quickly and are expensive to replace. Almost all of my customers who had them were not planning to replace their burned out catalytic units. Stoves featuring pre-heated combustion air (no catalytic unit) seem to me to offer the best features in terms of firebox longevity...but they, too, will burn out eventually. I should also note that EPA recently announced new air pollution standards. The new standards apply only to commercial manufacturing companies producing a certain number of stoves per year. DIY stoves, to my knowledge at this writing, are still exempt. Also exempt from air quality management agencies' no-burn requirements are solid fuel heaters used as the primary source of heat. BUT you need to check with your local authorities to see if that is true in your area. As for your idea to install firebricks in the firebox, I wouldn't. The bricks may or may not increase your combustion efficiency, but your heat transfer efficiency will definitely be decreased in terms of actual burn time.

I've been experimenting with the stove to see how it operated and get a better idea of how fire works. My stove is double walled with blowers. I've added red bricks (because that's what I have on hand) to the sides of the exceptionally large firebox. I figured the most of the heat transfer was probably going on at the roof of the stove. I've noticed that the air coming out is just as hot but with a smaller fire required. The fire stays hotter and burns better but I do have to feed it more often.

I'm really torn as to what to do with that stove. On one hand I want to replace it with a nice, shiny, new EPA certified stove and call it a day. I love the idea of a rocket heater but I am very busy with trying to get our farm running and don't want to babysit a stove feeding it sticks for hours. A large masonry installation seems to make more sense for my application. The Dragon core seems to be very rocket-esque but will burn larger wood. We also have too large of a house (2500 sf), but that's another thread. I'm okay with the house getting cold in the bedrooms and comfortable in the kitchen/dining area where the stove is.

Would running a stainless steel chimney liner help? It seems like I send a lot of heat up that standard clay-in-brick chimney to get it drafting well enough to keep the fire going to shut the doors of the stove.

The Dragon core seems to be very rocket-esque but will burn larger wood.

Do you have room for a large masonry installation? If so, you can build a 8" Dragon Heater castle build and line it with fireclay bricks (not red bricks). Depending on how many bells you add to it, you can store lots of heat. Once the system (all those firebricks) is (are) charged (with heat), you won't have to spend all day tending the fire. About the same amount of time which you spend in meal preparation and consumption.

The castle build using chimney flue liners as the structural element to hold the fire clay bricks is much cheaper than hiring a mason to build the whole thing. Also, this build uses bells rather than a flue (like is done in the standard rocket mass heater). Bells along more efficient and complete transfer of heat to the storage mechanism (the fireclay bricks).

Would running a stainless steel chimney liner help?

I don't know anything about this. I would check with Northline Express. They sell the liners and they would know what problems the liners solve. They also have a lot of free information on their website.