Absolute Steel Hybrid: Clearances

After a fairly long wait, we finally have the official report on the Absolute Steel Hybrid clearances from the independent UL Safety and Clearance testing.

Hybrid Clarifications

After our last hybrid email and blog post, we received requests for additional clarification about hybrid combustion. What is the difference between a hybrid stove and a stove with a catalytic combustor or secondary combustion?

Q&A about Hybrid Stoves

As a factory direct company, we get calls everyday with customers asking questions about our hybrid technology. If you are curious about hybrid technology but have not called, please read our Q&A addressing some of the most commonly asked questions, and be sure to comment below if you have a question that we have not addressed here.

Q . What is the biggest difference between the new hybrid stoves and the more traditional wood stoves made at the Woodstock Soapstone Company (the Fireview, Keystone and Palladian)?

Durafoil Steel Catalyst

A. The biggest difference is that hybrid technology allows us to achieve ultra-low emissions.  We developed hybrid stoves as part of a quest to achieve lower wood stove emissions. The traditional stoves (Fireview, Keystone and Palladian), are similar to a reliable, fuel-efficient, family sedan (think Honda Civic). The smaller fireboxes and stainless steel Durafoil catalysts allow our catalytic soapstone stoves to burn cleanly and have great low emissions performance.  The hybrids have bigger fireboxes and spectacular emissions performance at all outputs.

Nine years ago (in 2007) we started to research ways to make our stoves burn cleaner.  We knew that new EPA regulations were coming.  Our 2007 wood stoves (Fireview, Keystone, and Palladian) already met what are now the 2020 emissions standards, but we made them even better.  We improved the longevity and performance of the catalysts in these stoves by making them out of Durafoil.

But until we introduced the Progress Hybrid Stove in 2011, all wood stoves used either a catalytic combustor or a secondary combustion system to meet EPA emission standards.  By combining the two (catalytic combustor and secondary system) we were able to achieve efficient, low-emission burns throughout the entire burn range (low-medium-high) that were dramatic, consistent and predictable.

Q . What is the difference between catalytic combustion and secondary combustion? Is one better than the other, or better together?

A.The difference really comes down to the temperature needed for each system (catalytic system and secondary air system) to begin to clean up emissions.  A stainless steel catalytic combustor heats up very quickly and depending on conditions (wood, draft, operator) it can be engaged within minutes of loading the stove, or as soon as the internal temperature is around 400°. Catalytic stoves burn best at low to moderate burn rates, which tends to be the most common way to operate a wood stove for overnight and daytime burns while at work. Catalytic only stoves test with lower emissions and greater efficiency than their secondary air system counterparts, and do require periodic cleaning of the catalyst to work most effectively.

The secondary air system requires higher temperatures (between 1000-1100° internally) to start burning the combustible gases in the wood smoke. Most secondary air only stoves burn best at moderate to moderately high burn rates, when the internal temperature is at its peak. Due to the high temperature requirement, secondary air only stoves have a higher tested emissions level and a lower efficiency than catalytic style stoves.

If you combine these two separate means of lowering emissions and increasing efficiency, you have what we would consider the “Gold Standard”, a hybrid stove that will burn cleanly throughout all the variations in burn rates and customer operation.  Hybrid technology is uniquely effective at high burn rates where either system may struggle alone.  The hybrid design is also very effective with large firebox stoves, which can produce a large volume of combustible gas and particulates.

Cutaway view of the Progress Hybrid

Q . Are there differences, other than emissions, between the hybrid woodstoves and your traditional woodstoves?

One example of custom art for the Ideal Steel Hybrid

A.  Yes.  As part of the hybrid development process, we have developed ways to make our new hybrid stoves more efficient, more affordable, and customizable.  These are all “extra” benefits from doing extended R&D over the last nine years.

Focusing on combustion design to reduce emissions also helped us to make our stoves more efficient.  We discovered that we could make high performance stoves that were completely affordable if they were fabricated from steel rather than cast iron.  Then we realized that we could customize stoves made of steel, which would have been prohibitively expensive with cast iron. Plus, making stoves out of steel has the added benefit of being fully fabricated in our NH factory, which ties back into greater affordability and the ability to customize.

Q . Why are you promoting only the Hybrids during this sale?

A. Partly it’s to focus attention on the Absolute Steel Hybrid, which we are introducing during this promotion.  We’ve made a big commitment of time, energy, and resources to cleaning up wood burning technology, so of course we want to talk about it and of course, we want to keep improving our combustion designs.

Outdoor Air Quality vs Indoor Air Quality. What is the difference?

The short answer is that there is no difference.  Outdoor Air becomes Indoor Air.  Most of us spend more time indoors than out, so the quality of Indoor Air is just as important as Outdoor Air.  Perhaps even more.

A lot of people don’t give much thought to Outdoor Air Quality.  We at Woodstock Soapstone think about it all of the time.  We have to.  It’s part of our business.  But we also think about Indoor Air Quality.  Many people use Indoor Air to satisfy the combustion air for their wood stoves.  In very tightly sealed houses, the amount of Indoor Air available for a wood stove may be limited or reduced by the tightness of the house (super insulated) or by negative house pressure.

Some houses are so tight that turning on the kitchen exhaust fan can compromise performance of the wood stove.  In other cases, wood stove performance can be enhanced by cracking a window.  In both cases, house tightness/negative pressure affects stove performance. This is why every stove we build has the option of an outside air adapter, and our new Absolute Steel Hybrid has an Outside Air Adapter built-in.

More and more people have been installing outside air adapters on their stoves.  Some States and local jurisdictions now require it.  All mobile homes require a wood stove to have an outside air adapter installed. Check your local laws if you are in doubt.

The outside air adapter on our new Absolute Steel stove is unique in the wood stove industry for two reasons:  1) All of the air that goes into the stove (primary, secondary, and catalyst air) has to go through this one opening, and 2) it is virtually leak proof.  If you really want to protect the air to your stove from other air circulation in your house, and more importantly the negative pressure in a tight house, this is the best way to do it.

Click to Enlarge 

Our wood stoves are not only clean burning, our new Absolute Steel stove is one of the best stoves ever made at maintaining clean inside air as well as clean outside air.  We feel the Absolute Steel has the finest outside air adapter design in the industry.

And, to make life easier for you, we build an outside air adapter into each Absolute Steel stove we make.  If you don’t need it now, you may need it later, and it will be there for you.

R2Z Detour: Introducing Our First Test Stove

As we noted in an earlier post, we are taking a slight detour in our Race to Zero.  We are testing older stoves made in the 1970s and 1980s.  Stoves which were made with little technology to reduce emissions or improve efficiency.

Huntsman Stove - Before the Makeover

Our first test stove is a “Huntsman” (made by Atlanta Stove Works, circa 1977) step stove.  It is a welded steel stove, with a huge firebox (well over 5 cubic feet), and cast iron doors with six draft controls.  The bottom is lined with firebrick, and the side walls are also lined up to 9” in height (one layer of firebrick).  As with many steel stoves of this vintage, the stove itself is almost “overbuilt” (1/4” and 5/16” boiler plate), and as long as the firebricks are replaced periodically, it will last for a few more decades.  

Huntsman Stove - After the makeover

Interestingly, even though the Huntsman stove is physically huge, it weighs almost exactly the same as our new Absolute Steel Hybrid - just over 500 pounds.

Our first goal is to establish a careful baseline profile for these older stoves in terms of heat output, emissions, and efficiency at low, medium, and high burn rates.

Next, we want to see if we can design a retrofit catalyst and heat exchanger that will improve the performance metrics (heat output, emissions, and efficiency) enough to make them competitive with some of the stoves in today’s market.

We have wired the Huntsman in our research department so that we can measure surface and gas (O2, CO2, and CO) temperatures  at multiple locations.  The stove is on a scale, which allows us to measure the rate of fuel consumption.  Finally, we can (and will) get particulate catches for most of the baseline runs using Method 5G3 (the same Method the EPA uses).  We will use the Canadian CSA B415 algorithm for calculating efficiency.  We are trying to run parallel to the same methods and standards used by the EPA.

We are using cordwood for our testing because it is readily available, and is much less expensive than the cribs the EPA has used for emissions testing during the last few decades.

We’re doing our best to keep loads representative in terms of species, weight, and moisture content.  One of the nice things about R&D testing is that we can operate with a certain degree of informality that you can’t do with certification testing.  We are just interested in seeing if we can “move the needle” before we pay attention to all of the fine points of test protocol.

We hope to get baseline data on at least three stoves (one small; one medium; and one large), and we hope to develop mathematical models for different aspects of our testing – an example would be the heat transfer achieved by our device.  More on that in the next week or so.

Obviously, there is an opportunity to reduce emissions in old stoves.  We think there are additional opportunities to extract more heat (both by radiation and convection) around the area where our device would be installed.  Without getting too esoteric, we may be able to point the catalyst directly at the surface(s) we want to use as primary radiators.  Finally, the catalyst should introduce a considerable pressure drop into the system.  This resistance may result in increased stack temperatures around our device, a reduction in stack flow, a lowering of the burn rate, and some improvement in efficiency.  Well - we are hoping to get these results!

The questions are: (1) How much of a reduction in emissions?  (2) How much additional heat extraction?  (3) How much of an improvement in efficiency?  (4) At what cost and what degree of difficulty?

We expect to be busy in the lab with this project for 3 to 4 months, at least. We don’t know if we can succeed with this R2Z Detour, but we think it’s worth making a serious effort.