Hybrid. Theory and discussion. Not comparison

[JA600L]

First off, I am not doing this to try and tell you to buy my stove. I do not want to hear about NON-HYBRIDS and why they are better. Please stay on topic.

I want to discuss the theory and operation of hybrid technology being developed in wood stoves.


One topic on my mind is air flow rate. If a catalyst is designed to a certain dimension with a certain amount of surface area of stainless steel foil, is it possible that it can not keep up during a high burn rate? Will too high a flow of emissions keep it from burning ALL of the smoke? Is the intake of a cat stove designed to allow only enough air to keep the cat efficient? This leads me to question if the secondary burn of a hybrid is used to further increase flow rate while being able to handle a higher flow of emissions? Would anybody like to respectfully discuss this with me?

 

[TheRambler]

Yes a cat can be overwhelmed or saturated and be unable to burn everything passing through it under certain conditions.

This is where the secondary burn portion of a hybrid would kick in, in theory, as long as the temp is high enough to support secondarily combustion.

A hybrid stove, in my case a PH, does have a specific minimum amout of air supplied to the cat at all times even with the primary air control fully closed. This air will be used by the cat only, there is also a continuous feed to the secdary baffle so if temps are high enough it will feed the secondary burn.

If secondaries are active, this is taking place before the smoke/emissions pass through the cat.

 

[begreen]

Previous threads on this topic:
https://www.hearth.com/talk/threads/hybrid-stoves.117103/
https://www.hearth.com/talk/threads/can-someone-explain-a-hybrid-stove-to-me.91242/

 

[JA600L]

I'm sure that a lot of thought process goes into figuring out how much air the stove should get at various burn rates as well as how to smoothly transition from a secondary to a cat burn. I feel like the Ideal Steel is very well controlled. You would need the right amount of primary to work with the perfect secondary and automatic cat feed. I can see how it would be tricky to get it correct with low emissions.

 

[Highbeam]

how to smoothly transition from a secondary to a cat burn.

This is a common misconception. It's not either or. It is always cat and secondary. The ratio moves but there's no switch between the two technologies.

 

[Isaiah53]

As an engineer that has worked with natural gas combustion systems for 35 years, I am interested in the discussion. Never worked with solid fuels other than my fireplace and stove.

The thing that intrigues me most is what is the true benefit of the hybrid. It must have one for the manufacturers to develop them. Theoretically, I cannot see a reason why the cat cannot work at high fire rates. You just need more cross-sectional area and surface area, and I don't see how you can oversize a cat. Just saying the cat is "saturated" or "overloaded" is not really an explanation by itself. Maybe it is just the impracticality of providing a sufficient size cat, but I think it might also be that the stove needs additional oxygen at high fire. If the oxygen concentration gets too low, the residual smoke will not combust no matter how much catalyst there is. The secondary combustors in a secondary burn or a hybrid would provide supplemental oxygen for both the secondary burn and the cat. On other hand, adding additional air above what is needed, reduces your overall efficiency due to additional mass of hot air going up the chimney.

I have often wondered how much residual oxygen is in the stack of a wood stove since it has the largest bearing on the efficiency along with the stack temperature. I would think it must be very low to nil. Otherwise, I don't see any reason for a secondary combustion system to even work. It would just be burned in the firebox ahead of the secondary tubes. On the other hand, 80-82% efficiency is not that high for a combustion system operating at 400-500 F stack temperatures, unless it is based on the higher heating value of the wood (in which case it would agree with a low oxygen value). It has never been clear to me what is the basis for the reported heat release and efficiencies from the manufacturer literature (higher or lower heating value). Efficiencies on a lower heating value are going be better by about 10%, so combustion equipment manufacturers like to advertise them. Consequently, I have generally assumed that they are LHV, but maybe not. The rated heat release would be higher on an HHV basis by about the same margin.

 

[begreen]

Some stoves like the PH and the IS regulate secondary air mix via the position of the air control.

 

[JA600L]

This is a common misconception. It's not either or. It is always cat and secondary. The ratio moves but there's no switch between the two technologies.

I realize that air will be pulled through the secondaries at all times, however, as the system enters a deep cat burn where the firebox falls below 1000F inside there cannot be any secondary activity other then the catalyst. You can't have secondary air burn and heavy creosote build up within the firebox. The Ideal Steel definitely goes all cat when demanded. I do agree that air is still pulled through the secondaries.

 

[JA600L]

http://blog.woodstove.com/2013/09/under-hood-4-catalyst-air-we-apply-old.html?m=1

This might help explain a little what is going on.

 

[Oldman47]

That reference actually explains how the IS uses a balance between secondaries and the cat. No place does it even imply that the stove goes to an all cat mode. What it does explain is that at lower burn rates it favors the cat over the secondaries in terms of which is doing the most good at that time.

 

[JA600L]

I guess it's all down to how you look at it. I believe they both "can" work at all stages. I'm just saying the temperatures definitely fall below secondary thresholds.
If you look at the webpage it lists a lowest btu rating at 13,017. At this point IMO
the cat is doing all the work. I may be wrong.

 

[JA600L]

Soapstone. This stove is insulated like crazy with soapstone.

It's it possible that all of that insulation then becomes a hindrance once the cat/ secondaries die out? I realize that it will soften the heat output, but is it maybe accurate to say that an insulated firebox is less efficient once the fire goes out? Maybe this helps throw more heat out the door too. Any thoughts?

 

[TheRambler]

I wouldnt call soapstone an insulator. It is in a way, sure, but in my opinion the best way to think of it is as a thermal battery. Your not losing any efficiency, your gaining it. All of that extra mass is storing excess heat energy that would otherwise go up the flue or be lost in varying other ways and slowly release it over several hours.

Sure soapstone takes a little longer to heat up fully, but that doesnt mean its less efficient. A well designed stove will transfer more heat to the stove body and mass in general rather than to be lost up the flue.

The only thing that comes close to this is cast iron, and its about half as "efficient" as soapstone in this regard.

 

[Oldman47]

Soapstone. This stove is insulated like crazy with soapstone.

It's it possible that all of that insulation then becomes a hindrance once the cat/ secondaries die out? I realize that it will soften the heat output, but is it maybe accurate to say that an insulated firebox is less efficient once the fire goes out? Maybe this helps throw more heat out the door too. Any thoughts?

I would go further than TheRambler. As long as the stove interior is hotter than the outer surface of the soapstone it will still be transferring heat outward, albeit slower than a steel stove would.

 

[JA600L]

I took a few pictures of the stove during what I consider a "cat only" burn.

I turned the flash off so they are a little dark. This is about 45 minutes after the bypass was closed after a reload.

My temp gauges aren't the most accurate so give or take a little.

I can hold my hand on the double wall pipe meaning stack temperatures dropped.

 

[webby3650]

I took a few pictures of the stove during what I consider a "cat only" burn.

I turned the flash off so they are a little dark. This is about 45 minutes after the bypass was closed after a reload.

My temp gauges aren't the most accurate so give or take a little.

I can hold my hand on the double wall pipe meaning stack temperatures dropped.

How bout a picture of that cat glowing?

 

[webby3650]

I know everyone thinks that Hybrid stove technology is a new concept. Blaze King actually produced a few thousand of them about 30 years ago...

 

[JA600L]

 

[kennyp2339]

Wow very interesting, there def has to be a base line, tests that measure temps of the stove cat only, then air tube reburn only, also co produced, o2 levels in fire box , mix them together to get the proper tiers, like the one above poster stated about a high fire box temp with lower o2 levels would not let a cat burn, it would just remain rich, but wouldn't that cause the super hot smoke to flame out once it leaves the chimney into a regular environment? Or how does the high temps in the fire box with secondary reburn effect the cat if all the smoke is burned off in the secondaries? Or is the stove designed with the perfect air balance to get some secondaries to keep the cat at optimal temps so the rest of the smoke can get burned going threw the cat? I'm thinking to hard and aloud at the point, sorry for the nonsense rambling

 

[Idahonative]

My input on this subject can best be communicated by my comments from another forum, mostly in response to JA600L:

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I'm going to make a prediction: A few short years from now we will all look back and make a judgement on hybrid technology. Without some major improvements, hybrid technology will fail to be a real player in the industry and I say that regardless of who makes the stove. There just isn't any significant advantage over other, proven technologies. Will time prove me right or wrong? We shall see.

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I agree hybrid's are clean burning but no cleaner than other cat stoves currently on the market. In fact, I can think of three cat stoves off the top of my head that are cleaner burning than the Ideal Steel (& will heat a home twice as long on the same wood).

"If we are talking about lower emissions and more total heat staying in your house then yes there is an advantage." If that statement were true, the Ideal Steel, with it's 3.2 cf firebox, would be enjoying legendary real world burn/heat your home times. It is not.

"The advantage is during low burns you get the best technology." I'm pretty sure cat stoves thrive during low burns.

"A high burn rate can overwhelm a cat only stove." Uhh...What?

IMO, there is no advantage to hybrid technology and I think time will prove this.

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Does the EPA require manufacturers to pass different burn rate tests? If not, what's the point? Wouldn't the manufacturers want to focus on passing what the EPA requires?

[/QUOTE].If I were one of their competitors I would try out some of the principles they have used to try to improve my own product line[/QUOTE]

Why in the world would a competitor want to? What's the benefit? If hybrid technology was the benchmark in the industry, I could see your point. The fact is (at least at this time), there just isn't any significant benefit over current technology.

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Uhh...you do realize you are referring to the King model right? "Running low and slow isn't an option for everybody" If you think the King is only for low and slow, there really is no point in this conversation continuing. There are many King owners living in Alaska and Canada that experience far harsher conditions than us that would laugh at your statement.

"Only concerned about long burn times..." No, we are concerned with long burn times that keep our house above 70 f. If keeping a 1250 sf house like ours at 70 degrees in single digit weather for 40+ hours is so easy, then by all means find an Ideal Steel owner with a small house and try it. After you do, I don't think you will be tossing our results out like they're meaningless any longer. Under the exact same conditions, I would guess your Ideal Steel would come in somewhere around 12-14 hours. I realize the King has a bigger firebox but you are making the comparison.

I'm not trying to change your mind either. I really, really, really, really want someone to explain it to me. Yes, your stove is capable of producing lots of heat for 8-10 hours but so are many stoves. Yes, your stove burns cleanly but so do many stoves as well. And that's the point...hybrid technology really isn't capable of doing anything that's not already being done and in the area of burn time, being done much better.

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Ok, I'll take your word for it the Ideal Steel uses the same cat as the Fireview. So, I will try to make my point and this isn't necessarily directed at you.

In this thread, I've talked a lot about a wood stoves heating efficiency (how well it burns, extracts, and delivers heat to the home). My point being that every model stove will have a different heating efficiency even if it were possible to burn the exact same load of wood (others will disagree). Which brings me to my question about cats and the possibility that different sized cats may play a major role in a stove's heating efficiency.

1. The Ideal Steel's cat measures 43.71 cubic inches.

2. The Princess, Ashford, Chinook, and Sirocco's cat measures 84.80 cubic inches (194% larger).

3. The King's cat measures 105 cubic inches (240% larger).

If it is the job of the cat to produce heat (using smoke as fuel), is it safe to assume that a larger cat will have the ability to burn more smoke and thus, produce more heat? I am obviously not an expert on cats but the above figures kind of blow my mind. We aren't talking about 10-20% larger, we are talking approximately 200% larger. I made a claim earlier in this thread that any of the stoves in #2 would heat twice as long as #1 with the same load of wood. Can a correlation be made between my claim and a cat that is nearly 200% larger?

Common sense tells me there must be a positive benefit to using a larger cat. If not, why would a company spend the extra money? Granted, the smaller cat in the Ideal Steel may have something to do with it being a hybrid. Maybe it doesn't need as large of cat since it also has tubes? I really don't know but these are interesting points and I would like to learn more.

My other question was the cost of the cats. Members have made comments that BK's cats cost twice as much and that justifies eliminating them from purchase consideration. The materials used in cats are expensive and a larger cat will obviously cost more to manufacture. If a cat is twice as big, is it unreasonable to ask twice as much?

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Brian described that burn as a typical winter burn for his Ideal Steel. What you have just stated completely relates to the point I was trying to make about hybrid technology. If that burn was "cat and secondary" as you say and the results were as the graph represents, then there really is no point to hybrid technology in my opinion. And PLEASE understand, I am not trying to argue or down any particular stove. I just do not see the real world benefits. That is why I made the prediction that in a few short years, we will all look back and talk about the failure of hybrid technology (unless there is a major breakthrough).

 

[begreen]

It appears that the intent of the hybrid is to reduce emissions over a wide range of burning conditions while using a smaller cat. This is laudable if it's a method to keep costs down while meeting stricter emissions requirements and that savings is passed on to the consumer. That seems to be the intent of the IS. Long burn time is a good attribute for any stove, but there are several other factors that go into a stove purchase decision, cost being primary for many people. Looks, ease of operation, long term operating costs, local dealer network, fire view, flue size, clearances, etc. are also deciding factors in stove purchases. If hybrids succeed in allowing manufacturers to meet the new EPA regs without a drastic price increase then we may well be talking about hybrids for many years to come. We'll see.

 

[Dune]

As an engineer that has worked with natural gas combustion systems for 35 years, I am interested in the discussion. Never worked with solid fuels other than my fireplace and stove.

The thing that intrigues me most is what is the true benefit of the hybrid. It must have one for the manufacturers to develop them. Theoretically, I cannot see a reason why the cat cannot work at high fire rates. You just need more cross-sectional area and surface area, and I don't see how you can oversize a cat. Just saying the cat is "saturated" or "overloaded" is not really an explanation by itself. Maybe it is just the impracticality of providing a sufficient size cat, but I think it might also be that the stove needs additional oxygen at high fire. If the oxygen concentration gets too low, the residual smoke will not combust no matter how much catalyst there is. The secondary combustors in a secondary burn or a hybrid would provide supplemental oxygen for both the secondary burn and the cat. On other hand, adding additional air above what is needed, reduces your overall efficiency due to additional mass of hot air going up the chimney.

I have often wondered how much residual oxygen is in the stack of a wood stove since it has the largest bearing on the efficiency along with the stack temperature. I would think it must be very low to nil. Otherwise, I don't see any reason for a secondary combustion system to even work. It would just be burned in the firebox ahead of the secondary tubes. On the other hand, 80-82% efficiency is not that high for a combustion system operating at 400-500 F stack temperatures, unless it is based on the higher heating value of the wood (in which case it would agree with a low oxygen value). It has never been clear to me what is the basis for the reported heat release and efficiencies from the manufacturer literature (higher or lower heating value). Efficiencies on a lower heating value are going be better by about 10%, so combustion equipment manufacturers like to advertise them. Consequently, I have generally assumed that they are LHV, but maybe not. The rated heat release would be higher on an HHV basis by about the same margin.

80-82% is actually remarkable considering that wood, unlike most other fuel sources is about 20% noncombustible. Therefore, considering a good amount of heat MUST be wasted to ensure a functioning chimney effect, I would guess that even claims of 80-82% are bogus.
From an engineering standpoint, (as you are likely aware) cost is a major factor, hence a cat large enough to handle anything thrown at it would be an unneeded extra expense. Perhaps it is time for oxygen sensors in the stove and pipes, and a little autoregulation of combustion air?

The point is that stoves must be learned and then run properly. Sometimes I put too much highly volatile fuel (pine) in the stove and must give it extra air to compensate.

 

[Idahonative]

Some stoves like the PH and the IS regulate secondary air mix via the position of the air control.

I agree, and that is a big reason why the Ideal Steel DOES NOT possess "record efficiency" in the real world (aka HEATING EFFICIENCY). "Position of the air control" is operated manually and that's part of the problem. "High efficiency" and "Manual controls" don't belong in the same sentance. Without an auto damper, true (real world) efficiency can not be achieved IMO. Hybrid's are highly efficient when it comes to particulate emitted (EPA). But when it comes to the transfer of heat from inside the stove to inside the home, the results are just average (real world efficiency).

The point of my input is the fact that manufacturers will sometimes market a stove (or a technology) in such a way to mislead the public. The people pushing hybrids want you to believe this is some sort of new, highly efficient technology. Even from an emissions standpoint, hybrid's don't own exclusive rights to efficiency since many cat stoves will burn just as (or more) cleanly. I think hybrid's are more about selling stoves than selling a better product.

 

[begreen]

Actually I thought the PH does have a thermostatic valve. Some folks complained about it tapping with the old design. I think that hybrid designs are here to stay, primarily due to upcoming EPA regs. Are they the only solution, no. Are they the most cost effective solution, we'll see. It will take a several years for the market to adapt and embrace the new regs. For sure on some stoves we'll see more automated technology. We may see even more solutions that don't involve cat technology. New Zealand stove makers appear to have taken this approach. Personally I like their KISS solutions. FWIW, cat stoves also require an important manual control. The bypass damper must be closed for it to function.

What the IS has done that is remarkable is that it provides very clean burning technology in a large stove at an affordable price. WS has taken a leadership role here. More will follow.

 

[Idahonative]

Actually I thought the PH does have a thermostatic valve. Some folks complained about it tapping with the old design. I think that hybrid designs are here to stay, primarily due to upcoming EPA regs. Are they the only solution, no. Are they the most cost effective solution, we'll see. It will take a several years for the market to adapt and embrace the new regs. For sure on some stoves we'll see more automated technology. We may see even more solutions that don't involve cat technology. New Zealand stove makers appear to have taken this approach. Personally I like their KISS solutions. FWIW, cat stoves also require an important manual control. The bypass damper must be closed for it to function.

What the IS has done that is remarkable is that it provides very clean burning technology in a large stove at an affordable price. WS has taken a leadership role here. More will follow.

You bring up some interesting points although some may argue $2000 isn't all that affordable. Truthfully, I can't fault any company who invests the time and money to try and improve the equipment we love so much. I guess the thing that has really gotten under my skin is how these stoves are being marketed. In Woodstock's case, they are riding the "it won an award" horse and the "record efficiency" horse but in reality, neither of those mean much in the real world (at least right now).

It's probably the IS cheerleaders on the various forums that have antagonized me into calling them out and that part IS NOT Woodstock's fault. It's very confusing for the average person to sift through all the technical stuff and stating half truths on these forums doesn't help anything. But you are right, the new regs will change things and we will see how the industry adapts.

P.S.- The manual controls I was referring to was air control, not firebox