PE Spectrum - How Low Do You Go (with the inlet air)?

If it is like my hampton/regency insert...the colder it is outside the better the chimney draw and the lower you can choke it down. A new stove is a learning curve.

Wow, I am in Wisconsin and its supposed to be 80 today. I am trying to get all the outside painting done before the snow flies...The woodpile will be moved next week so I am set for the end of Oct when it cools down. Good luck and enjoy.

Burntime is right as it cools down your draft will increase. Also a deep bed of coals and a full load of wood will yield the best results on a low burn. Running the air shut all the way down with only a few splits and little to no coal bed (especially in relatively warm temps) is not going to work. Burn a smaller "brisk" fire is the fall/spring. The spectrum is an awesome stove.

To turn the primary air all the way down, you need to have enough secondary combustion to keep the firebox hot enough to keep the secondary combustion going. That usually requires a good coal bed, a decent size load of dry wood, and a hot fire prior to closing the air. It's definitely a tricky business sometimes; you'll think it's ready to turn down, and then 5 minutes later the flame snuffs right out. That's where the thermostatic controls on some models must be really handy.

rjustice4 said:

Do you get secondary combustion when the air inlet is wide open, or are you just blowin those gases right through? One of the reasons I've been trying to close down the air inlet is to assist with the secondary burn. Or in terms of the secondary burn, is more air better?

Also, is it normal to have unburned coals at the end of a burn, or should you just have ash?

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You are getting secondary with the air wide open...it's just harder to see cause the whole firebox is full of flame.

In terms of secondary burn, I find a med-low setting is best but it depends on many factors. Keep experimenting, you will find what works for you.

Generally there is very few unburnt coals left.

By chance are you burning wood from that load of logs you posted a few months back?

I do get secondary combustion even with the air wide open, but with it closed I get more (unless it goes out altogether). I assume this is because more air is pulled out of the burn tubes with the primary closed (anyone know for sure?), there is more unburned smoke produced, and the gases are moving slower. I'm still learning what "optimum" is; it seems to take a very hot firebox (say 550-700 F) to keep the secondary burn going with the primary closed, and thus I get a really hot fire for a couple of hours and then it tapers off. (Which matches the literature on my stove.) What I'd prefer is to keep the firebox a little cooler up front, but have that level maintain for a longer period. It may be that running wide open for a shorter period after reloading and then closing the air only most of the way down will work better. More experimenting for this year, if it ever gets cold.

I think wood moisture content has a lot to do with it too, as really dry wood gassifies a lot better and makes for a better secondary burn. This year I have lots of 2-year seasoned wood to play with, last year it was mostly just 1 year. Last year I found that the super dry locust I used for overnight burns burned completely to ash if given enough time. But the oak I had that wasn't quite as dry often left black coals. Of course, this could be a wood species difference too. So many variables to attempt to control for! All part of the fun.

Disco: You have to imagine the airflow through your stove being able to enter at 2 different points
1) the opening regulated by the primary air control, which can be wide open (very little resistance to flow) all the way to almost fully closed (considerable resistance to flow)
2) The secondary air inlet, which has a fixed inlet size and thus a fixed resistance to airflow

The AMOUNT of air flow through the stove is determined by the chimney draft AND the total resistance of the primary and secondary air paths

So, in the spring, air is warm outside, so there is a smaller temperature differential = less draft.

Or, our friend in CA at 4000 ft, where it is colder, he still has thin air because of the altitude, thus to compensate, the total height of his chimney has to be higher than for us "flatlanders" in order to get good draft. Also, he would benefit from a chimney inside the center of the home, where the stack will stay warmer compared to an external chase (sorry, I don't recall the details of the install).

Conversely, in deep winter at -10f the air is dense and cold, the stack effect is strong and the chimney will draft like crazy.

OK, after all that distraction, getting back to secondary air flow: So one has two air flow paths. With the primary air open wide, the resistance of the secondary air path is going to be high compared to the primary path, so most of the air will flow through the primary path. As you close the primary air down, its resistance grows and more of the flow will go through the secondary path. With the primary air shut down to 20% or less, the secondary path may in fact offer less resistance and more air will flow through the secondary than the primary. This whole discussion assumes that the wood has "taken" and the chimney is hot to begin with.

DiscoInferno said:

I assume this is because more air is pulled out of the burn tubes with the primary closed (anyone know for sure ?)

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KeithO, that is a reasoned and well written description of how the primary and secondary air inlets work on most non-cat EPA stoves. However, it doesn't apply to the Pacific Super Series stoves.

RJ, if you pull your ash drawer all the way out and set it aside, then get down on your belly and look up at the underside of your firebox floor, you'll see a slider plate that covers the primary intake hole. Slide your draft handle left and right, and you'll see how the plate covers the hole on the L setting (although not quite all the way), and opens it up on the H setting to regulate the primary air supply.

Now, look at the back of that slider plate. You'll see a linkage bar that extends backward into a little rectangular chamber at the rear of the stove. Slide your draft handle left and right some more, and you'll see that the linkage bar moves forward and backward at the same time the primary air slider plate opens and closes. That linkage bar is attached to another slider plate at the bottom of the air tube that supplies combustion air to the secondary burn chamber.

This is part of the Super Series design: in your stove, the secondary combustion airflow is regulated along with the primary combustion air when you actuate your draft handle.

I have a Spectrum at home, and here's how I usually work it: I kindle the fresh load (lots of air) until my wood is engulfed in flames and the secondaries light off. Then I turn the draft control down as far as it will go for the duration of the fire. Both the primary and secondary air sliders are designed so they still allow sufficient air for efficient combustion (the secondary burn goes on for hours).

If you're finding unburned chunks at the end of your fire, here's some things to check:


You're burning room air, so make sure there's sufficient air to be had. Make sure your room air knockout is completely knocked out, then try cracking a window a bit, and see if your low fire doesn't perk up. If it does, consider connecting to outside combustion air.

Double-check your fuelwood's moisture level. Split the piece and check it at the freshly exposed surface. Wet wood is a big cause of unburned chunks.

Tom,

Does the Pacific insert work in the same manner? I have air coming up the old ash trap in my fireplace and the surround is not sealed. Dont think we knocked out the inside air plate as it said it was not necessary in most installations in the manual.

Thanks

Wxman, the Pacific insert uses the same D1 firebox as the Spectrum: your draft control regulates both the primary and secondary air inlets in the same way as described above. Your Insert draws its combustion air through a rectangular opening in the backplate of the air plenum by default: if you supply combustion air into the fireplace through the ash cleanout, that's the air your insert will burn. The knockout should be left in place unless you want to burn room air.

Great information Tom. Can you explain how the Summit works. Does the EBT control the primary air or secondary air or both?

In the Summit design, there is no linkage to a second slider plate: the Summit's secondary air tube is open to a free supply of air at all times, although the intake rate automatically fluctuates with changes in chimney updraft (which in turn is driven by the temperature of the fire), exactly as described by KeithO in his post above. The EBT mechanism giveth and taketh away primary combustion air only.

I read about EBT from the link on your site and it say's:

The EBT technology was designed to maximize burn times for each load of fuel by delivering a burst of combustion air to the main and secondary fires when needed, even when the draft control is turned down far enough to create the longest possible burn.


So is it primary air only or both? I'm just curious.

It is both, but not via direct input of secondary air through the EBT intake port. In the Summit firebox, secondary air intake is regulated by chimney updraft. During phase I and phase II of the fire, whenever the firebox cools to a certain temperature the EBT mechanism opens the intake port and supplies additional primary air: this rejuvinates the fire, causing the firebox temp to rise. The hotter primary fire raises the exhaust temp, which increases the chimney updraft, which in turn draws more combustion air into the secondary air inlet to feed the secondary fire.

Not dissing PE or the EBT, but I don't get it. So if the EBT opens up and gives you more air, how the heck does it extend your burn? And why doesn't PE put it in all their stoves if it works so well? Maybe it's in the summit to clean up emissions a little more for that big fire box?

Todd, you're right: the EBT technology is all about the big firebox. We used to heat our showroom with a non-catalytic, EPA approved Avalon 990. It did a good job, but wouldn't hold the fire as long as we wanted it to. So, we switched to the larger 1190 model, thinking the bigger firebox would give us the longer burn we were looking for. We experienced the opposite: while the larger firebox did put out more heat while it was burning, the burn time for each load actually decreased! We finally figured out that the draft control on the big stove didn't shut down as much as the draft control on the smaller model, presumably because the larger firebox needed more air to pass EPA emissions testing at the lowest setting.

When PE designed the 3 cu.ft. Summit firebox, they incorporated the EBT technology to enable a lower draft control setting for longer burns. The EBT only kicks in when the draft control is turned down for the long burn, and only if this draft setting should fail to supply sufficient combustion air to maintain secondary burn temperatures. If, over extended time on the low setting, the firebox cools below secondary lightoff temperature, the EBT opens to inject a shot of air until the firebox comes back up to temp, then shuts back down, returning control of combustion air to the manual draft control. During Stage III of the burn, the charcoaling stage, the EBT doesn't open at all. In this way, the absolute minimum amount of combustion air needed for clean burning is supplied, and the maximum burn time is achieved.

Todd said:

Not dissing PE or the EBT, but I don't get it. So if the EBT opens up and gives you more air, how the heck does it extend your burn? And why doesn't PE put it in all their stoves if it works so well? Maybe it's in the summit to clean up emissions a little more for that big fire box?

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It extends the burn by allowing you to run with the air completely shut down. The EBT allows the fire to "teeter" with just enough air to maintain a clean burn. If there was no EBT one would have to set the air control higher to avoid a smoldering fire...resulting in a shorter burn time.


Edit: Tom we were posting at the same time...I like you explanation better.

Gunner, I've spent about 20 hours on the graphics and description on my EBT page, and about a gazillion postings on this forum to try to explain the EBT technology as I understand it, and you summed it up in three sentences. The owner's manual people at PE should put you on the payroll.

Gunner said:

Todd said:

Not dissing PE or the EBT, but I don't get it. So if the EBT opens up and gives you more air, how the heck does it extend your burn? And why doesn't PE put it in all their stoves if it works so well? Maybe it's in the summit to clean up emissions a little more for that big fire box?

Click to expand...

It extends the burn by allowing you to run with the air completely shut down. The EBT allows the fire to "teeter" with just enough air to maintain a clean burn. If there was no EBT one would have to set the air control higher to avoid a smoldering fire...resulting in a shorter burn time.


Edit: Tom we were posting at the same time...I like you explanation better.

Click to expand...

Ok, I get it. It has alot to do with draft as well as clean burning. I was wondering if EBT would be good for a cat stove, but I guess you don't have to worry about a smoldering clean burn as long as the cat is engaged.