Pellet Driven WFO?

[mendozer]

I just learned of an oven called the Uuni 2s. It's a stainless steel portable pellet driven oven. It reaches 900 F within 10 minutes. However, I would assume it regularly needs pellets to maintain temps since it doesn't have insulation like the brick/refractory ones on here do.

However, I'm intrigued on the idea of a brick or refractory oven using pellets as fuel. Pellets are cheap and provide a pretty fixed BTU output, making calculating resources for heat easy. Has anyone tried a dome oven with a pellet hopper chute in the back using gravity to draw in pellets for burning?

[mendozer]

The roccbox is another option for design inspiration. It has a wood stove that screws into the back. Heat flows back to front, no chimney. If it were bigger I would be game for using pellets instead of wood sticks for heat.

What I'm wondering is if pellets being used in a gasifying stove can generate enough BTUs for heating a dome with refractory lining. I think it would be cool to use pellets to heat a refractory WFO because a variety of different pellets are available easier than logs at least where I am. I don't have the ability to buy a yard of apple wood, but a $8 bag of pellets at Cash and Carry can be had easily. A few design questions:

1) attaching the wood stove to the bottom of the oven. Either opening at the back of the hearth, or centrally under the hearth (elevated cooking surface) with redirecting paths around the hearth and a central chimney of the dome
2) gravity fed hopper design for pellets would be preferable
3) is there a calculator anywhere that tells me how many BTUs are required to maintain X temperature for X volume? I feel like there's some math out there justifying these oven designs but don't know where it is.

Is this feasible or do you think you'd burn through pellets just to maintain temps? Keep in mind I'm thinking of a traditional WFO so there will be insulating materials under the hearth and around the dome as well.

[david s]

I feel like you are wasting your time, although I'd like you to prove me wrong. For a WFO you need flame and pellets are designed to give up their energy slowly. With wood we often talk about "nothing thicker than your wrist", this is so there is more surface area to get more flame. If the wood is a big log it burns too slowly and the oven either does not reach temperature or it takes too long. The same thing happens with pellets. This might be fine if you are roasting or baking at lower temperatures or if you are happy to cook six minute pizzas, but not much use for 90 sec or 2 min pizza.
Regarding a calculator, the BTU's in the fuel are roughly directly proportional to the weight, ie a softwood burns faster than a hardwood but you need more of it. I'd guess that a given weight of pellets would have the same BTU's as the same weight of softwood or hardwood.
The fuel consumption is also roughly directly proportional to the volume of the chamber.

[mendozer]

well items like the Uuni or Roccbox cook 90-120 second pizzas, however they're much smaller. They have stone hearths and metal surroundings, so I see them as being good for getting hot really fast, but not necessarily staying hot for long.

I discussed this with my wife and we honestly don't see ourselves heating up the oven for an hour, then eating a few pizzas, then cooking a chicken, vegetables, bread for the next few hours. We want a pizza oven outside, that's pretty much it. What I'm curious about is whether pellets will heat up a larger space (like my 30" dome plan) for pizzas. I see the value in these portable ones, but I want to know if the pellets at 8000 BTU/pound will heat a 30" dome.

[david s]

The problem is that brick or any other refractory material is not particularly conductive. It takes roughly one hour per inch for heat to travel through brick or any other dense refractory. Stainless steel is some 10 x more conductive than brick.

http://www.engineeringtoolbox.com/th...ity-d_429.html

[deejayoh]

It appears that firewood and pellets have about the same BTU per pound. But pellets appear to cost more per BTU than firewood.

http://www.evergreenbioenergy.com/facts.html

The magic of a pellet oven is more the ability for it to feed itself pellets and burn at a constant temp all day.

If the BTU content is the same, then as David points out, you aren't really going to save any time heating your oven. The stainless ovens heat fast because the material is conductive. But they don't retain heat. There are plenty of models to be had if you don't care about retained heat. Refractory materials take time to heat but also discharge the heat slower

Bottom line is that you aren't going to beat the laws of thermodynamics just by changing the format of wood.

[Gulf]

If you haven't seen it here is another recent thread on the subject.

Two way Heat idea for Pompeii Oven.

[mendozer]

ok as far as the metal vs brick goes, why not combine the two?

Steel inner oven (whether that be dome, barrel, etc) with 4" of perlcrete outer for insulation? I'm not trying to argue against the tried and true WFO but rather think of newer ideas.

[RandyJ]

You are still not in the same ball park. A true WFO will hold it's heat for days. My oven will take 10 days to cool down to under 100F. I cooked a beer can chicken 2 days after I had a fire and the temp was 631F. You also will never get to the same high temperature with the pellet burner. I love to cook steaks with my oven with temps in the 13-1400F range. They are done in 2 min a side for a nice thick portarhouse. If you want a pellet smoker then get a pellet smoker. If you want a WFO the build a WFO they are 2 completely different things and do different kinds of cooking.

Randy

[UtahBeehiver]

V12Spirit did a steel inner dome with brick or refractory on the outside. Whether it works well or not, the jury is still out. But here is his link.

https://community.fornobravo.com/for...to-a-steel-one

Dr_Mausse aka Loren had a friend do a steel oven here is the link

https://community.fornobravo.com/for...-journey/page7

Nothing wrong with thinking outside the box but our forum members are giving you practical and real world advice and since you only want to heat oven, cook a pizza then call it good. Then maybe these traditional WFOs are not for you.

[deejayoh]

ok as far as the metal vs brick goes, why not combine the two?

Steel inner oven (whether that be dome, barrel, etc) with 4" of perlcrete outer for insulation? I'm not trying to argue against the tried and true WFO but rather think of newer ideas.

That's not a combo, perlcrete would just be insulation around a steel oven. Since the steel doesn't store much heat, it would make the oven more efficient while burning - but it wouldn't stay hot for long.

If you add a layer of refractory, it will stay hot - but it won't get hot fast. You really aren't going to fool the laws of physics. Even if you think out of the box...

I think what you might want is a gas fired stainless oven... Gets hot fast and you don't have to deal with firewood. Me, I kind of like the firewood.

Edit:

here is a company that makes pellet-fired commercial ovens. I note that they claim the BTU content of pellets to be higher than that of firewood - but I think that's because they're using pellets w/6-8% moisture content vs. typical ~20% for firewood. The water adds to weight while providing no BTU content

http://www.maranaforni.it/en/forni-p...one-a-pellet/#

[mendozer]

so you're saying that even if I use metal, it won't heat up to 900F? Is that based off of expereince or is there some thermodynamic formula out there saying that a source of X BTU/hr can't heat up X area?

[deejayoh]

so you're saying that even if I use metal, it won't heat up to 900F? Is that based off of expereince or is there some thermodynamic formula out there saying that a source of X BTU/hr can't heat up X area?

Not sure to whom you're responding - but I'll give my take. You're asking about two things: build material and fuel

Build material:

• Steel or refractory, either oven will get to 900 degrees.
• A steel oven (based on other examples and what we know about conductivity) can get to that temp in ~30 minutes. But once the heat source is gone - it will only hold cooking temps for a few hours.
• A refractory oven (4.5 inches thick like most on this site) will take ~90-120 minutes to get up to that temp. But it will hold cooking temps for several days. Next day will be 500, 400 the next, 300...

Fuel

• Wood or pellet, either fuel should work, and both will take the same amount of time. You may be able to use slightly lower weight in pellets, but they will probably cost you more.

Does that help?

[mendozer]

OK that helps me. I'm not worried about carryover over temps for days and days. I'm also planning to incorporate a rocket stove design that can use either wood or pellets. And with regards to that, Costco here has 33 lb pellet bags for $20 and Cash and Carry has 20 lb for 10.33 or 33 lb for 16.25. I don't have a source for wood other than chips really. People do sell applewood by the cord, but that's too much.

[RandyJ]

I am sure there has to be someone in the area that sells fire wood. It might not hurt to check it out. And remember that even in pellet form you should not be burning soft wood. Make sure it is oak or something like that.

Randy