How much wood to use to fire Primavera 70

[Lowensteinis]

Hello,

I have had my oven for just over a month and have used it very often. I have the Primavera 70 and the specs say you can get the oven to temperature in 20min.
I have managed to this once, however not been able to do it again.

Any help on how many logs to start with could very much help me out here. If anyone by chance has a pic of the wood before they light it where it got up to temp in that time would also be appreciated.
Any other help would be welcomed too :-) I seem to spend an hour or more trying to get the oven up to temp.

Thanks so much in advance,
Regards.

Bradley

[Gulf]

Hi Lowensteinis,

Congratulations on the Primavera. I don't have an oven exactly like yours, so I can't accurately compare apples to apples. But here is my opinion. The dryer the wood, the hotter it burns. The smaller the diameter of the wood the faster it burns. However, I'm not sure how good that is for a new oven as a very fast heat up can damage some refractories. That said, I have access to lot of dead fall limbs. Dead fall limbs have usually dried on the tree before a high wind dislodges them. After a few more weeks drying in my shed they burn hot and fast. I have a large oven which can take over 2 hours to clear the dome. The exteremely dry dead falls cut that time by about 30 minutes. I usually swap over to cut dry seasoned wood toward the end.

[Petter]

Did some calculations . It should take about 7 kg of wood and 1 h to heat once the fire gets going.

If you have time and the possibility to limit air flow (i. e. optimize combustion) , you could get away with burning 4 kg in total during 2 h.

Why? Because heat soaks slowly in refractory and burning fast will only heat the very surface. The residual heat goes up the chimeny If burning faster.

20 minutes, sure with a large fire and lots of kindling but please be kind to your refractory. It likes ho heat slowly not to crack.

[Lowensteinis]

Thanks so much Gulf I had the oven cured at the factory before delivery to make sure the curing was as accurate as can be. Since having the oven, I have used it at least 10 times. I am taking anywhere from 60-90 min to get up to temp, and this is really bothering me. I have been trying to cut my logs down to a smaller diameter, how ever I have not yet found the sweet spot. Hopefully you and Petter I will be able to come up with something in less time.

[Lowensteinis]

Petter I did not realize the slower the better for these ovens. I thought that the purpose of refactories was the quick heat up time. I generally do have the time to fire slower, it's all about when I start the fire as to when the family want to heat. I will make sure to do this slower in the future and count on more time to get to a clear dome.

[david s]

Plus one on being kind to your refractory. Any dense refractory does not take kindly to sudden temperature rise particularly in the lower range <400C. Increasing heat means expansion and if one side hasn’t had time to catch up a bit the uneven expansion can lead to micro cracking which may not even be visible. If moisture is present in part of the refractory it holds the temperature down in that part which exacerbated the problem and at worst can cause steam spalling which creates catastrophic damage, worst in the case of unforced clay which is swollen and blocks the path of moisture escape. As all potters know, the standard safe firing schedule should not exceed a 100 C/hr ramp to avoid damage of the wares, but it also applies to dense kiln refractory (although not ceramic fibre lining). A fire brick that may be designed for 1500C service is still vulnerable to over quick firings. WFO owners tend to fire ovens up at a rate of 300C/hr. Reducing the air intake to get the oven temp up faster is a mistake IMO and while it will get the temp up quicker will burn more ferociously resulting in the same fuel consumption. Better to crack a beer and let the oven do its work more comfortably. (unless you are really in a rush)

[Petter]

Reducing the air intake to get the oven temp up faster is a mistake IMO and while it will get the temp up quicker will burn more ferociously resulting in the same fuel consumption. Better to crack a beer and let the oven do its work more comfortably. (unless you are really in a rush)

Sorry I was'nt clear, the point is not to heat faster. With controlled air intake, the amout of fuel can be reduced for equal heat output. It's more trickey than an ordinary fire though. Still time for a beer!

[david s]

The problem with firing with wood is that the amount of fuel delivered is extremely difficult to control. With a reduced air intake the power of the flue sucks oxygen in creating a ferocious burn. In a gas kiln with a much reduced air intake area compared to a WFO, the fuel control is far better controlled with a tap and the atmosphere can easily be controlled to produce either an oxidation or reduction atmosphere. When you throw a lump of wood into a WFO it’s consumption is variable. Not much is consumed at first then away it goes, more fire than you need. A reduced intake area magnifies the reaction as more oxygen is delivered. A much wider intake area actually reduces the oxygen intake and the fire simply takes what oxygen it needs resulting in a more gentle fire.

[Petter]

Good answer, thanks. I guess I'm too biased from the batch rocket stove-thinking. Will try later on though, just for curiosity ;-)