Hearth temperature

[jinsko]

Hi Everyone-

I am in the process of converting a wood stove to a wfo. It's going pretty well, but not yet as well as I'd like.

I'm having no trouble getting the oven good and hot. But my current problem is that the hearth is not getting as hot as I'd like. The pizzas are cooking faster on top than on bottom.

I'm not sure why I'm not getting more heat on the hearth. As you can see from the photos, the bottom is a little weird. I insulated the underside very thoroughly (with ceramic fiber board and blanket). Then put down steel plate to cover the existing hole (maybe not the best solution?) and then thick set 2.5" fire brick on top of that.

Perhaps I'm losing heat from the bottom yet? Perhaps I'm not firing long enough (about an hour)? Is it possible that the fire brick is poor quality?

Any advice would be very much appreciated.

[boerwarrior]

Re: Hearth temperature

Hi jinsko (Jeff?)

It's a little hard to tell from the photo's exactly how you insulated. Normally you would put the firebrick directly on top of the insulation. It looks like you have the firebrick on top of the steel plate? If that is the case you are likely losing heat through the metal-metal contact with the front and sides of the oven.

Have you tried measuring the floor temperature with an infra-red heat gun? Only $25 from harbor Freight. You will be able to quickly tell if you are getting your floor the to the right temperature. One hour is the quickest I can heat my 32 inch oven. Normally I fire it longer than that before cooking

Neil

[jinsko]

Re: Hearth temperature

Thanks, Neil (yes, it's Jeff; sorry). I haven't picked up a heat gun yet, though I keep meaning to. The reason for the steel plate was to cover up that hole you see in one of the pics; it was the simplest solution. I filled that hole with a piece of ceramic fiber board and then put a great deal of ceramic fiber blanket underneath that.

But you think maybe that so-called simple solution is working against me? The brick is transferring heat to the steel, which is transferring it to the steel on the sides, etc?

I figured (because I'm a know-nothing rookie) that it would be the opposite: that having nowhere else to go the heat from the steel plate beneath would be absorbed the brick...

-j.

[boerwarrior]

Re: Hearth temperature

Hi Jeff,

yes, that sounds to me like it could be the problem. Someone told me (before I built my oven) that the best way to think of a pizza oven is to think of a firebrick chamber COMPLETELY enclosed (top, bottom and sides) with insulation. This mental model was helpful to me when I was building my oven.

It looks like you can modify your oven pretty easily... could you place insulation in top of the steel floor and put the firebrick floor on top of that?

[jinsko]

Re: Hearth temperature

Thanks again. That helps. I don't think I can insulate over the steel as that would raise the floor/hearth up too high. I could eliminate the steel and use something else for the hearth: larger-sized fire brick, maybe soapstone. Do you think that would help?

[boerwarrior]

Re: Hearth temperature

I'm still having trouble visualizing what is under the steel plate. How about removing the steel and then using a thinner layer of insulation plus split firebrick for the floor? That way you wouldn't be up so high. You wouldn't have as much thermal mass in the floor but I think the floor would get much hotter than it does currently.

[v12spirit]

Re: Hearth temperature

I think the extra mass of steel under the hearth (maybe where a drawer or something in the stove was once installed) IS the responsible for heat loss through the hearth. IMHO insulating it as you show in the last picture may just add a little improvement to the oven warming up. Sure the oven will eventually saturate when all the things (including the lazy bricks) are hot enough. But that extra mass underneath is slowing your oven down. I don't know if it is easy to get rid of it. It is just useful in stoves and it is far unfamiliar in ovens.

[GianniFocaccia]

Re: Hearth temperature

I'm curious of you're really losing that much heat through your insulated metal floor and dome.

It appears your oven interior is completely metal except for the firebrick floor, which will normally take much longer to get to proper operating temp. Are you getting a good coal bed from your 60-min fires? My oven too, suffered from boiled pizza toppings until I learned to bring the floor (3.75"-thick) up to temp by spreading a thick layer of coals across it.

If this doesn't work, then you could always place a layer of CF board directly under your bricks and try again.

The only other consideration is that your oven may not breathe properly. You may be able to get a fire hot enough to heat your dome, but ovens that are wider than they are deep notoriously suffer from breathing problems.

[jinsko]

Re: Hearth temperature

Thanks, everybody. Let me try to answer everyone's questions at once:

If you look at the second picture, Neil, you can see the wood burner as it was when I bought it. There's some fire brick, then a big square opening in the middle for venting but also (I assume) for ash removal. There was a drawer to catch them.

I filled all of that up. There's insulating board more or less flush with the original fire brick. Below that, lot's of insulating blanker.

I guess I could have put the fire brick right on top of the old fire brick (and the bit of insulating board), but I put the steel down first. I can try removing it; maybe that's worth a try. Is that your suggestion, v12spirit? I'd be better off without the steel? (Yes, everything else is steel; on the whole, it actually retains heat surprisingly well).

As for firing, Gianni, I guess I'm not entirely sure what a "good coal bed" really is. I get a really good fire going. I let it go for a good while. But I do NOT spread a layer of hot coals over the entire hearth. Is that what you mean? I'd love to see a picture of that if you (or anyone else) has one.

I should also add that this is not a MAJOR problem. It's not as if the bottom isn't getting cooked at all. The pizza is perfectly good. The bottom is just not getting as crispy as I want it. And I'm pretty sure the hearth is not getting as hot as the rest of the oven

[cobblerdave]

Re: Hearth temperature

G'day
I'd not try to change too many things at once.
First thing I'd do is' I'd fire that thing for a least 2 hours with a good fire with heaps of flame, sticks rather than logs. You'd then find you have put a decent load of heat in those firebrick they take way longer to heat up than the shell.
As far as I can tell from you pics your door is oversized. Rough rule of thumb is that the door height be under 2/3 the height of the inside of the shell. Door width is smig under 1/2 the interior width. This will give you the best efficiency, but a touch either way it will still work of course.
Regards Dave

[jinsko]

Re: Hearth temperature

Thanks, cobblerdave. The longer heat time is what I'll try next.

For the record, my door height is perfect. I'm at about 65% in terms of that ratio. Width is another matter and tougher to rectify, though I suppose I could weld dome steel to make it smaller...

But I'm now thinking you may be right and that I'm being misled by how fast the steel/interior heats up. I'll be more patient. So you think the steel plate under the bricks is inconsequential?

Grateful for everyone's experience and knowledge!

[GianniFocaccia]

Re: Hearth temperature

Ok - here's what I would do: fit the square hole with firebrick flush with the level of your refractory floor. Hopefully the original floor is close to the 2.5" depth of a firebrick. Either lay your firebrick back down or get something thinner (1.25" splits or soapstone). Insulate underneath with CF board.

You're done, except closing your door down a smidge per Cobblerdave.

The way I get a good coal bed is to build my fire(s) on the side(s) of my oven (your oven is perfect for this) and keep stoking the fire until I get a good pile of glowing embers. Once large enough, I drag 50% of these coals to the middle of my oven and spread them out before adding smaller pieces of wood to burn. After 20-25min the floor is at least 500-600F and quickly recharges from dome heat the rest of the night. My floor is soapstone, but I feel firebrick will charge in a similar fashion. If you try this (sorry I don't have a pic), I think you'll be pleased with the results.

[cobblerdave]

Re: Hearth temperature

G'day
The steel under the firebrick shouldn't cause any problems as long as you have it decently insulated under that.
Good to here that the door height is on the mark!, as for door width I wouldn't be rushing in and welding it up without a little experimenting. You could stack a few bricks at the sides to cut down the width. doesn't have to be anything fancy as it only has to last one go.
Make sure you post your results, good or bad, it just adds to the knowledge base.
Regards Dave

[Mitchamus]

Re: Hearth temperature

ok - I'll weight in.

I agree with Dave & GianniFocaccia.

It's definitely not your hearth design - the steel under the bricks is fine.

I suspect it's a easily fixed with the way you fire the oven

Next time, fire the oven and when you're ready for pizza - move your fire to another spot, brush the ash from the floor (or just bounce the peel up and down to push the ash out the chimney)

And put your pizza directly where the fire just was.

It may be too hot to start with - but I guarantee the bottom will cook first.

You may want to move the pizza after a few seconds if it's too hot.

If the base is cooked and the top needs finishing off - 'Dome' the pizza by lifting it on the peel and inch or so away from the dome (without touching it obviously) to finish it in a few seconds.

I would try this before you change anything.

[v12spirit]

Re: Hearth temperature

I guess I could have put the fire brick right on top of the old fire brick (and the bit of insulating board), but I put the steel down first. I can try removing it; maybe that's worth a try. Is that your suggestion, v12spirit? I'd be better off without the steel? (Yes, everything else is steel; on the whole, it actually retains heat surprisingly well).

I didn't mean to remove the steel sheet you have put. I meant the steel room underneath the oven (where the ash drawer was) is soaking up the heat that is warming up the hearth causing your oven to take longer to heat up. A steel chamber like yours should heat up in less than an hour which is an advantage of having a steel chamber that you are not able to make use of it due to that block of steel underneath the heart.

I was just wondering if you can cut that "drawer room" off using an angle grinder or something so that the oven will be just the part over the yellow line here:



This way the oven will be more efficient. It should be because it is steel. It shouldn't take more than an hour to heat up.