How hot should underside of hearth be getting?

[jaymac2087]

Hello all.
Just wanted to know how hot should the underside of the hearth be getting?
I have 5" of vermicrete on top of the concrete slab but I feel that it is getting a little hot.
Also if cooking pizzas on an 800 degree floor, how long should it stay up to temp?

[blacknoir]

Re: How hot should underside of hearth be getting?

I can't speak for the underside of the hearth, I've never actually checked mine, but I can say that I can usually keep my floor around 650 when cooking pizzas and it's easy to maintain as long as I have a log burning on the side with some flames licking up to the top. 800 is a bit high for my oven, the bottoms would be burnt before the top was cooked but every oven is different.

[jaymac2087]

Re: How hot should underside of hearth be getting?

thank you for the reply, sounds like mine is working ok.

[kebwi]

Re: How hot should underside of hearth be getting?

My fairly new, but thoroughly cured, oven is also a little difficult to keep up to floor temps. I have to arduously maintain the fire on the side or it quickly drops down to the mid 500s. Likewise, my pizzas frequently cook (and start burning) on top before the bottom is blistering. With a good fire I can keep it up in the 700s, but it takes a conscious effort. I have three inches of InsBlock 19 under my floor so I don't know what else I could have done. It's a little dismaying but there's absolutely nothing I can do about it now.

And the underside of my hearth doesn't show any temperature rise, neither does the outside of my dome (the floor of my stucco planter beds). So the heat isn't being lost through the floor or dome of the oven, I have no idea what's wrong. Maybe you just can't keep an oven in the 700s without constant fire. I don't know.

[texassourdough]

Re: How hot should underside of hearth be getting?

An important factor not addressed in this discussion is how long and hard the fire burned before you cleared the oven to do pizza. My design is different from many of you in that my slab is above my insulation to give more mass for retained heat baking. IF I fire for 45 minutes my oven will clear and I can do pizza - for a while - but will have to rake coals out after a half hour or so to recharge the hearth. If I go for an hour and a half, the hearth will stay hot for hours with no extra recharge required.

I know some of you with insulation above the slab have no problems keeping a working temp hearth but others seem to. The question would then be, in part, how hard and long are you firing your oven before doing pizza? One of my concerns with a relatively low hearth mass would be that bread and pizzas significantly depress the surface temp of the hearth when they are placed in the oven. With less mass to supply heat, the lighter hearth could easily have problems and may need harder firing and/or occasional raking of coals to recharge the hearth.

[kebwi]

Re: How hot should underside of hearth be getting?

That's an excellent point. Admittedly, I am usually trying to cook pizza as fast as possible, so once it clears, I push the fire to the side to expose the floor, fire for a few more minutes, then go. So you're point is that my bricks probably aren't truly saturated, that if I fired longer, I could work heat deeper into the bricks. That's a good point.

Thanks.

[texassourdough]

Re: How hot should underside of hearth be getting?

Hi kebwi!

Right! I think it is easy to get complacent about heat loading the oven. I know I have been because last weekend my oven was wet so I fired it intermittently for 24 hours and it was smoking! The hearth sat at 750 or so all night and never needed any additional attention. I usually just fire for 45-50 minutes, let it burn a few minutes, clear the baking area and configure the "fire" and let the hearth cool to cooking temp (about five minues) and go. But I usually have to recharge the hearth when I do that after about an half hour to hour.

It is also easy to lose sight of the fact that ash is a very good insulator. You don't want to fire your oven on a bed of ash. The heat won't get into the hearth much. So be sure to pretty well clear your oven before starting - and it is not a bad idea to remove the ash before you rake the coals out at the end of firing and let them charge the hearth for five or ten minutes before you arrange the oven for cooking. That way you make sure the heat gets into the hearth. And yes, it will mean that you will then have to wait a few minutes for the hearth to cool before you are ready to cook!

Good luck!
Jay

[blacknoir]

Re: How hot should underside of hearth be getting?

you guys and your 45 minute firing.. I just can't get it that hot that quickly.. by the time my dome is clear my floor is nice and saturated.. I scrape the coals off and wait for the floor to cool a bit and then I'm good... but it usually takes me a couple hours. Might take less if I was more diligent with watching it but still not 45 minutes..

[texassourdough]

Re: How hot should underside of hearth be getting?

Hi Shay!

I probably have an advantage on you. I fire my oven with juniper which is a very hot, fast burning (trash) tree of the Texas Hill Country. It burns hot and bright and I practically stuff the oven with it when I fire up. It will burn most of an oven full in about an hour so at about 50 minutes (or more properly about ten minutes after it clears) I remove the ashes, keeping the better coals, spread them around and organize the fire area, remove the excess coals, sweep, wait, bake!

But the juniper definitely seems to speed the heating.
Jay

[kebwi]

Re: How hot should underside of hearth be getting?

I burn a substantial amount of newspaper and cardboard, followed by fir and maple. The oven makes a great incinerator for clean paper and cardboard that would otherwise go in the recycling.

[lwood]

Re: How hot should underside of hearth be getting?

It also is a function of "oven size" which ultimately boils down to mass inertia. The bigger the oven the longer it's going to take to get up to temp and even longer to maintain it. My 60" takes at least 2 hours to get up to temp and hold that temp. I suspect the 45 min are smaller ovens with less mass inertia to push the heat into and can maintain it with a little more heat soak.

That said, a large oven should maintain the heat longer. Well maybe, it depends on many things like amount of insulation, etc, etc. Each oven is different, so just because yours isn't heating up in 45 min doesn't mean there is something wrong. If you used the FB plans and did it the way they describe it here, you should be golden. Cheers John

[david s]

Re: How hot should underside of hearth be getting?

You might get the oven to clear in 45 mins but it takes longer for the heat to soak through to the full thickness of the brick floor. That is why the heat drops off fast because it is wicking its way to the outsides. My oven is small but I still use 1.5 Hrs for pizza cooking unles I'm only cooking 3-4 pizzas.

[jaymac2087]

Re: How hot should underside of hearth be getting?

Have any of you guys ever checked the temp of the underside of the hearth?
Mine seems to be a bit hot. I will be doing a couple of pizzas tonite so will check the temp but I think it is hotter than it should be.

[lwood]

Re: How hot should underside of hearth be getting?

I need to also check mine as well, will let you know.

[david s]

Re: How hot should underside of hearth be getting?

The underside of your hearth should never be any more than cosy warm. If it is then your insulation is either inadequate or your oven still has moisture. The moisture on the bottom always seems the hardest to eliminate.
Dave