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Offline miv

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Chinese countertop ovens mod thread
« on: November 12, 2021, 07:40:33 AM »
Hello everyone, recently I had a bit of time to spare, so I am back to trying to mod my oven(if only P134H was 400 euro and not 600-700 :drool:).

For those of you who are not acquainted with the ovens I am talking about:

There are many vendors who are selling those ovens. I've seen them for as low as 160 $
in Chinese sites up to around 230 euro, so prices can vary.
You can look them up at Aliexpress/Expondo/Royal catering/Ebay/GGM/etc.
I'd say around 180 euro is a pretty good deal unless you're in China in which case try to get it at 160 $ or lower.

So, I've seen the mods in other Italian/French forums and thought it's a good idea to make the mod myself too.
I am still in the process of gathering/modding stuff, but it's been really hard.
(translating/sourcing things/etc.)

My idea is to share my findings so far so that more people may attempt this if they want to.

So here we go:
These ovens come with 2x350 C thermostats.
There are cheap 400 C thermostats on Aliexpress for 3-5 $
A 500 C thermostat costs around 36 $.(or you can get an EGO branded one for about 50$)
Another idea is to use a PID controller, however you will need to add ventilation for that
or you can put in an outer box like some people do.
The good thing is that you can do precise control.
I am currently doing a swap on the bottom for 400 C and a bypass for the top.

The basic insulation on my oven is Fiberglass Wool - the cheapest kind.
The thermal conductivity is good below 300 C, but at 350 C the exterior gets kinda hot.
There are also places where the insulation is missing, so you will see a few random hot spots at 100-200 C.
Fiberglass Wool is not good for temperatures > 300 C and can melt at around 600 C,
so I am planning on changing it.
After looking at a lot of spec sheets, my findings so far are as follows(from more expensive/better to cheaper):
Aerogel 0.065 conductivity at 500 C
Ceramic Wool 0.125 conductivity at 500 C
Rock Wool 0.16 conductivity at 500 C

A lot of industrial deck ovens(62x62 cm 65 kg beasts) use Rock Wool because it's really cheap
and they place like 15-20 CM of it. However this Chinese countertop is space constrained,
so I am currently in the process of obtaining Ceramic wool/Aerogel.

Am I still unclear on whether I need to wrap the new insulation in aluminum foil
to reduce thermal losses further. Most premium ovens I've looked at had foil.
And most Rock wools have it too, but most Ceramic wools don't.

Safety note:
It seems that most Ceramic wool is a health hazard(so is Fiberglass wool),
so a respirator is needed when handling it and of course gloves.
I am still researching what respirator to buy.

Oven feet:
The feet are made out of rubber and screwed to the oven via M8(i think, have to check if it's m6 actually) bolts.
When running at 350 C they get pretty hot and one of them seems to be melting(very slowly, it could take years I guess).
I've seen that others replace the feet with longer screws so that the rubber does not touch the oven.
Perhaps this will be meaningless after I insulate the bottom of the oven but it can't hurt.

Oven lamp:
It seems that the standard bulb will not take more than 350-400 C so it needs to be replaced.
I bought 500 C lamps from Aliexpress but they were for the wrong socket, checking again it seems I need G9 lamps.
They are cheap and cost 1-2 $ per piece.

The oven comes with a thin Chamotte(also known as Grog) which is pretty conductive. It will burn the pizza at more than 400 C.
Hence the need for a Biscotto(lower conductive stone made out of clay, there are different types depending on the region where it's made).
For example I've seen Biscotto Casapulla and Biscoto Saputo.
I am in the process of obtaining 40x40x2.5 cm stone.
There are a lot of vendors selling 40x35 because that's the size of the P134H, but the chinese countertops are 40x40.
I've picked 2.5 cm thickness because I do not want to wait too much and I don't bake a lot in succession.

My plan here is pretty straightforward:
Attach a 230 VAC to 12 DC power supply to a CPU fan on the side of the oven.
I am thinking of using it pull in cold air hence the hot air will go out from the vent holes.

Resistances/heaters/elements(not sure what's the proper term) and cabling
Most of the time you get 2x1000W with those type of oven(some have 2x800W). 1000W seems to be perfectly acceptable for the bottom,
but it's not for the top. What's more - the oven is too tall, so I've currently lifted my stone via steel pieces.
However I am planning on replacing the top with 2300-2500 W so that I won't need to lift the stone.
The power cable is 3x1.5mm2, I am thinking of doing either 2.5mm2 or 4mm2. 3600W seems okay
with a Schuko power outlet mounted on 4mm2. And it's not above 16A either(assuming 230V mains).
On the inside 1.5mm2 per resistance is adequate even for 2500 W since it's really short.
Of course thermal protection for the cable is a good idea.

As for the resistance physical size - I am trying to get the biggest one that I can fit but ofc I want it to be uniform
so that the heating is balanced. There are a lot of top heaters for normal ovens which should do the job fine.
And then there are shops which can do a custom resistance for you.

Door glass
It seems the door glass loses a lot of heat. Other forum members have told me to stick a second piece of glass,
which is not a bad idea. However I am not sure the stock glass itself is suited to 500 C, and also it has a metal mount around it,
so I am not sure how to proceed with modding it. My idea so far is to just replace the glass with a solid metal piece or better yet two pieces
with Aerogel between them.

Door gasket/sealing
The door has no sealing, so I am sure a lot of heat is lost there. I am in the process of obtaining self-adhesive ceramic tape.
Of course I am thinking of leaving some tiny holes so that the moisture can escape and I don't get 500 C steam in my face when I open the door  :-D

Door handle
I've read reports of melting door handles, so I am planning to replace mine with a piece of wood as others have done.

Thermal mass
This really has bothered me and I believe because the oven's insides are thin, it does not have a lot of thermal mass.
We can fix the bottom by adding metal grills below the stone/etc. And the stone itself acts like thermal mass.
The top however is different, there's only a thin metal ceiling. So I am thinking of adding a steel plate above
the top heating element. However I am not sure what type of steel should I use, maybe 304 or 316 or something else?

Another safety note
Please do not do such modding if you do not have the relevant experience.
Everything you do is at your own risk.
I take no responsibility for any of the information in the thread.

So to summarize the biggest issues I am seeing so far:
  • What respirator to buy for using to change the insulation?
  • Whether to wrap the insulation in aluminum foil?
  • How to tell if the glass on the door can take 500 C without trying and getting glass in my face :-D?
    (open to idea regarding the glass)
  • Whether to increase the thermal mass of the oven by adding steel plates to the sides/top and what
    type of steel to use?

Offline barryvabeach

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Re: Chinese countertop ovens mod thread
« Reply #1 on: November 12, 2021, 09:08:04 PM »
I would suggest you also consider the wiring.  So long as you keep the oven fairly far away from any combustibles, the biggest concern with temps about OEM temp, IMO, is overheating the insulation on the wiring and causing a fire.  So for the ones I modded, i upgraded the wiring, both in gauge in in temp rating.    I saw that you were considering a bypass for the top thermostat.  I would advise against it, if you forget to switch it off, that means it will keep on heating, and could cause a fire or some other injury.  If you do go that route, make an evaluation of how high the temp can go before you run into problems, and wire in a "roll out" also called a thermal control switch or snap disc limit switch - you will want a NC that is designed to open at the upper temp,  as a fail safe. 

I didn't worry much about the insulation, though I did add some better insulation. Unless you are running it for extended period, insulation and thermal mass were not that important.  As long as the amount of heat generated by the elements exceeeds the heat loss from the chamber, the temp will keep climbing until the thermostat cuts off the element. 

As to the glass, it depends on how high you want to go, it is not all that hard to find very high temp glass - a lot of places sell custom glass for wood stoves that are rated up to 1,470 F

For the most part, the critical issues were getting an even heat on the stone so you would not have hot spots, and  an intense even blast of heat near the top of the pie while it was cooking.  I suggest you check out the Breville info on what they did to design their oven  https://www.breville.com/us/en/smart-oven-pizzaiolo/innovation.html   Since I did not go to the lengths they did, I just played with the thermo on the upper element and tried to have it set below max, so that when I went to load the pie,  I could turn it to max to get some browning.   I had a great deal of difficulty finding a good upper element for the last oven I tried to mod - getting the right size to fit the oven, getting enough bends to get an even heat, and getting a more powerful element, but staying within a safe amp load for the 240 outlet I was using, was quite challenging.  Good luck and let us know how it goes. 
« Last Edit: November 12, 2021, 09:18:47 PM by barryvabeach »
Current Ovens  -  BS, Ooni Koda 16
Mixers .   Electrolux ( ANK ) ,  Bosch Compact,
Mills - Retsel, Lee .

Offline miv

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Re: Chinese countertop ovens mod thread
« Reply #2 on: November 13, 2021, 04:13:12 PM »
I don't think that the cables will heat up, since I am planning to install a 12 V CPU fan to cool them and if one is not enough, I will add another. I also have 2 thermal limiters for up-to 55 C, so if the air temp gets above they will the stop the heat(and the fan will continue cooling). But for added safety all cabling will be put in fiberglass tubes rated at 600 C.

As for connecting to a power outlet 3x2.5mm2 wire should do just fine for 3.6 kW. 3.6kW at 230V means 15.65A which is almost the limit for Schuko power outlets - they are rated 250V 16A. So basically bottom 1000W, top 2500W plus fan and stuff ~3.6 kW.

I've started reading up on the physics of ovens - heat transfer/insulation/etc. So I am thinking I do need PIDs after all if I want to have stable temperature with high thermal losses.

One question that perhaps you can share ideas about: Is there such thing as too much top heater power? Say if I hardwire the oven to 4mm2 and do 4kW top/1kW bottom heaters, it will heat up very fast, but then perhaps it will burn the pizza if that 4kW monster turns on while baking it?(maybe the PID can solve that though - since it will control the amount of heat by PWM)

Also can you share what element did you choose for the top?

And about the glass - I can find a similar glass pane/etc., but what about the metal frame, where do you get a glass of the right size with a metal frame? Or better yet - two pieces of glass with air gap between them in a metal frame?

And one more thing - what if the top heater is not used at all while baking - say by installing a thick steel plate above the top heater, we store a lot of heat there, so it will compensate the opening of the door. And also I've seen people using thermal reflectors - basically a disc below the top heater?

Offline barryvabeach

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Re: Chinese countertop ovens mod thread
« Reply #3 on: November 13, 2021, 05:32:09 PM »
I have made a few PID's  -  I don't think they would be of much help in a pizza oven, and actually would be a negative for the top element.  PID's shine where you need to keep an extremely even temp, and don't want cycles that go up or down 25 of 50 F.  In our gas pizza ovens, we often have swings far in excess of that  ( depending on adjustments to the flame ) and that does not present a problem.  I would spend your money elsewhere since you are getting a thicker bottom stone, that will even out the heat enough for a pizza.

The physics of heat transfer suggest that the top element be much hotter than the stone since the stone is in physical contact with the bottom of the pie, so for me, I don't think there is too much top heat, though of course, you need to balance, so the top should be hotter than the bottom, by how much will depend on the style of pizza, contruction of the oven, and other factors.

https://www.pizzamaking.com/forum/index.php?topic=22190.msg225456#msg225456  This is where I got a replacement element first time around, I don't recall if I had any success on the next oven. 

For the glass, I didn't need to upgrade, but if I did, there were places that cut it to custom sizes, and I would use existing trim to hold the rear piece, and would probably fabricate a spacer out of stainless steel to wedge the front glass in place, assuming the opening at the rear was bigger than the front.

As to steel above the upper element, it appears that Breville used steel,  I doubt the thickness has much to do with anything ,you are really working on radiating heat and since the pie is usually only cooking for a few minutes of time, I would not think you would need to store much heat.
Current Ovens  -  BS, Ooni Koda 16
Mixers .   Electrolux ( ANK ) ,  Bosch Compact,
Mills - Retsel, Lee .

Offline miv

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  • Location: Bulgaria
  • I Love Pizza!
Re: Chinese countertop ovens mod thread
« Reply #4 on: November 13, 2021, 07:14:56 PM »
Thanks for the heads up on the PIDs.

Hm, I guess I will just try with 2x400 C thermostats then since they cost like 3 $ and not 30 $(like the ones for 500 C).
Plus bypass but with internal safety cutoff(50 C in electrics compartment).

After experimenting I will decide if I should go PID.

And ofc I will mount a 3-5mm steel piece on top of the top element, because why not, Breville are doing it :-)
(and calculating the energy needed to heat it with 2500W - it seems it won't increase the heat up time)

I think that for NP maybe you don't need thermal mass, but for NY pizza it's different.
I've used this oven for some NY pizzas.
One issue is that without modding it loses of a lot of heat, Yael has mentioned this too.
And also the heat is uneven. I need to rotate the pizza a lot.
So I am thinking that thermal mass + better insulation could solve it.
While the more powerful heating element will do the opposite, basically overcooking the toppings.
I wonder if the 2500W top element will still overcook the NY pizza top, then perhaps the PID will help.
But again - I will have to experiment to find out :-)

Btw what type of insulation did you use? I was confident in getting ceramic wool, but I've read on
some other forums people telling that's it's dangerous for your health/etc. Also did you wrap it in aluminum foil?